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  • View in gallery

    Sea surface temperature analysis for the Gulf Stream region based on an average of the JHU/APL composite of AVHRR channels 3B, 4, and 5. Based on a composite for the (a) 5.71 days prior to 2319 UTC 9 Apr, (b) 5.64 days prior to 2218 UTC 24 Apr 2002, (c) 5.57 days prior to 2159 UTC 10 Dec 2000, and (d) 5.57 days prior to 2243 UTC 18 Mar 2001

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    AVHRR channel-4 infrared image from 0713 UTC 8 Apr 2002

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    Locations and station identifiers for the meteorological buoys and Coastal-Marine Automated Network (C-MAN) stations within the study area. Locations of the coastal ME and offshore climatological data (black crosses). The Gulf Stream climatological point at 38°N, 62°W is approx 1° lon east and 1° lat north of the lower right corner of this map

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    GOES-8 channel-5 infrared image from 1345 UTC 8 Apr 2002. Gulf Stream meanders are noted (black dots)

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    A portion of the National Weather Service Northern Hemisphere 850-hPa analysis for 1200 UTC 8 Apr 2002. Satellite-derived winds are shown as stars and rawinsonde observations as circles

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    A portion of the National Weather Service Northern Hemisphere surface analysis for 1200 UTC 8 Apr 2002. The National Weather Service, Federal Aviation Administration, and volunteer ship observations upon which this analysis is basesd are not shown

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    As in Fig. 4 but from 1415 UTC 24 Apr 2002

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    As in Fig. 2 but from 0918 UTC 24 Apr 2002

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    As in Fig. 4 but from 1215 UTC 10 Dec 2000

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    As in Fig. 2 but from 1039 UTC 10 Dec 2000

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    As in Fig. 4 but from 0015 UTC 20 Mar 2001

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    As in Fig. 2 but from 2019 UTC 20 Mar 2001

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    Graph of the annual cycle of mean surface air temperature for coastal ME, the sea surface temperature off the coast of ME, and the Gulf Stream temperature southeast of ME

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Mesoscale Stratocumulus Bands Caused by Gulf Stream Meanders

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  • 1 The Pennsylvania State University, University Park, Pennsylvania
  • | 2 United States Naval Academy, Annapolis, Maryland
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Abstract

Examination of visible and infrared imagery from geosynchronous and polar orbiter satellites reveals the occasional existence of mesoscale cloud bands of unusual width and area, originating over the open northwest Atlantic Ocean during cold-air outbreaks. This phenomenon is of both dynamic and synoptic interest. As a dynamic phenomenon, it represents a mesoscale flow that is driven by transient surface features, which are meanders in the Gulf Stream. The forcing geometry and the resulting cloud pattern are similar in many respects to the anomalous cloud lines observed downwind of Chesapeake and Delaware Bays in similar conditions. These open ocean cloud bands are often of a larger scale, however, because the Gulf Stream meanders represent the largest-scale high-amplitude “coastal features” in the western North Atlantic. These cloud bands are of synoptic interest because, when present, they play a major role in determining the cloud pattern over much of this oceanic region.

Examination of surface and 850-hPa analyses demonstrates that these open ocean cloud bands occur during cold-air outbreaks and that they align approximately with the boundary layer wind. Comparison of visible and infrared satellite imagery with contemporaneous sea surface temperature analyses derived from infrared polar orbiter satellite imagery reveals that the open ocean cloud bands originate at the upwind end of Gulf Stream meanders. Climatological data and synoptic observations from land and sea indicate that these events occur only during that part of the spring season in which coastal temperature differences are small but cold-air outbreaks continue to reach the Gulf Stream. Examination of this observational evidence suggests that these open ocean cloud bands result from mesoscale solenoidal circulations driven by the horizontal gradients in sea surface temperature caused by Gulf Stream meanders.

