1. Introduction
It is tempting to think of clouds in the inner regions of tropical cyclones as cumulonimbus that just happen to be located in a spinning vortex. However, this view is oversimplified, as the clouds in a tropical cyclone are intricately connected with the dynamics of the cyclone itself. Perhaps up to now it has not been important to know the detailed inner workings of tropical cyclone clouds. However, as high-resolution full-physics models become more widely used, forecasting the probabilities of extreme weather and heavy precipitation at specific times and locations will become an increasingly feasible goal for landfalling cyclones. In addition, prediction of sudden and rapid changes in storm intensity remains perhaps the greatest challenge in tropical cyclone preparation and warning. Rapid intensity changes often occur in conjunction with rapid reorganization of the cyclone’s mesoscale cloud and precipitation structures. Realistic simulation of the clouds and precipitation, therefore, is a particularly important aspect of the models’ ability to make these forecasts accurately and for the right physical and dynamical reasons.
Accurate representation of tropical cyclone clouds in numerical models is also important from a climate dynamics standpoint. Tropical cyclones have been at the center of discussions of global warming effects. Although the issue remains unresolved, tropical cyclones may increase in number and/or intensity as the earth warms. Since tropical cyclones are major producers of both cloud cover and precipitation in the tropics and subtropics, the correct prediction of tropical cyclone behavior in global climate models will depend ultimately on the accuracy with which tropical cyclone clouds are represented.
For these reasons, it seems appropriate to synthesize and organize the available information on the diverse cloud processes within tropical cyclones. This review presents a dynamical and physical description of the main types of clouds in a tropical cyclone with an emphasis on how they relate to the dynamics of the cyclone itself. The paper will proceed by first examining the cloud structures associated with the formation of a tropical cyclone. Then the review will consider in detail how the mean symmetric structure of the vortex affects cloud structure. Following that will be a discussion of how cloud structures are affected and controlled by storm asymmetries, including those brought on by the storm’s translation, the large-scale environmental wind shear, the dynamics of rainbands (which may evolve into a new eyewall) and embedded convection. The review will extend to an even smaller scale by examining the microphysics and electrification of clouds within tropical cyclones.
2. Definitions, climatology, and the synoptic-scale contexts of tropical cyclones
According to the Glossary of Meteorology (Glickman 2000), a tropical cyclone is any low pressure system having a closed circulation and originating over a tropical ocean. It is further categorized according to its peak wind speed: a tropical depression has winds of <17 m s−1, a tropical storm has a peak wind of 18–32 m s−1, and a severe tropical cyclone has a peak wind of 33 m s−1 or more.1 In this article, we will use the term tropical cyclone to refer primarily to the stronger tropical storms and severe tropical cyclones. Severe tropical cyclones have local names. They are called hurricanes in the Atlantic and eastern North Pacific Oceans, typhoons in the western North Pacific Ocean, and cyclones in the South Pacific and Indian Oceans. They are all the same phenomenon.
Gray (1968) identified most of the environmental factors favoring tropical cyclones. They originate over oceans, as their primary energy source is the latent heat of water vapor in the atmospheric boundary layer. They nearly always form over regions where the sea surface temperature (SST) exceeds 26.5°C (Fig. 1a). Once tropical cyclones form, they tend to be advected by the large-scale wind. Figure 1b shows how the tracks are generally westward at lower latitudes, where easterlies dominate the large-scale flow. The storms often “recurve” toward the east, when they move poleward into the midlatitude westerlies. When they encounter land or colder water, they die out or (more rarely) transition into extratropical cyclones (Jones et al. 2003).
Although tropical cyclones form at low latitudes, generally between 5° and 20° (Fig. 1a), they rarely form within ±5° of the equator because the Coriolis force is too weak for low-level convergence to be able to generate relative vorticity.2 Gray (1968) found that genesis regions not only are located off the equator, where the Coriolis force is nonzero, but also have higher-than-average relative vorticity of the surface wind field and temperature stratification that is at least moderately conditionally unstable, and weak vertical shear of the horizontal wind. The strong background positive vorticity helps to trap energy released in convection so that it contributes via generation of potential vorticity to strengthening of the positive vorticity. Weak shear of the background flow allows the cyclone to develop vertical coherence. The climatological presence of strong shear over the South Atlantic, plus the lack of synoptic precursor disturbances, accounts for the extremely rare occurrence of tropical cyclones in that region. Besides the factors highlighted by Gray (1968), the relative humidity of the environment must not be too low (DeMaria et al. 2001). Otherwise, clouds cannot fully develop, and without clouds to transport moist static energy upward from the boundary layer in contact with the ocean, there can be no tropical cyclone.
The large-scale preconditions for tropical cyclogenesis noted by Gray (1968) and DeMaria et al. (2001) may be set up by a variety synoptic-scale processes. Bracken and Bosart (2000), Davis and Bosart (2001), and McTaggart-Cowan et al. (2008) have shown that about half of Atlantic tropical cyclones occur when African easterly waves (Burpee 1972; Reed et al. 1977, 1988; Thorncroft and Hodges 2001) move westward off the African continent, and a tropical cyclone spins up in the trough of the wave, provided the wave trough does not ingest dry midlevel air from the Saharan region (Dunion and Velden 2004). Other Atlantic tropical cyclones develop when midlatitude synoptic-scale dynamics operating over a warm ocean surface set up conditions of large-scale shear and vorticity that lead to tropical cyclogenesis. According to Bracken and Bosart (2000), Davis and Bosart (2001), and McTaggart-Cowan et al. (2008), these midlatitude conditions involve midlatitude troughs and/or fronts extending into low latitudes.
The ultimate formation of a tropical cyclone in a favorable synoptic-scale environment entails the concentration of perturbation kinetic energy into a low pressure system with a horizontal scale of a few hundred kilometers. The concentration of energy on this scale may be partly a downscale process, whereby a developing storm extracts energy from larger scales of motion. One idea suggested by Webster and Chang (1997) is that when a synoptic-scale wave in the easterlies enters a region of large-scale negative stretching deformation (∂
3. Clouds involved in tropical cyclogenesis
a. Idealization of the clouds in an intensifying depression
As noted in the previous section, tropical cyclogenesis is partly an upscale process, with convective-scale dynamics locally adding energy and vorticity to a preexisting cyclonic disturbance of the large-scale flow, in a region synoptically predisposed to development. Convective clouds are central in the upscale positive feedback to tropical cyclogenesis. Steranka et al. (1986) noted from satellite data that prolonged “convective bursts” occurred “in the near region surrounding the depression centers before the maximum winds initially increased.” These bursts of cirrus outflow from convective clouds stand out in satellite imagery and suggest that a very special type of cumulonimbus cloud is involved in tropical cyclogenesis.
Figure 2 illustrates conceptually the variety of convective entities that may exist in the region where a convective burst presages intensification of a preexisting vortex to tropical storm status. Some of these convective entities are individual towers, while others take the form of a mesoscale convective system (MCS), which contains both deep convective cells and stratiform clouds and precipitation (see Houze 2004 for a review of MCSs). The convective entities in Fig. 2d are depicted within a low-level preexisting cyclonic circulation. From Gray’s (1968) climatology we know that cyclogenesis occurs in an environment rich in vorticity at low levels, and the cloud-scale processes depicted in Fig. 2 can only operate effectively to enhance cyclogenesis in a preexisting region of low pressure and maximum vorticity such as indicated by the L and the dashed cyclonic streamline in the figure. As discussed in section 2, the work of Dunkerton et al. (2008) suggests further that cyclogenesis may be especially effective in a closed cyclonic circulation protected from the larger-scale environment.
Figures 2a–c give a simplified depiction of the life cycle of the cloud-scale MCS as it would occur within an assumed larger-scale environment rich in vorticity at lower levels. The general characteristics of the schematic life cycle are an adaptation of the generic MCS life cycle described by Houze (1982), but with an emphasis on the way vorticity might develop within the MCS. The MCS begins as one or more isolated deep convective towers (Fig. 2a). The vorticity of the low-level environment is stretched by convergence at the base of the buoyant convective updrafts and advected upward. The updrafts thus become upward extending centers of high positive vorticity, which Hendricks et al. (2004) called vortical hot towers (VHTs; see Fig. 2), as a special case of the “hot tower” terminology introduced by Riehl and Malkus (1958) to describe deep tropical convection in general. As an individual convective tower dies off, it weakens and becomes part of a precipitating stratiform cloud (Houze 1997). New towers form adjacent to the stratiform region, so that at its mature stage of development the MCS has both convective and stratiform components (Fig. 2b). The base of the stratiform cloud deck is in the midtroposphere, as it is mostly composed of the material of the upper portions of convective cells. The vertical profile of heating in the stratiform region leads to the development of a mesoscale convective vortex (MCV) in midlevels in association with the stratiform cloud deck (see Houze 2004). In the case depicted in Fig. 2, the stratiform region is composed of the remnants of convective cells, which themselves have considerable positive vorticity. These vortical hot tower remnants add more vorticity to the MCV. During the middle life cycle state in Fig. 2b, the MCS contains vorticity structures both in the form of convective-scale VHTs and in the form of the wider stratiform-region MCV. In the late stages of the MCS life cycle, the hot towers cease forming, but the stratiform cloud region containing MCV vorticity remains for some hours (Fig. 2c).
