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John Molinari and Steven Skubis

Abstract

The surface wind field in a developing tropical cyclone (Agnes, 1972) was analyzed over a 1660 km radius for four days using conventional surface data, as the storm evolved from a disorganized depression to a hurricane. The transition to hurricane intensity was characterized by a wavelike disturbance propagating inward at 15 m s−1 from the outermost radii to the storm core over a 36-hour period. This propagating disturbance was clearly visible in the radial and vertical motion fields as a surge of inflow and upward motion. Rapid intensification of the storm began within hours after the leading edge of the surge reached the storm center. The analysis of consecutive 12-hour periods without compositing of data from nonsynoptic times was essential for identification of this feature.

The surge had the same asymmetry as the upper-level outflow channel, indicating the possible involvement of the outflow layer in its initiation. No clear evidence of an external forcing mechanism for the surge, such as the passage of an easterly wave across the circulation, could be found. No instability theory could account for propagation of this feature across regions with such strongly varying dynamical properties. As a result, it remains uncertain whether the inflow surge represented an environmental trigger to hurricane formation or a manifestation of an internal instability.

The boundary layer momentum budget was dominated by Coriolis torque and frictional dissipation. The sum of these two terms acted as a momentum source primarily during the passage of the inflow surge across each radial region. Inward lateral flux of momentum contributed significantly only within 440 km of the center.

A distinct diurnal oscillation in pressure tendency occurred until hurricane strength was reached, with maximum deepening at 1200 local time, and minimum deepening at 0000 local time. Diurnal oscillations in other variables were more subtle and often at variance with those described in other tropical cyclones.

Because the inflow surge developed at outer radii 36 hours prior to rapid deepening and had a clear signature in the time change of radial mass flux, it provides a potential tool for forecasting tropical cyclogenesis 24 hours or more in advance which requires only the use of conventional data. More study is needed to determine whether such an early warning signal frequently occurs in intensifying tropical cyclones.

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Anantha Aiyyer and John Molinari

Abstract

The role of the Madden–Julian oscillation (MJO) in modulating the frequency and location of tropical cyclogenesis over the eastern Pacific and the Gulf of Mexico during August–September 1998 is examined. During the nonconvective phase of the MJO, convection and low-level cyclonic vorticity occurred primarily in conjunction with the intertropical convergence zone (ITCZ). During the convective phase, convection, low-level cyclonic vorticity, and convergence expanded into the northeastern Pacific and the Gulf of Mexico. This was accompanied by enhanced eddy kinetic energy and barotropic energy conversions as compared to the nonconvective phase, consistent with previous research. During the nonconvective phase of the MJO, vertical shear was relatively weaker but tropical cyclones tended to form mainly within the ITCZ. On the contrary, during the convective phase, vertical wind shear exceeded 10 m s−1 over much of this region and tropical cyclone development occurred north of the ITCZ, near the Mexican Pacific coast and the Gulf of Mexico.

Idealized numerical experiments are conducted using a barotropic model with time-invariant basic states representative of the nonconvective and convective phases of the MJO. The simulations indicate that the propagation paths as well as the amplification of the eddies differ substantially between the two phases. During the nonconvective phase, the waves tend to propagate westward into the eastern Pacific. During the convective phase, stronger southerlies steer the waves into the Gulf of Mexico. The MJO-related modulation of tropical cyclogenesis in the eastern Pacific and Gulf of Mexico thus appears to involve anomalous convergence, cyclonic vorticity, vertical wind shear, and differing tracks of easterly waves.

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John Molinari and David Vollaro

Abstract

Outflow layer winds were objectively analyzed every 12 h for 6 days during the life cycle of Hurricane Elena (1985). A high correlation was found between angular momentum fluxes by azimuthal eddies at large radii and central pressure changes in the storm 27–33 h later. Momentum flux by eddies exceeded that by the azimuthal mean outside the 800 km radius, while vortex spinup by the eddies reached instantaneous magnitudes as large as 25 m s−1/day. Outflow maxima and minima repeatedly appeared more than 1000 km from the hurricane center and tracked inward with time. The results provide evidence of significant environmental control on the behavior of the storm.

After reaching hurricane strength, Elena experienced a major secondary intensification associated with a large inward cyclonic eddy momentum flux produced by the passage of a middle latitude trough north of the hurricane. An outflow maximum appeared radially inside of the eddy momentum source, consistent with balanced vortex theory, and tracked inward with the eddy momentum source during the following 24 h. When the outflow maximum reached the storm core, an extended period of rapid pressure fails followed. It is speculated that these pressure falls represented a response to midlevel spinup forced by the outflow layer momentum sourcers.

Although environmental forcing dominated the later stages of Elena, the rapid initial intensification of the storm as it moved from land to water appeared to be a precursor to subsequent environmental interactions. The enhanced anticyclonic outflow from this initial deepening reduced the outflow-layer inertial stability, allowing a more radially extended region for external forcing. The secondary intensification of Elena is thus viewed as a cooperative interaction between mesoscale events at the hurricane core and synoptic-scale features in the upper tropospheric environment.