Corresponding author address: Dr. George S. Young, Department of Meteorology, The Pennsylvania State University, 503 Walker Building, University Park, PA 16802. Email: young@ems.psu.edu

Abstract

Examination of visible and infrared imagery from geosynchronous and polar orbiter satellites reveals the occasional existence of mesoscale cloud bands of unusual width and area, originating over the open northwest Atlantic Ocean during cold-air outbreaks. This phenomenon is of both dynamic and synoptic interest. As a dynamic phenomenon, it represents a mesoscale flow that is driven by transient surface features, which are meanders in the Gulf Stream. The forcing geometry and the resulting cloud pattern are similar in many respects to the anomalous cloud lines observed downwind of Chesapeake and Delaware Bays in similar conditions. These open ocean cloud bands are often of a larger scale, however, because the Gulf Stream meanders represent the largest-scale high-amplitude “coastal features” in the western North Atlantic. These cloud bands are of synoptic interest because, when present, they play a major role in determining the cloud pattern over much of this oceanic region.

Examination of surface and 850-hPa analyses demonstrates that these open ocean cloud bands occur during cold-air outbreaks and that they align approximately with the boundary layer wind. Comparison of visible and infrared satellite imagery with contemporaneous sea surface temperature analyses derived from infrared polar orbiter satellite imagery reveals that the open ocean cloud bands originate at the upwind end of Gulf Stream meanders. Climatological data and synoptic observations from land and sea indicate that these events occur only during that part of the spring season in which coastal temperature differences are small but cold-air outbreaks continue to reach the Gulf Stream. Examination of this observational evidence suggests that these open ocean cloud bands result from mesoscale solenoidal circulations driven by the horizontal gradients in sea surface temperature caused by Gulf Stream meanders.

Corresponding author address: Dr. George S. Young, Department of Meteorology, The Pennsylvania State University, 503 Walker Building, University Park, PA 16802. Email: young@ems.psu.edu

1. Introduction

Mesoscale patterns in surface temperature have long been known to give rise to mesoscale solenoidal circulations in the atmospheric boundary layer (e.g., Holton 1992). The common sea/land breeze is an example of such a phenomenon (e.g., Arritt 1993). Other types of mesoscale solenoidal circulations produce distinctive cloud features. Take, for example, midlake cloud bands (Passarelli and Braham 1981; Hjelmfelt 1982; Niziol et al. 1995), and cloud lines downwind of bays and bights (Sikora et al. 2001; Sikora and Halverson 2002). For each of these phenomena, spatially fixed surface features control the mesoscale pattern of surface temperature (Arritt 1993). Thus, the location of the resulting mesoscale solenoidal circulation depends primarily upon the synoptic-scale wind direction in the planetary boundary layer. Similarly, the existence of these phenomena depends upon the solenoidal forcing being strong enough to overcome both drag and synoptic-scale advection (Sikora et al. 2001). Thus, they represent one of the easiest classes of mesoscale phenomena to forecast via numerical or statistical methods. Forecasting such phenomena would be more challenging if the surface features themselves moved with time, as is the case with meanders and rings in the western boundary currents of midlatitude oceans (e.g., Halliwell and Mooers 1983; Robinson et al. 1988; Lee and Cornillon 1996). The multiday Advanced Very High Resolution Radiometer (AVHRR) composites shown in Fig. 1 provide four examples of the mesoscale pattern of sea surface temperature (SST) that can result from these meanders. A smaller-scale single AVHRR image (Fig. 2) reveals the potential complexity of the SST field in the vicinity of such meanders.

For the discussion that is to follow, it is useful to define carefully what is meant by mesoscale and how the scale of cloud bands is to be quantified. The scale nomenclature used herein follows Orlanski (1975), with the mesoscale extending somewhat arbitrarily from 2 to 2000 km. Orlanski subdivides this range into the meso γ (2–20 km), meso β (20–200 km), and meso α (200–2000 km). At smaller scales Orlanski defines the micro γ (2–20 m), micro β (20–200 m), and micro α (200–2000 m), while at scales beyond the meso lie the macro β (2000–10 000 km) and macro α (+ 10 000 km). These scales are applied to the quasi-two-dimensional cloud features discussed herein by measuring the horizontal wavelength from band center to band center. Because of the clear space between any two bands, the actual cloud features are somewhat narrower than the scale given. As will be shown below, a dynamical distinction between convective rolls and solenoidal-circulation bands is more enlightening than mere horizontal scale in distinguishing between these two cold-air outbreak phenomena in the Gulf Stream region.