b. Example of a vortical hot tower
An example of vortical hot tower has been documented with airborne Doppler radar by Houze et al. (2009) in an MCS located in the depression that became Hurricane Ophelia (2005). The basic structure of the updraft is illustrated in Fig. 3. Doppler radar showed a deep, wide, intense convective cell of a type that has been previously thought to occur in intensifying tropical depressions (Zipser and Gautier 1978) but had not been documented in detail until high-resolution airborne Doppler radar data were collected in the pre-Ophelia depression. The observed updraft of the cell was 10 km wide and 17 km deep and had updrafts of 10–20 m s−1 throughout its mid- to upper levels (Figs. 3a,b). It contained a cyclonic vorticity maximum (shown at 8-km altitude in Fig. 3c). This vorticity maximum, centered on the updraft, confirms that the cell was a vortical hot tower. The massive convective updraft was maintained by strong positive buoyancy (virtual potential temperature perturbation >5°C at 10 km; Fig. 3a), probably aided by latent heat of freezing at higher altitudes. The convective updraft was fed by a layer of strong inflow several kilometers deep. Wind-induced turbulence, just above the ocean surface, enriched the equivalent potential temperature of the boundary layer of the inflow air, thus creating an unstable layer with little convective inhibition. This air was raised to its level of free convection when it encountered the denser air located at low levels in the rainy core of the convection. Although evaporative cooling and precipitation drag occurred in the rain shower of the cell (see the negative temperature perturbation below 2 km in Fig. 3a), the negative buoyancy was insufficient to produce a strong downdraft or gust front outflow to force the updraft. Instead the updraft air was forced initially upward when the ambient low-level wind of the developing depression encountered the density gradient at the edge of the rainshower. This type of updraft formation without the aid of a gravity current outflow from a downdraft is not common, but its possibility of occurrence was pointed out in numerical simulations by Crook and Moncrieff (1988). The lack of a divergent spreading cool-air pool made this intense convection an extremely effective feedback mechanism for cyclogenesis in the pre-Ophelia depression. The production of low-level mass convergence, and associated stretching and upward advection of vorticity in the lower troposphere of the region where cyclogenesis was occurring, was not offset by strong downdraft divergence, and air of low equivalent potential temperature was not transported into the boundary layer to offset the gain of moist static energy from the sea surface.
The dominance of updraft motion and relative lack of downdraft motions in the convective burst of convection in the intensifying depression was also noted by Zipser and Gautier (1978) in their analysis of flight-level data. Nolan (2007) conducted idealized model simulations of convection in a tropical cyclogenesis environment and found that updraft mass transport dominated as the depression approached tropical storm stage. Zipser and Gautier (1978) further noted that the mesoscale convergence feeding the massive convective updraft present in the MCS they investigated was sufficient to account for the intensification of the synoptic-scale depression vortex in which the MCS was located. A sharp increase of upward mass flux with height occurred throughout the lower portion of the updraft in the pre-Ophelia convection (see the isolines of the vertical derivative of the vertical mass flux in Fig. 3b). The concomitant stretching of vorticity and generation of potential vorticity3 in the lower portion of the updraft, and the strong vertical motions of the updraft advecting the concentrated vorticity vertically, created a deep convective-scale vorticity perturbation manifested as a convective-scale cyclonic vortex—that is, a vortical hot tower. This vortex is evident in Fig. 3c.
The vorticity field shown in Fig. 3c shows both negative and positive vorticity centers near the center of the updraft. This structure is consistent with the updraft tilting the environmental horizontal vorticity into the vertical to produce a vortex couplet in midlevels, as described by Rotunno (1981) and Montgomery et al. (2006). For simplicity, this tilting-produced couplet is not shown in Fig. 2a since the positive member of the tilting-produced couplet dominates, probably because it combines with the positive vorticity of the stretched and upward-advected boundary layer vorticity to produce deep updrafts that are centers of cyclonic rotation. Moreover, numerical experiments suggest that the negative vorticity anomalies tend to be expelled from the developing tropical cyclone vortex while the positive anomalies are retained (Montgomery and Enagonio 1998). Thus, the negative anomalies are not relevant to the developing cyclone as positive feedback elements.
c. Ensemble of clouds in a developing storm
The ensemble of clouds in the intensifying depression shown in idealized form in Fig. 2d is based on real examples. Simpson et al. (1997) and Ritchie et al. (2003) showed satellite imagery of MCSs rotating around the center of the developing Southern Hemisphere Tropical Storm Oliver (1993). Sippel et al. (2006) described a similar set of MCSs seen in both satellite imagery and coastal radar observations as the newly formed Tropical Storm Allison (2001) made landfall in Texas. The MCSs in Allison were distributed around the developing cyclone, and the coastal radar showed positive vorticity anomalies in the active convective elements of the MCSs, probably similar to the vortical hot tower illustrated in Fig. 3.
Convective-ensemble behavior similar to Allison and Oliver occurred during the genesis stage of Hurricane Ophelia (2005). Figure 4 shows infrared satellite and coastal radar imagery just prior to the pre-Ophelia tropical depression intensifying to tropical storm intensity. A day before the depression reached tropical storm intensity (Figs. 4a–d), strong convection was prevalent over a wide area, but the clouds and precipitation exhibited no tropical storm–like structure. At the time of Figs. 4c,d, the convection seen on radar had grouped into three MCSs ∼200 km in horizontal dimension, each having both active convective cells and areas of stratiform precipitation. The vortical hot tower cell shown in Fig. 3 was located in the MCS located northeast of the Melbourne, Florida, coastal radar. Thus, the pre-Ophelia depression had a cloud population containing intense rotational convective cells and MCSs scattered about in the low-pressure area as depicted schematically in Fig. 2d.
By the time of Figs. 4e,f, the overall area covered by deep convection in the pre-Ophelia depression had decreased and became focused on a single very intense MCS. In the next several hours, this particular mesoscale precipitation area radically changed its shape and took on the structure of a tropical storm (Figs. 4g,h). Nolan’s (2007) idealized model simulations suggest that the long period of moistening by prior convection (e.g., Figs. 4a–d) may facilitate conversion from MCS to tropical storm structure. The radar echo exhibited an incipient eyewall and a well-defined principal rainband (as defined by Willoughby et al. 1984b; Willoughby 1988) extending from south to east to north of the storm and having a convective structure upwind and a stratiform structure downwind, as described in papers on mature hurricane rainbands (Atlas et al. 1963; Barnes et al. 1983; Hence and Houze 2008). Details of principal rainbands will be discussed in section 8c.
d. Cloud feedback in cyclogenesis
Figure 2d depicts an idealized scenario in which a population of clouds occurs in a region where large-scale conditions have dictated the presence of a preexisting low-level weak cyclonic circulation. The cloud population within this region consists of a combination of isolated deep convective cells with cyclonic vorticity maxima in the updrafts (as in Fig. 2a), one or more mature MCSs with both cyclonically rotating convective cells and MCVs (as in Fig. 2b), and older MCSs with residual MCVs (as in Fig. 2c). Each of the clouds contains significant vorticity perturbations in the form of convective-scale vortical hot towers and/or stratiform-region MCVs. These in-cloud vorticity perturbations can feed back positively to the larger-scale cyclonic vorticity. The deep convection generates potential vorticity. Collectively, the clouds accumulate and distribute vorticity derived from the boundary layer and low levels of the free environment throughout a deep layer. By these means they help to organize the preexisting weaker synoptic-scale cyclonic circulation into a tropical storm. The background positive vorticity helps to reduce the Rossby radius (Houze 1993, p. 53) within which the effect of the clouds is combined.
Tropical cyclogenesis is not just a question of strengthening the preexisting vortex. The vortex must reorganize to become a tropical cyclone. An important question is how a cloud population such as that depicted conceptually in Fig. 2d, or shown by example in Fig. 4, helps convert a preexisting benign synoptic-scale cyclonic circulation to a structure having the specific configuration of a tropical cyclone. One of the distinctive features of a tropical cyclone is a narrow annulus of maximum wind at some distance (usually between 10 and 100 km) from the storm center. This zone is referred to as the radius of maximum wind (RMW), and it is kept in near–thermal wind balance by a strong secondary circulation, which in turn produces an eyewall cloud. Outside the eyewall precipitation occurs in mesoscale spiral rainbands. These cloud and precipitation features of the mature tropical cyclone will be discussed in detail in subsequent sections of this paper. As a cyclonic disturbance reaches tropical storm strength, it also develops an RMW and incipient eye, eyewall, and rainbands.