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Michael Dickinson and John Molinari

Abstract

A large-amplitude mixed Rossby–gravity wave packet is identified in the western Pacific using 6–10-day bandpass-filtered winds. Individual disturbances of 2300–3000-km wavelength propagated westward as the packet moved slowly eastward. The packet first appeared, and subsequently amplified, within a region of active convection associated with the Madden–Julian oscillation (MJO), which was isolated by low-pass-filtered outgoing longwave radiation. The packet lasted about 5 weeks, then rapidly dispersed as the active MJO moved away from it to the east.

West of 150°E, individual disturbances within the packet turned northwestward away from the equator, indicating an apparent transition from mixed Rossby–gravity waves to off-equatorial tropical depression (TD)-type disturbances. Cyclones filled with cloud and anticyclones cleared during the transition. Nevertheless, convective structure consistent with mixed Rossby–gravity waves remained outside the circulation centers, and three tropical cyclones formed on the edges of three consecutive cyclonic gyres as they moved off the equator. Although the expected Rossby–Kelvin wave structure was present in the background winds within the active MJO, tropical cyclone genesis did not occur within the trailing Rossby gyres, but 2500 km to the west and north.

This case study provides evidence that equatorial modes, under the right conditions, can supply precursor disturbances for repeated formation of tropical cyclones. It is argued based on previous work in the literature that this sequence of events is not uncommon.

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John Molinari and David Vollaro

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The vertical structure of the interaction of Hurricane Elena (1985) with a baroclinic wave was evaluated using analyses from the European Centre for Medium Range Weather Forecasting. During the period of interaction, azimuthal eddies produced a localized flux convergence of cyclonic angular momentum in the upper troposphere which shifted to progressively smaller radii prior to major secondary deepening of the storm. These momentum fluxes decayed above and below the outflow layer. Eddy heat fluxes showed maximum cooling in the middle and upper troposphere and warming in the lower stratosphere, reflecting the temperature structure of the baroclinic wave as it moved into the hurricane volume.

The response of the hurricane vortex to the fluxes of heat and angular momentum was determined by solution of Eliassen's balanced vortex equation. The balanced solutions showed a band of upward motion, with deep inflow and narrow outflow, which shifted inward from the 500 km radius to the hurricane core in the 24 hours prior to the secondary deepening. The position and timing of this feature corresponded to the contracting outflow maximum found in Part I. Eddy heat fluxes contributed to the induced circulation in the same manner as momentum fluxes near the core, but with smaller magnitude and areal coverage. The contracting outflow maximum thus appeared to represent the upper branch of a secondary circulation excited primarily by the eddy momentum fluxes.

The reintensification of hurricanes is often directly associated with formation of a wind maximum at inner radii which replaces or reinforces the original eye wall as it contracts. Such a feature was seen in reconnaissance data in Elena at the time the secondary circulation reached inner radii. It is speculated that the relatively weak secondary circulation evolved into a local wind maximum through the actions of diabatic heat sources. The approaching trough is thus viewed not as a direct cause of deepening, but as a catalyst which organized the diabatic sources in such a way as to excite internal instabilities of the system.

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John Molinari and David Vollaro

Abstract

A set of 327 dropsondes from the NOAA G-IV aircraft was used to create a composite analysis of the azimuthally averaged absolute angular momentum in the outflow layer of major Hurricane Ivan (2004). Inertial instability existed over a narrow layer in the upper troposphere between the 350- and 450-km radii. Isolines of potential and equivalent potential temperature showed that the conditions for both dry and moist symmetric instability were satisfied in the same region, but over a deeper layer from 9 to 12 km. The radial flow maximized at the outer edge of the unstable region. The symmetrically unstable state existed above a region of outward decrease of temperature between the cirrus overcast of the storm and clear air outside. It is hypothesized that the temperature gradient was created as a result of longwave heating within the cirrus overcast and longwave cooling outside the cloudy region. This produced isentropes that sloped upward with radius in the same region that absolute momentum surfaces were flat or sloping downward, thus creating symmetric instability. Although this instability typically follows rather than precedes intensification, limited numerical evidence suggests that the reestablishment of a symmetrically neutral state might influence the length of the intensification period.

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Patrick Duran and John Molinari

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High-vertical-resolution rawinsondes were used to document the existence of low–bulk Richardson number (R b) layers in tropical cyclones. The largest frequency of low R b existed in the inner 200 km at the 13.5-km level. This peak extended more than 1000 km from the storm center and sloped downward with radius. The presence of an extensive upper-tropospheric low-R b layer supports the assumption of Richardson number criticality in tropical cyclone outflow by Emanuel and Rotunno.