The most basic feature of a cold-air outbreak over warm water is the creation of a thermal internal boundary layer (Chang and Braham 1991). When a cold dry air mass advects over a warm body of water a sequence of processes occur that can (and frequently does) lead to the formation of stratocumulus or related clouds. First, the combination of wind with air–sea temperature and vapor pressure differences lead to upward surface fluxes of heat and humidity into the surface layer. Second, this heating and moistening of the surface layer destabilizes a deeper layer of the air mass resulting in convective mixing. The convection carries the heat and moisture upward, warming and moistening this new convective mixed layer. Because the convection transports turbulent kinetic energy to the top of the mixed layer, the mixed-layer growth occurs via entrainment rather than encroachment (Young et al. 2000). Thus, there is a downward flux of heat at the top of the boundary layer, but an upward flux of humidity. If the surface moisture flux exceeds the entrainment moisture flux by a sufficient amount, the actual vapor pressure will increase faster than the saturation vapor pressure, leading eventually to cloud formation. Such is nearly always the case when continental polar or arctic air advects over warm water. The difference between the clear and cloudy scenarios stems from both the nature of the air being entrained (dry versus moist) and the Bowen ratio of the surface fluxes. Thus, for the dry continental polar or arctic air masses associated with cold-air outbreaks, the essential difference between a convective boundary layer that clears and one that becomes cloudy is the surface Bowen ratio.

Surface-driven solenoidal circulations occur when mesoscale gradients in the surface heat flux lead to horizontal mesoscale gradients in boundary layer temperature (Pielke 1984). This situation typically occurs when a synoptic-scale air mass moves over a localized region with surface temperatures greater than that of the air mass. Bays, islands, or peninsulas can provide the localized heat source depending on the time of year (e.g., the Chesapeake and Delaware Bays during early autumn; Sikora and Halverson 2002). From bulk aerodynamic arguments (Fairall et al. 1996), it follows that the warmer the surface the greater the surface heat flux, all else being equal. If the initial boundary layer depth is similar throughout the region, the vertical flux convergence, and thus boundary layer heating, will also be greater the warmer the surface (Stull 1988).

Several processes complicate this issue without, in general, changing the outcome. First, entrainment at the top of a growing boundary layer provides an additional heat source. Because the boundary layer grows fastest over the region of highest surface temperature, the entrainment flux of sensible heat acts to further enhance the buoyancy there. Second, this entrainment results in an upward flux of moisture and a corresponding downward flux of dry air into the boundary layer. This process can result in evaporation of cloud water as described earlier, thus reducing the buoyancy over the region of highest surface temperature. Unless the criteria for entrainment instability are met (Stull 1988), the net of these two effects will be to enhance the buoyancy rather than decrease it. Third, latent heat release will enhance the buoyancy over the region of highest surface temperature if the resulting solenoidal circulation is sufficient to form cloud bands. If these processes work in unison to enhance buoyancy, a mesoscale region of buoyant boundary layer air is created over the warm surface. This region of buoyant air results in a hydrostatic low at the base of the boundary layer and a hydrostatic high at its top. The resulting horizontal pressure gradients drive inflow near the surface and outflow near the boundary layer top (Pielke 1984). Mass continuity requires upward motion of the warm air, as one would expect from buoyancy considerations.

If there is a synoptic-scale wind, the solenoidal circulation advects downwind of the original surface hot spot (Hjelmfelt 1982). The buoyant plume and its associated updraft lie between two plumes of compensating subsidence with the boundaries between the updraft and downdrafts forming a pair of counterrotating horizontal vortices. Schär and Smith (1993) discuss the dynamics and longevity of such plumes of potential vorticity. As a result of this longevity, the surface-driven solenoidal circulations can extend for meso-β- to -α-scale distances downwind of the region of higher SST.

Thus, boundary layer cloud bands can form in response to mesoscale hot spots by either of two mechanisms: entrainment deepening of the boundary layer over the heated surface and advective deepening of the boundary layer by the mesoscale circulation. Indeed, the physical linkage between these processes is such that the two mechanisms work in concert. Details of the resulting cloud field can however provide insight into which mechanism is dominant in a particular case (Niziol et al. 1995).