The foregoing question then reduces more specifically to how upscale feedback by the cloud population depicted in Fig. 2d helps to convert the preexisting cyclonic circulation to a cyclonic disturbance exhibiting an RMW, eye, eyewall, and rainbands. One idea about how this conversion occurs is that the convective and mesoscale vorticity perturbations contained within the members of the cloud population, as depicted in Fig. 2d, are eventually sheared apart by the radial gradient of the vortex wind and mixed into the mean flow around the low in a process of axisymmetrization.4 Hendricks et al. (2004) and Montgomery et al. (2006) present evidence from a model that the axisymmetrization process distributes subsynoptic-scale vorticity perturbations, such as those contained in the vortical hot towers and MCVs, into a ring of high vorticity at a specific distance from the storm center, giving the enhanced low pressure system an RMW and thus a structure like that of a tropical cyclone.
While axisymmetrization must play a role, a remaining curious fact is that the tropical cyclone center has been observed to occur definitively within a particular member of the cloud population rather than as a collective smearing of the effects of the whole population around the preexisting low (Fig. 4h). Bister and Emanuel (1997) have suggested that cooling due to melting and evaporation of precipitation below the base of the stratiform cloud is involved in the extension of the middle level MCV downward. However, their idea requires a particularly strong stratiform-region downdraft, and observations of MCSs in intensifying tropical depressions indicate that these MCSs have relatively weak downdrafts (Zipser and Gautier 1978; Houze et al. 2009). Another way for one MCS to become a storm center was demonstrated in a numerical model of a midlatitude MCS by Chen and Frank (1993). In their simulation, the MCV strengthened and built downward as a result of humidification in the region of the MCV; the humidification lowered the buoyancy frequency and thus led to a reduction in the Rossby radius. This occurred in the absence of a lasting downdraft. Finally, there may be a stochastic aspect to the process. As the larger depression gathers strength from all the ongoing convective activity and gradually takes on tropical storm structure in its wind field, the probability increases that one of the MCSs with rotational convective cells and/or MCV will occur in the ideal spot (presumably the exact center of the depression) where the MCS’s cloud structure interacts with the vortex center to metamorphose into a structure that has an incipient eye, eyewall, and spiral rainbands.
4. Overview of the mature tropical cyclone
a. Visible clouds
In satellite pictures, the clouds in a mature tropical cyclone are dominated by upper-level cirrus and cirrostratus spiraling anticyclonically outward. In the visible image of Hurricane Katrina (2005) in Fig. 5a, the rough tops of convective clouds penetrate through the smooth cirrus outflow. The most striking feature is the open eye of the storm located in the middle of the spiraling cloud pattern. A zoomed-in satellite view of the eye in Fig. 5b shows that the cloud top surrounding the eye slopes downward toward the ocean surface. Seen from an aircraft flying inside the eye (Fig. 5c), the cloud surface bounding the eye region and sloping at about a 45° angle gives an observer on the plane the impression of being inside a giant circular sports stadium with the grandstand banking upward and outward. This huge sloping cloud bank, called the eyewall, is highlighted by the sunlight on the east side of the eye in all three panels of Fig. 5. The ocean surface is not visible in Figs. 5b,c. Rather it is obscured by low stratus or stratocumulus clouds. This low cloud cover is common in the eye of a strong tropical cyclone. The dynamics of clouds in the eye and eyewall regions will be discussed further in sections 5–7.
b. Three-dimensional wind field
The visible cloud pattern seen in Fig. 5 is determined by the wind and thermodynamic structure of the cyclone. An example of the low-level wind field in a tropical cyclone (Gloria 1985) is shown in Figs. 6a,b. The winds were constructed from rawinsonde, dropsonde, and aircraft Doppler radar measurements. In Fig. 6a, the wind data have been filtered to remove wavelengths less than about 150 km near the center of the storm and 440 km in the outer portions of the figure. This view emphasizes the large-scale flow pattern in which the storm is embedded. In Fig. 6b, the analysis retains wavelengths down to about 16 km in the center of the figure and down to about 44 km in the outer part of the figure. In this higher-resolution analysis, the tropical cyclone vortex itself is highlighted. The streamlines show boundary layer air spiraling inward toward the center of the storm. The shaded isotachs (Fig. 6b) show the roughly annular zone corresponding to RMW at ∼20 km (0.18° latitude) from the storm center. The air parcels spiraling inward at 900 hPa tend to increase their angular momentum, and the RMW occurs where the radial inflow rate abruptly slows down. Radial convergence, increased tangential wind speed, and sudden upward turning of the air current occur at this location and produce the eyewall cloud. Inside the RMW is the eye, where the wind speeds drop off almost immediately to nearly zero, and the vertical air motion is downward, producing the eye of the storm by suppressing clouds, except for the low-level stratus capping the mixed layer (Fig. 5). The dynamics of the eye are discussed further in section 6.
The 200-hPa wind pattern (Figs. 6c,d) illustrates that although the tropical cyclone’s outflow is strong, it is generally asymmetric. In this case, the outflow is concentrated in a channel northeast of the storm center. The depth of the cyclone core is also apparent; even at this high altitude, the flow is cyclonic near the storm center but changes to anticyclonic ∼100 km from the storm center. Details of the cyclonic flow near the center of the storm are illustrated by the high-resolution analysis in Fig. 6d.
Vertical cross sections of the broadscale mean radial u and tangential υ components of the wind of a tropical cyclone are shown in Figs. 7a,b, which are composites of data collected in many storms. Most evident in the radial wind field (Fig. 7a) is the increase in the inward directed component at low levels as one approaches the storm center. Strong radial outflow is evident at the top of the storm, at about the 200-hPa level. An important feature missed by these old, low-resolution composite observations is the occurrence of a shallow radial outflow layer at the top of the boundary layer, which is related to the fact that the strong winds near the center of the vortex become supergradient (Smith et al. 2008; this will be discussed in section 6). The strongest tangential wind field (Fig. 7b), defining the RMW, is at very low levels. The average dropsonde profile obtained on aircraft penetrations of tropical cyclones is characterized by a broad maximum centered ∼500 m above the surface (Franklin et al. 2003). The strong convergence in the eyewall at low levels and the strong outflow aloft must be balanced by strong upward motion. The broadscale pattern of vertical motion in a tropical cyclone is shown in Fig. 7c. The overall cloud and precipitation amounts are determined by this vertical mass transport, which on average is upward over the region located within 400 km of the storm center. However, this large-scale mean vertical motion pattern does not have the spatial resolution to provide much insight into cloud structures, nor does it show the downward motion in the eye. To see these details, special aircraft observations and radar instrumentation are required.
c. Equivalent potential temperature and angular momentum in relation to the eye and eyewall
In the large-scale environment, far from the center of a tropical cyclone, the typical sounding (Jordan 1958) shows that stratification of equivalent potential temperature θe is dominated by potential instability (decrease of θe with height) in the lower troposphere, with θe reaching a minimum at about the 650-hPa level. Above that level, the air is potentially stable. The pattern of θe changes markedly as one proceeds inward toward the center of a tropical cyclone (Bogner et al. 2000). The typical pattern of θe within a tropical cyclone is indicated by the example in Fig. 8a. In the low levels, the values of θe increase steadily to a maximum in the eye of the storm. In the vicinity of the eyewall (∼10–40 km from the storm center), the gradient of θe is the greatest, and the isotherms of θe rise nearly vertically through the lower troposphere, then flare outward as they extend into the upper troposphere. Since above the boundary layer θe is nearly conserved following a parcel, these contours reflect the flow of air upward and outward in the eyewall. In the very center of the storm, θe decreases strongly with height. The center of low θe at 500 hPa is evidence of subsidence concentrated in the eye of the storm.
The vertical circulation of the tropical cyclone in relation to θe is illustrated qualitatively in Fig. 8b, where the low-level radial flow is depicted as converging into the center of the storm (consistent with Fig. 7a) in the boundary layer below cloud base. As it flows inward, turbulence produces a well-mixed boundary layer of high θe. When this air enters cloud in the eyewall zone, it ascends to the upper troposphere approximately along the lines of constant θe. The θe lines also reflect a concentrated descent of air in the eye of the storm, which will be discussed further in section 5.
The dynamical necessity of the funnel-like shape of the outward-sloping flow lines in the eyewall region was reasoned out by early meteorologists. Haurwitz (1935) noted that a vanishing pressure gradient at some high level indicates that the strong low-level pressure gradient in a tropical cyclone must be associated with an outward slope of the boundary of the core of warm air in the center of the storm. Durst and Sutcliffe (1938) argued that rising rings of air in a warm-core storm in which the radial pressure gradient decreases upward must move outward for the centrifugal and Coriolis forces (dictated by conservation of their surface angular momentum) to balance the weaker pressure gradients aloft. This reasoning foreshadowed today’s view of the inner core of the storm (Emanuel 1986; Smith et al. 2008).