The low-R b layers were found to be more common in hurricanes than in tropical depressions and tropical storms. This sensitivity to intensity was attributed to a reduction of upper-tropospheric static stability as tropical cyclones intensify. The causes of this destabilization include upper-level cooling that is related to an elevation of the tropopause in hurricanes and greater longwave radiative warming in the well-developed hurricane cirrus canopy. Decreased mean static stability makes the production of low R b by gravity waves and other perturbations easier to attain.

The mean static stability and vertical wind shear do not exhibit diurnal variability. There is some indication, however, that low Richardson numbers are more common in the early morning than in the early evening, especially near the 200–300-km radius. The location and timing of this diurnal variability is consistent with previous studies that found a diurnal cycle of infrared brightness temperature and rainfall in tropical cyclones.

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John Molinari and David Vollaro

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The previous study of helicity, CAPE, and shear in Hurricane Bonnie (1998) was extended to all eight tropical cyclones sampled by NASA during the Convection and Moisture Experiments (CAMEX). Storms were categorized as having large or small ambient vertical wind shear, with 10 m s−1 as the dividing line. In strongly sheared storms, the downshear mean helicity exceeded the upshear mean by a factor of 4. As in the previous study, the helicity differences resulted directly from the tropical cyclone response to ambient shear, with enhanced in-up-out flow and veering of the wind with height present downshear. CAPE in strongly sheared storms was 60% larger downshear. Mean inflow near the surface and the depth of the inflow layer each were 4 times larger downshear. At more than 30% of observation points outside the 100-km radius in the downshear right quadrant, midlatitude empirical parameters indicated a strong likelihood of supercells. No such points existed upshear in highly sheared storms. Much smaller upshear–downshear differences and little likelihood of severe cells occurred in storms with ambient wind shear below 10 m s−1. In addition to these azimuthal asymmetries, highly sheared storms produced 30% larger area-averaged CAPE and double the area-averaged helicity versus relatively unsheared storms. The vortex-scale increase in these quantities lessens the negative impact of large vertical wind shear.

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Anantha R. Aiyyer and John Molinari

Abstract

A linear shallow water model is used to simulate the evolution of mixed Rossby–gravity (MRG) waves in background states representative of the convective phase of the Madden–Julian oscillation (MJO). Initial MRG wave structures are obtained analytically. The MJO basic state is defined by the steady response of the tropical atmosphere to localized heating. Results from the simulations reveal that variations in the background flow play a significant role in the evolution of the MRG waves. When the basic state is symmetric about the equator, the MRG wave amplifies within the convergent region of the background flow and the ensuing development remains symmetric. When the heating is asymmetric, both the basic state and the MRG wave evolution exhibit significant asymmetries. Prominent features of this simulation are the development and growth of a series of small-scale, off-equatorial eddies that resemble tropical-depression-type disturbances.

The results suggest that a persistent large-scale heating that is asymmetric with respect to the equator may lead to the growth of off-equatorial disturbances from an equatorial mode. These disturbances, approximately 1000–2000 km in scale, are considerably smaller than the initial wavelength of the MRG wave. It is suggested that the cyclonic elements among them could serve as seedlings for tropical cyclones. This process may be particularly relevant to cyclogenesis in the tropical western Pacific, a region where the MJO and MRG waves are frequently observed.

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Kristen L. Corbosiero and John Molinari

Abstract

The influence of the direction of storm motion on the azimuthal distribution of electrified convection in 35 Atlantic basin tropical cyclones from 1985 to 1999 was examined using data from the National Lightning Detection Network. In the inner 100 km, flashes most often occurred in the front half of storms, with a preference for the right-front quadrant. In the outer rainbands (r = 100–300 km), flashes occurred predominantly to the right of motion, although the maximum remained in the right-front quadrant. The results are shown to be consistent with previous studies of asymmetries in rainfall, radar reflectivity, and vertical motion with respect to tropical cyclone motion. The motion effect has been attributed to the influence of asymmetric friction in the tropical cyclone boundary layer.

The authors previously found a strong signature in the azimuthal distribution of lightning with respect to vertical wind shear. Because both effects show clearly, vertical wind shear and storm motion must themselves be systematically related. It was found that more than three-quarters of 12-hourly periods contained a storm motion vector that was left of (i.e., counterclockwise from) the shear vector. These results support the importance of a downshear shift in the upper anticyclone, which produces motion left of shear for all directions of shear. The results are further broken down by direction of shear, and it is shown that the beta effect also plays a significant role in the relationship between motion and vertical wind shear. These results also suggest that substantial downshear tilt of the cyclonic part of the tropical cyclone vortex is uncommon, because that alone produces motion right of shear.

The relative importance of asymmetric friction and vertical wind shear on the azimuthal asymmetry of convection was determined by examining circumstances in which the two effects would place maximum lightning in different quadrants. Without exception, the influence of vertical wind shear dominated the distribution. Although asymmetric friction creates vertical motion asymmetries at the top of the boundary layer, these apparently do not produce deep convection if vertical wind shear–induced circulations oppose them.

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