Fixed coastal features are not the only source of mesoscale hot spots for solenoidal-circulation formation. Meanders and rings arising from the Gulf Stream can also create mesoscale regions of elevated surface temperature (e.g., Halliwell and Mooers 1983; Robinson et al. 1988; Lee and Cornillon 1996). The Gulf Stream is the western boundary current of the North Atlantic. As such, it sweeps a band of high-temperature water northeastward from the southeast coast of the United States to the region well south of Nova Scotia (e.g., Fig. 1). The SST gradient along the northwest side of the Gulf Stream is particularly intense (e.g., Fig. 1), earning it the name North Wall (Warnecke et al. 1971). With typical temperature differences of 5°–10°C across a few kilometers, the Gulf Stream North Wall is as thermodynamically significant as most of the surface features known to drive mesoscale solenoidal circulations. Indeed, for synoptic-scale flow parallel to the North Wall, Sublette and Young (1996) observed and simulated sea-breeze-type fronts resulting from these circulations.

The mesoscale pattern of surface forcing becomes more complicated when the Gulf Stream's northeastward progress breaks down into a series of meanders such as those in Figs. 1 and 2. Robinson et al. (1988) document the common occurrence of these meanders and the detached rings they may form. The typical meander has a wavelength of 330–350 km and amplitude of 200 km (Halliwell and Mooers 1983; Lee and Cornillon 1996). Thus, the typical Gulf Stream meander is similar in spatial scale and baroclinicity to one of the Great Lakes and is markedly larger than the Chesapeake and Delaware Bays. Their origin and evolution via differential advection by the quasigeostrophic ocean currents results, however, in their evolving on a scale of weeks rather than being quasi-permanent features (Halliwell and Mooers 1983; Robinson et al. 1988; Lee and Cornillon 1996). These characteristics suggest that Gulf Stream meanders, and quite possibly rings, could provide sufficient forcing for mesoscale solenoidal circulations during cold-air outbreaks.

Mesoscale solenoidal circulations forming in cold-air outbreaks over warm water would be subject to several diabatic feedbacks. Latent heat release would provide additional buoyancy to drive the circulation's updraft while entrainment could lead to breaks in the overcast or affect band wavelength (Young et al. 2002). The band thus represents a local intensification of the cold-air outbreak thermal internal boundary layer discussed earlier.

Solenoidal circulations originating at surface hot spots are not, however, the only common form of buoyantly driven boundary layer circulations observed during cold-air outbreaks (Hjelmfelt 1990). Longitudinal roll vortices occur over both lakes and oceans in those cases where horizontal gradients of SST are small in the crosswind direction (Hjelmfelt 1990). They differ from the mesoscale solenoidal circulations discussed earlier in that their buoyancy results from vertical advection in an unstable boundary layer (i.e., gradient production as part of convective instability) rather than from localized variations in the surface heat flux and moisture fluxes. In this gradient production mechanism, the unstably stratified layer is perturbed in the vertical resulting in the creation of positively buoyant updrafts and negatively buoyant downdrafts via vertical advection of potential temperature, or equivalent potential temperature if the layer is saturated. The resulting parcel buoyancy accelerates the up- and downdrafts resulting in buoyant production of turbulent kinetic energy and, therefore, enhancement of the gradient production of buoyancy. As a result of this internal instability mechanism, roll vortices do not require the external forcing of a surface hot spot. Likewise, their horizontal wavelength is determined by convective dynamics rather than the scale of an underlying SST feature (Miura 1986; Young et al. 2002). These authors demonstrate that open ocean cold-air outbreak roll vortices result in cloud streets for which the cross-roll wavelength increases as 3.22Zi + 0.0028Z2i, where Zi is the boundary layer depth, reaching 25 km for a boundary layer depth of 2.5 km. Young et al. (2002) refer to these features as wide-mode rolls to distinguish them from the overland rolls wherein wavelength varies as 3.15Zi. The existence of the scaling relationship between the wavelength of the cloud features and the boundary layer depth will prove useful in distinguishing the mesoscale solenoidal circulations from wide-mode rolls in the following discussion.

The current paper presents observational evidence for the occurrence of mesoscale solenoidal circulations and their associated cloud bands in response to cold-air outbreaks over Gulf Stream meanders. It discusses the oceanographic and atmospheric conditions required for their formation and the evidence that they are indeed surface-driven solenoidal circulations. The synoptic and climatological data are then used to explain the limited season in which these open ocean cloud bands form.