Although the outward spreading of the m and θe lines is consistent with the theory of a balance vortex and is a typically observed characteristic of tropical cyclones, there are nonetheless observations and model simulations that indicate that vertical convection is often embedded in an eyewall cloud, modifies its structure, and significantly affects the overall intensity of the tropical cyclone. To fully examine the eye and eyewall dynamics, sections 5 and 6 will discuss how the basic or mean structure of the eye and eyewall cloud can be thought of in terms of the slantwise conditionally symmetrically neutral component of motion in a thermal wind–balanced tropical cyclone vortex. Section 7 will then examine the buoyancy driven vertical drafts superimposed on the eyewall cloud.
5. The eye
As noted in section 4a and illustrated by Fig. 5, the eye is a region of unique and strong dynamics that involves two distinct and different types of clouds, a layer of low-topped stratus and/or stratocumulus clouds extending horizontally over the eye region and an outward-sloping eyewall cloud. These two types of clouds are depicted schematically in Fig. 9 in relation to the wind field in the tropical cyclone. Both are related to the dynamics of the tropical cyclone vortex. The depiction in Fig. 9 is from a thorough examination of observations in the eye region of hurricanes by Willoughby (1998).
The conditions in the eye region are ideal for the formation of a cloud-topped mixed layer (Lilly 1968; Stevens et al. 2003; Bretherton et al. 2004) because subsidence in the eye produces a stable layer at the top of a vigorous mixed layer over a warm ocean surface. The top of the stratiform cloud with a few cumuliform turrets appears below the aircraft in Fig. 5c at the top of the mixed layer cloud, near the upper extent of the subsidence-induced stable layer.
The sloping eyewall cloud highlighted by sunlight in Fig. 5 is primarily a manifestation of the secondary circulation, which, as shown in Fig. 9, is a radial-vertical overturning transverse to the cyclone’s primary circulation, which refers to the general cyclonic rotation of the wind around the eye. In a balanced tropical cyclone, the secondary circulation consisting of the radial u and vertical w components of the wind keeps the primary circulation (i.e., the υ component of the wind) in gradient-wind equilibrium. The low-level radial inflow branch of the secondary circulation gains latent heat energy via upward turbulent flux of sensible and (primarily) latent heat (i.e., moist enthalpy) as it flows over the ocean surface toward the eyewall. Latent heat released in the vertical component of the eyewall circulation provides the energy that powers the vertical circulation and maintains the tropical cyclone strength and structure.
The secondary circulation is characterized both by rising motion in the eyewall zone and compensating subsidence, occurring partly concentrated in the eye region and partly more diffusely in the far field radially well outside the eyewall. Figure 10 shows the secondary circulation computed by considering point sources of heat and momentum placed near the RMW in an idealized tropical cyclone–like vortex (Shapiro and Willoughby 1982). From this basic result, it is evident that the concentrated subsidence in the eye region is a consequence of the heating in the eyewall cloud. The subsidence in the eye, conceptualized and labeled as “forced dry descent” in Fig. 9, keeps the eye cloud free except for the mixed-layer stratus/stratocumulus at the top of the boundary layer. Pendergrass and Willoughby (2009) and Willoughby (2009) have shown that variations in hypothesized heating patterns associated with the eyewall and different environmental conditions lead to a variety of variations. The latter paper indicates that transient bursts of heating in the eyewall, for example from embedded buoyant convective towers, might produce significant alterations in the storm structure. The subsidence computed for the eye region is accompanied by divergence at low levels within the eye region. Figure 9 indicates that the eyewall is fed in part by the low-level radial outflow from the eye region as well as by radial inflow from outside the eyewall. Important implications of this ingestion by the eyewall of air from the boundary layer of the eye region are discussed in section 7.
Shapiro and Willoughby’s (1982) idealized tropical cyclone circulation shown in Fig. 10 is not in a steady state because the RMW contracts over time. Since the RMW is collocated with the eyewall, this behavior is referred to as eyewall contraction. The nonlinear response to the eyewall heat and momentum source has the maximum rate of increase of tangential wind velocity located slightly radially inward of the RMW. This result implies that the RMW (Fig. 8b) of a storm in thermal wind balance should decrease in time, and observations and numerical models confirm that many eyewalls contract (Willoughby et al. 1982; Willoughby 1990, 1998; Houze et al. 2006, 2007). The dashed lines in Fig. 9 indicate an earlier position of the inner edge of the eyewall cloud. As the eyewall contracts, the winds in the eyewall zone increase according to conservation of angular momentum and other factors, and the central pressure of the storm decreases as the pressure gradient associated with the new location and strength of the maximum wind continually adjusts toward gradient-wind balance.
The downward air motion in the eye region and divergence of mass away from the eye at low levels seen in Fig. 9 can be thought of partly as a compensation for the gradual contraction of the eyewall boundary over time. Also contributing to the subsidence in the eye is turbulent mixing of angular momentum at the interface of the rapidly rotating eyewall and more quiescent eye region. This mixing is suggested by the entrainment arrows at the inner edge of the eyewall cloud in Fig. 9. To maintain thermal wind balance in the eye region the increase of tangential wind velocity and relative change in the vertical shear of the horizontal wind in the eye needs to be matched by a stronger horizontal temperature gradient, which can only be provided by adiabatic subsidence warming in the center of the vortex (Emanuel 1997).
Figure 9 includes two types of downdrafts based on close-up observations of tropical cyclones during aircraft penetrations. On the outer side of the eyewall, negatively buoyant precipitation driven downdrafts (see further discussion in section 7d) occur at lower levels. Below cloud base, they merge with the frictional inflow beneath the eyewall and become part of the boundary layer air spreading into the eye region. At higher levels along the inner edge of the eyewall, a thin layer of moist air cascades down the eyewall (Willoughby 1998). Mixing of the dry air in the eye and moist air in the eyewall cloud occurs in this thin layer. Evaporation of cloud particles cools the air and produces a cascade of negatively buoyant air down the side of the eyewall cloud. This cascade is visible as a mist of particles mixed into the dry air adjacent to the eyewall cloud (Fig. 11, from Willoughby 1998). This downdraft on the inner side of the eyewall has also been well documented by aircraft measurements of the vertical velocity (Jorgensen 1984, see also footnote in section 7c).
6. Dynamics of the mean eyewall cloud
a. Sloping angular momentum surfaces
The eyewall surrounding the eye of the storm is like no other cloud. Although it is often described as a “convective” cloud, the eyewall does not simply follow the dynamics of ordinary cumulus and cumulonimbus, which arise from purely vertical accelerations releasing buoyant (conditional or potential) instability. Rather, as noted in section 4c, the eyewall cloud is shaped to great measure by slantwise convective motions, where the combination of vertical and horizontal accelerations (responding to both buoyancy and inertial forces) maintain a state of near–conditional symmetric neutrality. In this section, we summarize the dynamics of the slantwise component of the eyewall circulation. Section 7 will describe important buoyant convective perturbations of this structure.
The essential dynamics of the slantwise circulation of the sloping eyewall cloud can be inferred from Emanuel (1986) and Smith et al. (2008), who provide an interconnected theoretical linkage of air–sea interaction at the surface, atmospheric boundary layer structure, the secondary circulation of the balanced vortex, and the moist conditionally symmetrically neutral upflow in the eyewall. Emanuel’s (1986) approach is often parameterized in terms of an approximate Carnot cycle (section 3 of his paper), which gives insight into tropical cyclone intensity. However, the focus of the present review is cloud dynamics, not storm intensity. To understand the air motions giving rise to the eyewall cloud, the following discussion makes use of the more explicit version of Emanuel’s approach rather than its Carnot parameterization.5
Equation (2) provides a basis for obtaining mathematical expressions that describe the dynamics of the eyewall cloud. An expression for m(r, p) is substituted into (2) to determine the geometry of the streamlines. With the help of the basic equations of a symmetric vortex, one can obtain the magnitudes of the wind components, and the thermodynamic properties of the circulation of an idealized tropical cyclone vortex containing the eyewall cloud. The solution is based on the observation that the tropical cyclone circulation in the eyewall region tends to be conditionally symmetrically neutral [∂θes/∂z = 0 on a surface of constant m, where z is height and θes is the saturation equivalent potential temperature defined by (B1)], but not especially strongly conditionally unstable (∂θes/∂z usually only slightly negative in the vertical direction). Consistent with these observations, the eyewall cloud is assumed to be undergoing slantwise, nearly neutral vertical motions above the boundary layer. The ascending air is assumed further to be emanating from a boundary layer whose value of θe has been determined by upward flux of sensible and latent heat from the ocean surface. For an idealized circularly symmetric tropical cyclone vortex, this approach yields both the primary and secondary circulation by connecting the eyewall dynamics directly to the air–sea interaction that occurs in the boundary layer over the warm ocean.
b. Boundary layer assumptions and implications
Emanuel (1986) divides the boundary layer of the idealized two-dimensional axisymmetric vortex into the regions I, II, and III shown in Fig. 12a. All three regions are assumed to be well mixed by turbulence and to be of constant depth h. The production of turbulent kinetic energy in the boundary layer by shear is substantial because of the strong low-level winds circulating around the center of the storm, while the generation of turbulence by buoyancy is strongly promoted by the warm ocean surface. The turbulence produces strong upward eddy flux of latent heat (and to a lesser degree sensible heat) everywhere in the lower part of the boundary layer. The flux at the top of the boundary layer, however, is assumed to vary significantly from one region of the storm to the next. In region III, rainbands (to be discussed in section 8) are expected to be important contributors to the turbulent flux at the top of the boundary layer, with convective updrafts and downdrafts transporting high and low θe out of the boundary layer, respectively. Region II is the region of the eyewall cloud, and the flux at the top of the boundary layer there is assumed to be entirely positive and dominated by the mean upward flux associated with the secondary circulation of the tropical cyclone vortex. Turbulent flux at the top of the boundary layer here is considered to be very small relative to this mean upward flux. Region I is the eye of the storm (section 5).
Emanuel’s (1986) simplified treatment of the basics of eyewall dynamics (Fig. 12a) does not consider any interaction between the eyewall and region I. However, in real storms the air diverging out of the eye region at low levels (Fig. 9) can and does feed extremely high-θe air into the base of the eyewall cloud. The eyewall cloud vertical motions are intensified by the entrainment of high-θe air from the boundary layer of the eye region, and local convective cells of buoyant updraft motion can form within the eyewall in locations where the entrainment particularly strongly enhances the buoyancy. To take the entrainment of high-θe air from the eye into account, Smith et al. (2008) replaced Emanuel’s (1986) boundary layer model with one that allows for air from the eye region to enter the base of the eyewall cloud (Fig. 12b). Implications for enhancing buoyancy within the eyewall in this manner will be discussed further in sections 6e and 7a–c.
A subtle but significant feature of Emanuel’s (1986) boundary layer treatment is that the wind at the top of the boundary layer is assumed to be in gradient balance. Smith et al. (2008) employed a boundary layer model that allows for frictionally induced cross-isobaric flow in the boundary layer. In their model, the frictionally enhanced radial inflow in the boundary layer at the boundary of regions A and B (Fig. 12b) is much stronger than the radial inflow in Emanuel’s version. One result is that in region A the tangential wind (into the plane of the cross section) becomes supergradient (as a consequence of the Coriolis force acting on the radial inflow). Outward-directed Coriolis and centrifugal forces acting on the supergradient tangential wind in turn slow the boundary layer inflow. Convergence in region A, resulting in part from this effect, feeds the eyewall cloud with higher-θe air than would be supplied from directly beneath the eyewall as in Emanuel’s (1986) model. But the rising supergradient flow turns radially outward in a layer atop the inflow layer as it seeks gradient balance. This low-level radial flow reversal allows air to flow out of the eye region into the base of the eyewall cloud.
c. Connecting the balanced vortex with a simplified boundary layer
Although Emanuel’s (1986) model underestimates the θe of the air entering the base of the eyewall (cf. Figs. 12a,b), the overall effect of this underestimate is slight on the mean structure of the tropical cyclone. Bryan and Rotunno (2009) show that the parcels of air entering from the eye make the eye only about 0.3 K warmer on average and compose only about 8% of the air rising in the eyewall. The individual parcels entering from the eye do have increased in buoyancy and, as will be discussed in later sections, may account for locally intense updrafts embedded within the eyewall. But since the mean structure of the eyewall cloud is largely accounted for by Emanuel’s approach, it remains useful and instructive to utilize his approach to illustrate the basic sloping structure of the streamlines (i.e., the m and θe surfaces) in the eyewall cloud. This slantwise circulation remains the primary determinant of the sloping nature of the eyewall cloud, and following this simplified approach we can obtain a view of how the overturning of air required to balance the tropical cyclone vortex takes on the structure made visible by the outward-sloping eyewall cloud (Figs. 5c and 11).
The secondary circulation consistent with the boundary layer regions II and III in Fig. 12a is obtained by considering the equations for a vortex in both hydrostatic and gradient-wind balance (appendix A). The secondary circulation is indicated by mapping the isolines of angular momentum m in the r–p plane; since m is conserved, the m surfaces are also the trajectories of the mean air motion. Following Emanuel (1986), we obtain the surfaces of angular momentum m(r, p) implied by the conservation of m by substituting in (2) and integrating upward, using the value of m at the top of the boundary layer as a boundary condition and assuming that the mature vortex has adjusted to a state of conditional symmetric neutrality everywhere above the boundary layer.
d. Emanuel’s solutions for the m and θes surfaces in the eyewall cloud
The aim of this paper is to identify the characteristics of clouds of tropical cyclones that distinguish them from other types of convective clouds. Emanuel’s (1986) approach to determining the pattern of m and θe surfaces is valuable in this regard because it shows that the basic slantwise nature of the eyewall is an unavoidable feature of a strong vortex that is simultaneously in thermal wind balance, conditionally symmetrically neutral, and drawing energy from the sea surface. While buoyant convective motions may be intermittently and importantly superimposed on it, the eyewall cloud owes its fundamental slantwise convective nature to the vortex itself. Equations (3), (6), (7), and (A3) are the heart of Emanuel’s approach, and they yield the pattern of m and θe surfaces, provided that the SST, upper-level outflow temperature To, ambient surface relative humidity, and several other quantities ( f, pa, ra, CD/Cθ, and TB) are specified. The assumption about relative humidity is the most questionable. The relative humidity is directly proportional to the ratio θe/θes(SST) implied by (8). Emanuel (1986) assumes a fixed value of this ratio in region III. The assumption about relative humidity only works because of the above discussed unrealistic assumption of gradient-wind balance at the top of the boundary layer (Smith et al. 2008). Furthermore, he assumes a constant relative humidity of 80%, whereas buoy data analyzed by Cione et al. (2000) show that the mean relative humidity varies significantly as a function of distance from the center of a tropical cyclone, increasing from ∼85% at ∼200 km from the storm center to ∼95% within ∼50 km of the storm center. Despite these serious limitations regarding the quantitatively realistic nature of the eyewall cloud, Emanuel’s (1986) assumptions nevertheless lead to an uncluttered view of the basic factors controlling the prevailing slantwise nature of vertical motions in the eyewall cloud. For further details of the solution strategy for the simplified model, see Emanuel (1986) or chapter 10 of Houze (1993). For a more quantitatively accurate treatment see Smith et al. (2008).
An example of the results of this type of calculation is shown in Fig. 13. The location of maximum winds near the center of the storm (Fig. 13a) is consistent with Fig. 7b, although the winds are stronger in the calculated case than in the composite section, where averaging from many cases has smeared the basic pattern. The modification of the Emanuel model by Smith et al. (2008) indicates that the maximum winds could be even stronger if the interaction with the eye region (Fig. 12b) were taken into account. The m and θes surfaces in Figs. 13b,c may be regarded as the streamlines of the radial cross-sectional flow above the boundary layer, and near the center of the storm they depict the streamlines of flow within the eyewall cloud. Although the interaction with the eye region is ignored, the m and θes surfaces of this stripped down model of the symmetric vortex provide a useful view of the essential features that produce the eyewall cloud of a tropical cyclone—namely, the overturning required to keep the storm in thermal-wind balance when the boundary layer is well mixed and the circulation above the boundary layer is conditionally symmetrically neutral. Near the center of the storm, where the m and θes surfaces depict the upward and outward air motion within the eyewall cloud, the surfaces are packed closely together. This enhanced gradient of m and θes in the eyewall cloud is the result of a form of frontogenesis, where radial frictional inflow in the boundary layer advects moist enthalpy inward, thus increasing the gradient of θes in the eyewall region (Emanuel 1997).
e. Temporal development and stability of the mean two-dimensional eyewall cloud
The theoretical picture of a two-dimensional steady tropical cyclone just discussed is, of course, an idealization. In nature, the cyclone and its eyewall cloud are in a perpetual state of evolution. The idealized structure appears to be a state toward which the mature storm tends to adjust. Rotunno and Emanuel (1987) used a two-dimensional axisymmetric nonhydrostatic model to show this adjustment process (Fig. 14). To be consistent with the idealized case described above, the environment was assumed to be initially conditionally neutral but unstable in the conditionally symmetric sense. This assumption represents an actual tropical cyclone environment fairly well, except that in reality the environment usually has some degree of conditional instability. Figures 14a–c show that the fields of m and θes gradually become more parallel as time passes. Figure 14d is a 20-h average of the m and θes fields after the storm reaches a quasi-steady state. Having reached this equilibrium state the streamlines in the eyewall region are generally parallel to the m and θes surfaces, indicating that the eyewall region of the model has the form of a conditionally symmetrically neutral circulation connected to a boundary layer in contact with the warm sea surface, as in the idealized model calculations illustrated in Figs. 13.