2. Data

In order to undertake this study it is necessary to observe the open ocean cloud bands and the meteorological and oceanographic environment in which they form. This study relies primarily on satellite imagery for cloud band detection and mapping as well as analysis of the SST pattern. Operational analyses from the National Weather Service (NWS) and individual buoy observations from the National Data Buoy Center (NDBC) are used to document the synoptic setting. The locations of the relevant buoys are shown in Fig. 3. Land-based climatological records from the National Oceanic and Atmospheric Administration (NOAA) supplement the Comprehensive Ocean–Atmosphere Data Set (COADS) in describing the climatological conditions necessary for cloud band formation.

Geostationary Operational Environmental Satellite GOES-8 infrared imagery (channel 4, 10.20–11.20 μm), archived by Unisys Weather Information Services (http://weather.unisys.com/), were examined for the fall 2000 through spring 2002 time period in order to detect open ocean cloud band cases. These 4-km resolution images proved sufficient to distinguish the phenomenon of interest from the cloud streets caused by the more common wide-mode boundary layer rolls (Young et al. 2002). Once illustrative cases and countercases were selected, a more robust selection of GOES-8 and NOAA AVHRR images were obtained for subsequent analysis.

The GOES-8 images were provided by the Cooperative Institute for Research in the Atmosphere (CIRA) of Colorado State University (http://www.cira.colostate.edu/). Of the five channels available, channels 1 (0.55–0.75 μm, 1-km resolution), 2 (3.80–4.00 μm, 4-km resolution), 4, and 5 (11.50–12.50 μm, 4-km resolution) proved the most useful. The cloud bands could be seen on channel 1 during the day, and on the other three channels day or night. They were typically most apparent on channel 5, as were the other boundary layer cloud features such as wide-mode rolls.

AVHRR images were provided by the Applied Physics Laboratory of The Johns Hopkins University (JHU/APL) (http://fermi.jhuapl.edu/). Channel 1 (0.58–0.68 μm), 2 (0.725–1.10 μm), 3B (3.55–3.93 μm), 4 (10.30–11.30 μm), and 5 (11.50–12.50 μm) data were obtained. The resolution of the AVHRR imagery is 1.1 km. The infrared imagery was particularly useful as it captured both the cloud bands and the sea surface temperature pattern in the surrounding clear-sky areas.

AVHRR-derived SST analyses of the Gulf Stream region were also obtained from JHU/APL. Channels 3B, 4, and 5 are used by JHU/APL to generate their SST images. Because of the persistent cloudiness associated with cold-air outbreaks, it was necessary to use their multiday composites despite the loss of temporal resolution. The meanders in the Gulf Stream evolved slowly enough that this did not present a problem.

Synoptic conditions were documented using the NWS operational Northern Hemisphere charts for 850 hPa and the surface. These fax charts were downloaded from the National Climatic Data Center (NCDC) (http://lwf.ncdc.noaa.gov/oa/ncdc.html). Boundary layer winds were determined from these charts and from moored buoy observations downloaded from NDBC (http://www.ndbc.noaa.gov/index.shtml). Climatological SST values were obtained from the Columbia University online catalog of COADS monthly averages (http://ingrid.ldgo.columbia.edu/SOURCES/OBERHUBER/) while the land-based air temperature climatology was obtained from NOAA's 30-yr means (NOAA 1974). Small-scale variability was reduced by using the averages for climatological divisions within a state rather than those for individual stations.

3. Results and discussion

This section will document the occurrence and origin of open ocean cloud bands using two case studies and two illustrative countercases. Each of the countercases was selected to highlight the role of a particular feature in the nonoccurrence of ultrawide open ocean cloud bands.

a. 8 April 2002

Both visible (not shown) and infrared (Fig. 4) GOES imagery show bands of enhanced cloudiness extending southeast from the warm meanders of the Gulf Stream at 1345 UTC on 8 April. These bands are at least as long as the more common cloud streets associated with wide-mode boundary layer rolls (Miura 1986). They are, however, much wider than wide-mode rolls would be given any reasonable boundary layer depth, that is, 25 km for a 2.5-km-deep boundary layer as computed earlier. As discussed in the introduction, the relationship between the horizontal scale of the cloud features and boundary layer depth is quite robust for wide-mode rolls, so the extreme width of the current bands suggests that they represent a different phenomenon.