Examination of Figs. 14a–c further shows that the two-dimensional tropical cyclone circulation tended to create conditional instability where none initially existed. As the storm developed, air in the boundary layer flowing inward increased its value of θe, and conditional instability (∂θes/∂z < 0) developed near the center of the storm in the lower troposphere. Vertical convective motions took place in response to this instability and, in the process, transported high m upward. This transport produced local maxima of m, and hence local areas of inertial instability, which was released in horizontal accelerations (as suggested by Willoughby et al. 1984a). The combination of vertical convection in response to conditional instability and horizontal motion in response to local inertial instability created a final mixture in the eyewall region which was neutral to combined vertical and horizontal acceleration. Thus, in the process of achieving its conditionally symmetrically neutral final state, transient buoyant convective cells were superimposed on the developing slantwise circulation. These buoyant cells were apparently part of the mixing process that led to the slantwise conditionally symmetrically neutral air motion in the eyewall.
Even after a mature eyewall forms, a tropical cyclone tends to create zones of conditional instability in the eyewall. The boundary layer frictional inflow continues to import air of high θe from distant radii so that the process illustrated by Figs. 14b–d continues to create instability in the eyewall region. In a further investigation using Rotunno and Emanuel’s (1987) two-dimensional model in a high-resolution mode, Persing and Montgomery (2003) found that overturning eddies occur in the radial-vertical plane and draw low-level boundary layer air from the eye into the eyewall (Fig. 15). This behavior is especially powerful since the boundary layer air in the eye tends to have the highest values of θe in the tropical cyclone (Fig. 8a). This process is probably the two-dimensional model’s way of accomplishing the ingestion of high-θe air from the eye into the eyewall, which Smith et al. (2008) show is a necessary outcome of the development of supergeostrophic wind at the top of the boundary layer (Fig. 12b). This entrainment of high-θe air from the eye strengthens the eyewall circulation and creates buoyancy that can be released in strong vertical convection altering the structure of the otherwise moist symmetrically neutral eyewall cloud. Convection, however, is typically a three-dimensional process, and to consider convection superimposed on the eyewall it is essential to remove the constraint of two-dimensionality. The next section considers convection and other asymmetrical processes without this constraint.
7. Substructure and asymmetry of the eyewall cloud
a. Conditional instability within the eyewall cloud
While a circular outward-sloping eyewall cloud consisting of purely slantwise conditionally symmetric neutral motions (section 6) is a useful idealization that explains a great deal about the typical structure of an eyewall cloud, the eyewall cloud does not exist in reality without containing superimposed cells of buoyant convective updrafts arising from the local release of conditional buoyant instability. The air masses in which tropical cyclones occur are not exactly slantwise neutral; they usually exhibit some degree of conditional instability. In addition, conditionally unstable air processed by the eyewall cloud is not undisturbed large-scale environmental air but rather air whose thermodynamic structure has been modified by the tropical cyclone circulation. The processes by which this modified air enters the eyewall are typically associated with deviations from the idealized symmetrically neutral model of the hurricane dynamics. The two-dimensional modeling discussed in section 6e shows that the vertical overturning of the tropical cyclone in the r–z plane tends to create conditional instability within the eyewall cloud even if none initially existed in the environment. Observations (summarized in Fig. 9) verify that the eyewall is fed from its base by both frictional inflow from outer radii and by low-level outflow from the eye. The circular shape of the eyewall cloud is often distorted. Figure 16 shows an example where the eyewall exhibits a wavenumber-3 or -4 irregularity, and the undercast of stratus across the eye is highly distorted. These asymmetries are thought to be due at least in part to barotropic instability of the sheared flow in the RMW (Schubert et al. 1999; Kossin and Schubert 2001; Kossin et al. 2002). Vortices associated with eyewall asymmetries constitute a mechanism by which high θe of the air in the boundary layer may be entrained at low levels from the eye into the eyewall.
If the θe of the air flowing up into the base of the eyewall cloud as a result of any of these processes is sufficiently high, the eyewall cloud will develop pockets of conditionally unstable air, and small-scale buoyant updrafts will form in those locations. In a realistic three-dimensional simulation of Hurricane Bob (1991), Braun (2002) found that buoyant elements superimposed on the eyewall accounted for over 30% of the vertical mass flux in the eyewall, but that the buoyancy was most often manifested along outward-sloping rather than purely vertical paths because of being superimposed on the strong slantwise component of the eyewall circulation. These buoyant updrafts are highly three-dimensional and the constraint of two-dimensional modeling must be removed to fully simulate their behavior.
b. Eyewall vorticity maxima and strong updrafts
Braun et al. (2006) simulated Hurricane Bonnie (1998) with a three-dimensional full-physics model and found that strong vertically extensive updrafts located along the eyewall were associated with small-scale vortices in the horizontal wind field (Fig. 17). Marks et al. (2008) call such subvortices eyewall vorticity maxima (EVMs). The EVMs on the eyewall in the Bonnie simulation were ∼20 km, small enough in scale for four of these subvortices to form on the eyewall, which was very large in diameter, the eye of Bonnie being ∼100 km. In the first airborne dual-Doppler radar study of a tropical cyclone, Marks and Houze (1984) found an EVM on the developing eyewall of Hurricane Debby (1982) that was ∼5–10 km in horizontal scale. The primary vortex was ∼30 km in diameter. Marks et al. (2008) describe a harrowing flight directly across an intense EVM located on the inner side of the eyewall of Hurricane Hugo (1989). Figure 18 contains a record of the meteorological variables on this flight track directly across the subvortex. The mean vortex structure has been removed, so these data show only the perturbation values associated with the EVM. The tangential wind perturbation record in Fig. 18a shows that this subvortex was 6–7 km in overall horizontal extent although the windshift across the center from −18 to +25 m s−1 occurred over a distance of only ∼2 km. The primary vortex of Hugo was ∼25–30 km in diameter.
Braun et al. (2006) suggest that EVMs form along the eyewall in response to instability of the vortex ring associated with the RMW, similar to the way “suction vortices” form within the ring of maximum vorticity of a tornado vortex (Fujita 1981; Houze 1993, 298–303). Tornadic suction vortices are often hundreds of meters in dimension and are subvortices of a primary tornado that is hundreds–thousands of meters in diameter. Collectively, the model simulation of Bonnie, the observations of EVM in Debby and Hugo, and the breakdown of a tornado vortex into suction vortices suggest that EVMs are fundamental products of vortices and may come in a range of sizes related the width of the parent vortex.
The EVM described by Marks et al. (2008) contained a very strong updraft, reaching a peak velocity of ∼17 m s−1 (Fig. 18b). This strong updraft is consistent with the Bonnie simulation, in which Braun et al. (2006) found that each EVM coincided with a deep vertically erect updraft. The association of the strong updrafts with the EVMs in Bonnie and Hugo is not coincidental. The small-scale vorticity maxima entrain extremely high-θe air from the eye region into the base of the eyewall cloud. This entrainment of air from the eye is a primary mechanism for imparting buoyancy to the otherwise slantwise convective eyewall. The intense buoyant updraft associated with an EVM is a direct response to this local enhancement of buoyancy.
The model simulation of Braun et al. (2006) indicates that the convective-scale updrafts associated with the small-scale vortices remain vertically erect and coherent as they are advected around the eyewall. This erect structure of the rotating convective updrafts is somewhat surprising since we might expect the vertical shear of the horizontal wind in the tropical cyclone vortex to produce updraft cells that are distorted in the vertical. Braun et al. (2006) suggest that the small-scale rotating updrafts in the EVMs are inertially resistant to the surrounding airflow so that their central cores are protected and resistant to the shear of the mean tangential wind of the tropical cyclone vortex. The EVM observed by Marks and Houze (1984) in Debby was not vertically erect but rather was tilted in the direction of the shear. It may have been a weaker example of an EVM and therefore unable to withstand the shear in the tangential flow of the main vortex.
Since the EVMs derive their rotation from the vorticity on the inside edge of the zone of maximum wind of a fully formed tropical cyclone, these rotational updrafts are different from, and should not be confused with, the vortical hot towers discussed in section 3a, which occur prior to the formation of a tropical cyclone and derive their vorticity from the vorticity-rich boundary layer and/or tilting of the ambient shear. It is worth noting that the damage that could occur from the locally enhanced winds in an EVM in a landfalling storm is likely to be worse that what would be expected from the mean vortex winds.
c. Statistics of updrafts and downdrafts in eyewall clouds
We have seen that the EVMs contain strong updrafts associated with entrainment of high-θe air from the eye region. The vertical velocities in those drafts are on the upper extreme of updraft velocities in hurricane eyewall clouds. The general statistics of vertical air motions in hurricane eyewalls are known fairly well since research and reconnaissance aircraft have been flying through tropical cyclones and measuring vertical air motions for over 50 years (Sheets 2003; Aberson et al. 2006). Up to the mid-1980s, aircraft measurements of vertical air motions were obtained only in situ, along the flight tracks of aircraft, and spatial distributions of vertical velocity could be determined only by composites of data from different flight tracks (e.g., Fig. 7c). One of the best sets of flight-track vertical velocities were obtained in the extremely intense Hurricane Allen (1980). Figure 19a from Jorgensen (1984) shows aircraft-observed vertical velocities in the eyewall of Allen determined by mass continuity from the horizontal wind data along the flight track.6 Figure 19b shows vertical velocities indicated by the aircraft’s inertial navigation system. Both types of measurements indicate peak magnitudes of w of just over 6 m s−1. Figure 20 from Jorgensen et al. (1985) shows that the vertical velocities seen in Allen are typical of those seen at flight level in other tropical cyclones, although this larger sample shows slightly larger peak updrafts ∼7–10 m s−1.
Airborne Doppler radar measures the velocity of precipitation particles along the beam of the radar. For data collected with the beam pointed vertically, correction for particle fall speed leads to estimation of the vertical air motion. Black et al. (1996) analyzed vertically pointing Doppler radar collected by aircraft on 185 radial flight legs into and out of the eye regions of hurricanes. Their results are shown in Fig. 21 in the form of a two-dimensional contoured-frequency-by-altitude diagram (CFAD; Yuter and Houze 1995). Note that the broadening of the distributions with height is an artifact resulting from a sampling bias whereby the weaker drafts are difficult to detect at greater distance from the aircraft, which was most frequently flying at altitudes of ∼1.5–3 km. In general agreement with the flight-level data in Figs. 19 and 20, the peak updraft values are ∼8 m s−1. More than 70% of the Doppler-derived vertical velocities were in the range of ±2 m s−1. Drafts (both up and down) were defined by Black et al. (1996) as zones having |w| > 1.5 m s−1 continuously along the flight track and containing a maximum of w > 3 m s−1. Updrafts defined this way were nearly all <3 km in horizontal dimension; only 5% were wider than 6 km. More extreme updrafts such as in the EVM updraft encountered on the Hugo flight (Fig. 18) do not appear in statistics of the type shown in Figs. 19, 20, and 21 because they are rare and because they would normally not be penetrated since the aircraft normally avoids portions of eyewalls that appear dangerous either visually or on radar. The statistics in Fig. 21 show that the magnitudes of updraft velocity tend to increase with height, the largest values being above the 10-km level. It has been suggested that stronger vertical velocities should occur at higher levels because their buoyancy is boosted by the release of latent heat of fusion (Lord et al. 1984; Zipser 2003).
The airborne Doppler radar data of Black et al. (1996) provide information on the horizontal as well as the vertical structure of the upward motion zone in the eyewall. The statistics exhibit a coherent radially outward-sloping structure in two-dimensional autocorrelations relative to the updraft maxima in the eyewall for updrafts at 2.5- and 7.5-km flight altitudes (Figs. 22a,b). A difficulty in interpreting aircraft-based updraft data in tropical cyclones, whether obtained in situ or by radar, is that no determination can be made observationally between slantwise neutral vertical motions (see section 6) and embedded vertically accelerating buoyant updrafts. The Hugo penetration (Fig. 18) and Bonnie simulation (Fig. 17) demonstrate that locally intense updrafts that are primarily buoyancy phenomena can be superimposed on the eyewall. In numerical simulations of both Bonnie and Hurricane Bob (1991), Braun (2002, 2006) found that updrafts exhibiting buoyancy relative to the mean vortex circulation in which they were embedded accounted for over half7 of the condensation in the eyewall. If this result is representative of tropical cyclones in general, it implies that while the eyewall’s mean structure (including the sloping stadium-like structure exemplified in Fig. 5 and evident in Figs. 22a,b) is determined by the adjustment of the mean vortex motion toward an idealized slantwise symmetrically neutral state (see section 6), the dynamics of the eyewall cannot be accurately interpreted without considering the superimposed intense buoyant updrafts.
d. Downdrafts in the eyewall
As indicated in Fig. 9, convective-scale downdrafts occur in the zone of heavy rain within the eyewall. The CFAD in Fig. 21 attaches a magnitude to these downdrafts. The downdrafts are observed at all altitudes, but they are more frequent at lower levels than at higher altitudes. Overall they are outnumbered by updrafts by about a factor of 2. Figure 23 from Black et al. (1996) shows that the downdraft mass flux in the eyewall is about 20%–30% of the upward mass flux at any given altitude. Despite their frequent occurrence, neither the causes of the eyewall downdrafts nor their dynamical significance to the overall tropical cyclone circulation have been fully determined. Figures 22c,d show that unlike the upward vertical velocities in the eyewall, these downdrafts show no sloping coherent structure; the downdrafts are essentially uncorrelated in the radius–height plane. It seems likely that, as is the case in convection not associated with tropical cyclones (e.g., Yuter and Houze 1995), the downdrafts at upper levels are forced responses to updraft motions while those at lower levels are precipitation driven.
e. Eyewall asymmetry owing to storm motion and shear
The clouds and precipitation of the eyewall of a tropical cyclone are practically always distributed nonuniformly around the storm. This asymmetry arises for two reasons: (i) the movement of the storm through the surrounding atmosphere and (ii) the wind shear of the large-scale environment. The first reason is because the eyewall is closely associated with the cyclone’s strong circulation, which is inertially stable and resistant to the surrounding airflow. Consequently, the movement of the tropical cyclone through its environment creates convergence on the side of the storm toward which the eyewall is moving (Shapiro 1983). Thus, the mean vertical velocity pattern in the eyewall of a moving tropical cyclone is inherently asymmetric. Of interest is that there is a feedback between the wavenumber-1 asymmetry and the storm track; instabilities in the asymmetry are related to a trochoidal motion sometimes exhibited by tropical cyclone tracks (Nolan et al. 2001).The second reason for the wavenumber-1 asymmetry is vertical wind shear in the environment. Although shear is unfavorable to the development and intensification of tropical cyclones (see section 2), these storms usually exist within environments having some degree of shear. The storm-relative environmental flow is a function of height, and as such it redistributes cloud and precipitation particles around the tropical cyclone. These two factors acting together lead to a variety of wavenumber-1 asymmetries of the eyewall structure and intensity.
Analyzing data from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (Kummerow et al. 1998) along with large-scale wind databases for tropical cyclones over six ocean basins for a 3-yr period, Chen et al. (2006) have determined the expected rainfall patterns for different combinations of storm motion and wind shear vectors. They find that in general, the tropical cyclone rainfall asymmetry has larger amplitudes with respect to vertical wind shear than to storm motion. Regardless of the strength of the environment shear, the maximum in eyewall rainfall is on the downshear left side of the storm. When the shear is strong, this asymmetry is felt at all radial distances from the storm center. When the shear is weak, the shear-related asymmetry is confined to the radii nearer the storm center (including the eyewall), while at long distance from the center, the maximum rainfall is on the downshear side of the storm, as expected in the case of no shear. Rogers et al. (2003) have shown how these asymmetries of rainfall in the eyewall combine with the storm motion to produce a variety of surface rainfall patterns.
Analyzing airborne Doppler radar data and in situ flight track data collected in Hurricanes Jimena (1991) and Olivia (1994), Black et al. (2002) inferred the fate of cloud and precipitation particles produced in an updraft forced by convergence (due to storm motion) on the forward side of a tropical cyclone embedded in a sheared environment (Fig. 24). For illustration, they assumed the shear to consist of upper-level westerly flow and lower-level easterlies. Cloud and precipitation particles generated in the updraft on the downshear side (east) of the storm are advected cyclonically by the primary vortex circulation. Precipitation particles produced in the updraft formed on the east side of the eyewall are advected cyclonically around the north side of the storm, where they produce a strong 45–50 dBZ radar echo. This echo corresponds to the maximum rainfall found by Chen et al. (2006) to occur typically on the downshear left side of the storm. Since translation-induced vertical motion does not favor updraft formation on the west side of the eyewall (i.e., upshear), the radar echo is weaker on the south side of the eyewall. Figure 24 further shows how the upper-level cloud produced by the active convection is blown off in an asymmetric pattern. Ice particles are advected outward and cyclonically around to the south side of the storm and exit in a massive cirriform plume on the southeast side of the storm.
f. Cloud microphysical processes in the eyewall and inner-core region
In the upward branch of the secondary circulation of the tropical cyclone, precipitation particles are generated rapidly to produce the ring of heavy rainfall defining the eyewall. The basic elements of precipitation particle growth and fallout are illustrated by Fig. 25, which is based on Doppler radar observations in Hurricane Alicia (1983) (Marks and Houze 1987). Many of the precipitation particles in the eyewall are generated by the “warm rain” process—that is, drops are condensed, grow rapidly by drop coalescence below the 0°C level, and fall out before they have a chance to become ice particles. However, much of the rain generation in the eyewall and surrounding inner-core region of the cyclone involves the ice phase. Just above the 0°C level, ice particles occur and grow by riming to produce graupel. The heavier graupel particles fall out rapidly (fall speeds of several meters per second), melt to form raindrops, and fall out in the eyewall precipitation region (see trajectory 0–1′–2′ in Fig. 25). However, many of the ice particles formed in the eyewall cloud are ice-particle aggregates (bunches of ice particles stuck together), which fall more slowly (∼1 m s−1; see chapter 3 of Houze 1993). The aggregates and other slowly falling ice particles are advected outward by the radial wind component (Fig. 25) and swept great distances around the storm by the strong tangential winds of the cyclone (trajectory 0–1–2–3–4 in Fig. 26). As a result of the ice particles swirling outward in this way, the eyewall seeds the clouds throughout the inner-core region of the tropical cyclone.8 When the aggregates finally pass through the melting layer, usually within a rainband located some distance from the eyewall, they produce a bright band just below the 0°C level in the radar reflectivity, which is a signature of stratiform precipitation. This structure is indicated at radii of >20 km in Fig. 25.