Because the atmosphere upwind of the bands is cloud free, it is possible to see the sharp SST contrast at the Gulf Stream North Wall in Fig. 4. Each band can be seen to originate within one of a series of Gulf Stream meanders. The Gulf Stream and its meanders are more clearly evident in the corresponding AVHRR-derived SST analysis (Fig. 1a). The Gulf Stream appears as a mesoscale band of higher SST separating cool water to the northwest from warm waters to the southeast. It lies relatively near its climatological position. As is typical (Robinson et al. 1988; Lee and Cornillon 1996), these meanders grow in amplitude but decrease in intensity in the downstream (northeast) direction. A high-resolution AVHRR channel-4 image from this same period, 0713 UTC on 8 April, (Fig. 2) reveals even more detail of the SST field in and around these meanders. The North Wall can be seen as a sharp boundary looping across the image, often only 1 pixel across, but occasionally breaking down in the presence of smaller oceanographic eddies. The boundary layer cloud field is similar to that seen in cold-air outbreaks over the Great Lakes, small clouds form just downwind (southeast) of the North Wall and grow into larger and more complex structures to the southeast. The mesoscale atmospheric structure seen in Fig. 4 is clearly modulating microscale convective elements in Fig. 2. This pattern and its relationship to the mesoscale meanders of warm, Gulf Stream water are strongly reminiscent of the midlake convective bands observed during along-lake cold-air outbreaks (Hjelmfelt 1982; Niziol et al. 1995).

The atmospheric synoptic setting is also similar to that for lake-effect cloud bands. The 850-hPa analysis for 1200 UTC on 8 April (Fig. 5) shows a cold front extending south-southwest from a low near Greenland, with northerly winds extending behind the front to about 70°W longitude. Cold advection is occurring throughout this region. Farther west, an area of high heights is bringing light winds to the offshore waters and southerlies to the mid-Atlantic coast. The cloud bands described earlier are confined to the region of northerlies and cold advection between the cold front and this trailing high. The surface analysis (Fig. 6) is similar although the high is displaced farther to the east and the geostrophic winds in the cold-air outbreak are from the north-northeast instead of north. Frictional backing would, however, cause the true surface winds to more nearly parallel those at 850 hPa. Thus, the boundary layer wind in the region of the bands was approximately northerly, matching the alignment of the cloud bands.

The geographic correspondence of each of the cloud bands to a warm meander in the Gulf Stream is striking, as can be seen from a comparison of Figs. 1a, 2, and 4. Unlike traditional cold-air outbreak cloud streets, these bands originate near the Gulf Stream North Wall rather than the coast. The upwind end of each band lies near the tip of a warm meander. The bands are persistent enough to cause data gaps in the multiday SST composite found in Fig. 1a, which speaks to the robustness of their spatial relationship to the meanders.

The lack of clouds northwest of the Gulf Stream North Wall can be understood in terms of the change in surface heat flux across this oceanographic feature. Buoy 44011 lies well south of Nova Scotia but still to the north of the Gulf Stream's 8 April position. Observations from this buoy during the period of cold advection yield a bulk aerodynamic sensible heat flux of 10 W m−2 (1000 UTC on 8 April) using the method of Fairall et al. (1996) as implemented by Babin et al. (1997). This low value explains the lack of convective clouds to the north of the Gulf Stream. In contrast, situating the 3.1°C air from buoy 44011 over the Gulf Stream would result in a bulk aerodynamic sensible heat flux of 215 W m−2, a value similar to that seen in intense lake-effect convection (e.g., Hjelmfelt 1982; Niziol et al. 1995).