Figure 27 is a more detailed schematic of the microphysical processes in the mixed-phase region of the eyewall cloud (enclosed by the dashed circle). This diagram summarizes 20 years of research based on 230 aircraft missions collecting cloud microphysical observations in tropical cyclones (Black and Hallett 1986, 1999). It is consistent with Fig. 25 in that below a mixed-phase region of the cloud (a shallow layer zone between ∼0° and −5°C containing both liquid drops and ice particles), drops grow vigorously by coalescence, and many rain out before they can be lifted into the mixed-phase region. Above the mixed-phase region, the cloud is glaciated, apparently from the highly probable collisions of any newly condensed supercooled drops with the numerous ice particles already present at upper levels in the mature eyewall. In the mixed-phase layer of the eyewall cloud, supercooled liquid water content is generally low (<0.5 g m−3) and found primarily where the updraft intensity is >5 m s−1, although not even all updrafts of this strength contain supercooled drops (Black and Hallett 1986). This liquid water is likely condensed just below the 0°C level, lofted by the updraft above this level and exists only for a short period of time before coming into contact with preexisting ice particles. The inset of Fig. 27 magnifies the region of the eyewall cloud where supercooled drops are found. This zone can be divided into three subregions: the inner edge adjacent to the eye region, an interior zone, and an outer edge of the eyewall cloud. Supercooled drops are found primarily on the inner edge of the eyewall cloud, where the newly generated drops are unlikely to have yet collided with preexisting ice particles since they occur near the top of the outward-sloping eyewall cloud, where no ice particles are entering the layer from above.
The middle portion of the inset in Fig. 27 illustrates how ice particles from higher levels come into contact with the supercooled droplets in the mixed-phase region. The drops freeze by contact nucleation and grow further by riming as they accrete the continuously produced supercooled cloud droplets. Graupel particles result from this riming and continue to grow by collection of supercooled droplets. Riming at ∼0° and −5°C produces large numbers of secondary ice particles by the process identified by Hallett and Mossop (1974; see summary in chapter 3 of Houze 1993). High concentrations of ice particles (up to hundreds per liter) are produced by this process (Black and Hallett 1986). These secondary particles combined with the snow particles falling into the region from the eyewall outflow above, make the probability of the supercooled drops being scavenged by ice particles extremely high. The crystal habits of the tiny ice particles produced by the secondary ice-particle production mechanism and growing further by vapor deposition are usually difficult to discern from instruments aboard aircraft. Where identifiable, the predominant crystal habits are columns, consistent with the temperature regime of ∼0° and −5°C (Table 6.1 of Wallace and Hobbs 2006). As indicated in Fig. 27, these columns are part of a population that includes supercooled droplets, graupel and other ice particles in the interior of the updraft region of the eyewall cloud.
The inset in Fig. 27 indicates that the outer edge of the eyewall cloud is dominated by completely glaciated cloud, with high concentrations of ice particles (up to 200 L−1; see Black and Hallett 1986). Of these particles, many are small and likely produced by the secondary ice-particle production process. However, large aggregate ice particles also appear in this region. Figure 28 from Houze et al. (1992) shows the pattern of ice particles sampled extensively by aircraft flying at the 6-km level in the eyewall and inner-core regions of Hurricane Norbert (1984). This overall distribution of ice particles is consistent with the processes indicated in Fig. 27. High concentrations of small ice particles (0.05–0.5 mm in dimension) likely resulting from secondary ice-particle production were located just outside the eyewall. On either side of this zone of numerous small ice particles were regions where larger ice particles (>1.05 mm in dimension) were concentrated. The larger ice particles in the eyewall region were graupel particles, which because of their higher fall velocities remained in the eyewall region as they fell out. The larger ice particles found some 50 km outside the eyewall were aggregates, and in trajectories similar to 0–1–2–3–4 in Fig. 26, they were advected radially outward, circulated around the storm, and fell out as stratiform precipitation well outside the eyewall region.
From observations such as those described here, it is evident that the cloud microphysics in the tropical cyclone (and associated cloud electrification to be described below) are complex and linked to the details of the cloud dynamics. The extent to which these processes are represented accurately in numerical models is only just beginning to be explored. Rogers et al. (2007) have compared observational and model statistics of gross vertical velocity and radar reflectivity in tropical cyclones and found rough agreement. However, much remains to be determined about specific particle growth modes, mixing ratios of different hydrometeor species, nucleation and breakup processes, and particle trajectories in relation to both the larger-scale air motions in the cyclone and the mesoscale and convective air motions within specific cloud and precipitation features. Until models represent these details, precise forecasting of rainfall in landfalling hurricanes will remain a major challenge.
g. Electrification of the eyewall cloud
Cloud electrification is described in basic references such as Williams (1988) and Wallace and Hobbs (2006). When a smaller ice particle collides with a larger graupel particle the two particles take on opposite electric charges. Air motions in the cloud advect the smaller particles away from the falling graupel to a region of the cloud where they accumulate and give that part of the cloud an overall electric charge opposite in sign to the net charge in the region of the cloud where the graupel is concentrated. The sign of the charge transfer depends on the temperature and liquid water content of the region of the cloud where the ice-particle collisions occur. In the eyewall cloud of a tropical cyclone, graupel occurs generally in the ∼0° and −5°C temperature layer (Fig. 27, inset), and the liquid water contents there are nearly always low (<0.5 g m−3). Under these conditions, the graupel takes on positive charge, and the small colliding ice particles become negatively charged. The air motions in the eyewall tend to carry the small ice particles upward and outward, away from the region of graupel particles, leading to a negatively charged region of cloud at the −10° to −15°C level, slightly radially outward of the region where the ice-particle collisions occurred. Black and Hallett (1999) have determined, as indicated on the right-hand side of Fig. 27, that this sloping region of negative charge results in an upward component of the electric field vector (Ez) within the eyewall cloud. Component Ez points upward from a lower region of less negative to the upper zone of more negative charge that has accumulated from the upward and outward advection of small ice particles. When Ez becomes sufficiently large, a lightning discharge may result, as a negative strike at the ocean or ground surface. The (pos) in the figure indicates that on occasions when a portion of the eyewall cloud becomes buoyantly unstable, with larger vertical velocities and larger liquid water content (>0.5 g m−3), the small ice particles may become positively charged when they bounce off graupel particles. In that case positive charge is advected upward and outward, the sign of Ez is reversed, and positive surface lighting strikes may result at the ocean or ground surface. However, positive strikes are rare in tropical cyclones since the liquid water contents are predominantly low in the eyewall cloud, where, as noted above, any newly generated supercooled drops are quickly glaciated because of their highly probable contact with the abundantly present ice particles. An exception might be an extreme updraft of the type documented by Marks et al. (2008), but this has not been documented.
Figure 29 shows the distribution of negative surface lightning flashes observed in nine tropical cyclones (Molinari et al. 1999). These data indicate that a maximum of flashes occurs in the eyewall region, within 100 km of the storm center. A secondary maximum of flash occurrence is at a distance of >200 km from the storm center. As will be discussed in section 8b, the clouds in this region of a tropical cyclone appear to be less strongly influenced by the dynamics of the inner-core vortex, and this lightning is probably produced by processes similar to those observed in thunderstorms not associated with tropical cyclones (Williams 1988; Houze 1993; Wallace and Hobbs 2006) rather than by the eyewall electrification mechanism represented in Fig. 27.
8. The region beyond the eyewall: Rainbands and eyewall replacement
a. The eyewall–rainband complex—An overview
In a tropical cyclone, beneath the overriding cirrus canopy issuing from the primary eyewall, the rainfall outside the eyewall is primarily associated with a complex of rainbands, which have a spiral geometry as opposed to the quasi-circular geometry of the eyewall. Figure 30 shows an idealized but typical array of rainbands and eyewalls in a tropical cyclone. For purpose of discussion, Fig. 30 includes a circle delineating an approximate boundary between the outer environment of the tropical cyclone and an inner core, which is dynamically controlled by the cyclonic vortex circulation. Two eyewalls are shown bec