From the previous discussion, it is apparent that the open ocean cloud bands observed on 8 April 2002 bear a striking resemblance, in both form and environment, to midlake convective bands in lake-effect snowstorms (e.g., Passarelli and Braham 1981; Hjelmfelt 1982; Niziol et al. 1995) and to the dynamically similar bay-effect cloud bands (Sikora et al. 2001; Sikora and Halverson 2002). The open ocean cloud bands form only where a mesoscale region of large sensible heat flux is surrounded on the upwind and lateral sides by regions of modest sensible heat flux. From this point of origin, they trail downwind for hundreds of kilometers over the more nearly uniform SST field southeast of the Gulf Stream North Wall. Thus, while the existence of a mesoscale region of enhanced boundary layer instability gives rise to a band of convective cloudiness, the band can persist far downwind of its parent surface feature. Similar downwind persistence has been observed in lake-effect cloud bands (e.g., Niziol et al. 1995) and bay-effect cloud bands (Sikora et al. 2001), and is attributed to the existence of a mesoscale solenoidal circulation. The initial circulation forms in response to the localized surface heating as described in the introduction. The circulation then persists downwind for as long as sufficient warmth, and thus buoyancy, remains in its updraft plume to counteract drag. The several-hundred-kilometer persistence seen in this case is not unusual for phenomena of this kind.

b. 24 April 2002

A second example of this phenomenon was observed on 24 April 2002. Both visible (not shown) and infrared (Fig. 7) GOES-8 satellite imagery reveal mesoscale bands of convective cloud originating in meanders of the Gulf Stream at 1415 UTC 24 April. Unlike the previous case, high-amplitude warm meanders were present only to the south of Nova Scotia and to the east of Virginia (Fig. 1b). AVHRR imagery from 0918 UTC (Fig. 8) reveals a mesoscale cloud band originating in each of these meanders. The intervening zone, where the Gulf Stream is straight, is covered south of the Gulf Stream with a mix of cloud streets and cellular convection.

As with the 8 April case discussed above, the 24 April cloud bands occurred in a region of cold advection at 850 hPa (850-hPa analysis not shown), located between a cold front and the subsequent polar high. The cloud bands are aligned from the north-northwest to northwest, as are the 850-hPa geostrophic winds. The surface pattern (surface analysis not shown) is also similar to that of the 8 April case, so the cloud bands are aligned with the boundary layer wind in this case as well.

The observed difference in cloud cover across the Gulf Stream North Wall again corresponds to a change from sensible heat flux of 0 W m−2 to the northwest of the Gulf Stream North Wall (buoy 44011) to a relatively large sensible heat flux of 50 W m−2 over 16.7°C water near the North Wall (buoy 44004). Advecting the air from buoy 44004 over the core of the Gulf Stream results in a bulk aerodynamic estimate of the sensible heat flux of 125 W m−2.

c. 10 December 2000

The air–sea temperature difference and static stability associated with cold-air outbreaks are very different in midautumn to early winter than in late winter to early spring. This change occurs because, during the former period, the land (source of cold air) is much cooler than either the Gulf Stream or the inshore waters. During the cold-air outbreak of 10 December 2000, for example, the sensible heat flux northwest of the Gulf Stream (buoy 44011 at 1200 UTC) is approximately 60 W m−2, a sharp contrast to the weak inshore instability seen in the two mesoscale cloud band cases described earlier. Because of this much greater inshore instability, convective cloud cover began near the coast instead of near the Gulf Stream North Wall, as can be seen in the infrared GOES-8 image from 1215 UTC (Fig. 9) and the AVHRR image from 1039 UTC (Fig. 10).

Unlike the band cases discussed above, 10 December exhibits a typical cold-air outbreak cloud pattern with wide-mode rolls beginning a uniform distance off the coast as the thermal internal boundary layer grows to the lifting condensation level (Chang and Braham 1991). Because of the bight between Massachusetts and Nova Scotia, the upwind ends of the cloud streets are farther upwind (northwest) in that region. There is also some indication of mesoscale banding in Figs. 9 and 10. These bands begin well to the northwest of the Gulf Stream and have no spatial correspondence to the Gulf Stream meanders seen in Fig. 1c. The difference in cloud pattern between this and the previous cases cannot be tied to corresponding differences in the synoptic pattern (not shown). Indeed, the setting is similar to those of the Gulf Stream meander cloud band cases. That is, there was a cold-air outbreak occurring in the northwesterly boundary layer flow west of a cold front and east of a polar high. Thus, for this cold-air outbreak with large inshore sensible heat flux (60 W m−2 at buoy 44011 at 1200 UTC on 10 December), it is the coastal features rather than those of the Gulf Stream that control the mesoscale structure of the cloud field. Moreover, this difference in source feature was due to seasonal changes in the surface temperatures of land and sea rather than differences in the atmospheric synoptic setting or oceanographic mesoscale setting.

d. 20 March 2001

Even when the contrast in air–sea temperature differences across the Gulf Stream North Wall is sufficient for meander-generated cloud bands, it is possible to have cold-air outbreak cases in which meso-β-scale cloud bands do not form. The GOES infrared imagery (Fig. 11) for 0015 UTC 20 March illustrate one such case. Only meso-γ-scale cloudlines are present in the cold-air outbreak cloud field. These cloud lines are also illustrated in the AVHRR imagery from 2019 UTC (Fig. 12). These cloud lines do, however, begin at the Gulf Stream North Wall (Fig. 1d) instead of the coast. This positioning reflects the lack of a strong sensible heat flux over inshore waters during this case. Data from buoy 44011 northwest of the Gulf Stream North Wall yields a bulk surface sensible heat flux of 0 W m−2 while the inferred value over the Gulf Stream was 120 W m−2. Thus, the thermodynamic forcing was similar to that in the previously discussed mesoscale band cases. The synoptic setting was likewise similar with cold advection occurring in the northerly and northwesterly boundary layer flow west of a cold front (not shown). Yet, despite these similarities, the 20 March case lacked mesoscale-β cloud bands.

The structure of the Gulf Stream depicted in Figs. 12 and 1d provides an explanation for this difference in cloud structure. Whereas the meander-induced cloud band cases of this study had clearly defined high-amplitude, high-intensity meso-β- to -α-scale meanders in the Gulf Stream, the period around 20 March featured ill-defined, low-intensity meso-γ- to -β-scale eddies that were both convoluted and diffuse. The key observation is that meso-γ cloud lines appear to be initiating in two different ways depending on the nature of the North Wall of the Gulf Stream in this case (Fig. 12). While the majority of the meso-γ cloud lines in this case trigger over areas of uniformly warm SST south of smoothly curving sections of the North Wall, some of these cloud lines appear to trigger from small-scale hot spots where minor eddies disrupt the North Wall. Thus, cloud lines of the same size and shape appear to be arising in the same synoptic setting from both the wide-mode convective roll mechanism and the solenoidal-circulation mesoscale band mechanism. This observation yields insight into a possible synergy between the two buoyant updraft-producing mechanisms discussed earlier. For those meso-γ cloud lines triggering preferentially at SST features of corresponding scale, the phenomenological distinction between wide-mode convective rolls and solenoidal-circulation bands becomes blurred. As defined in the introduction, wide-mode rolls draw their buoyancy primarily from the vertical component of gradient production while solenoidal-circulation bands draw their buoyancy primarily from locally enhanced surface fluxes. For those instances in this case where cloud lines with the meso-γ width expected of wide-mode convective rolls trigger from correspondingly scaled SST hot spots, the flux-enhancement mechanism must dominate initially despite the relatively small roll wavelength, else the rolls would not prefer to initiate over the hot spots. Thus, the existence of several such instances in this case implies that there may not be purely gradient production forcing in regions where there are SST variations on any horizontal scale at which boundary layer convection can occur (roughly the meso γ scale). This mixing of mesoscale and convective dynamics in features with scales a few times the boundary layer depth has been dubbed the terra incognita problem (J. Wyngaard 2003, personal communication) because of the challenges it poses for mesoscale modelers.

e. Climatological considerations

The previous cases suggest that the formation of mesoscale cloud bands downwind of Gulf Stream meanders depends upon the interplay of atmospheric synoptic conditions, oceanic mesoscale conditions, and a particular climatological state of air–sea temperature difference. On the atmospheric synoptic scale and oceanic mesoscale, a cold-air outbreak must pass over high-amplitude, high-intensity meanders in the Gulf Stream. Examination of 2 yr of daily satellite imagery revealed three clear-cut cases (the two discussed earlier plus 8–9 March 2001), all of them in the months of March and April. This period is much more limited than that for the required synoptic conditions, roughly September through April. Earlier in the cool season, similar phenomena occur, but they begin at the coast instead of the Gulf Stream North Wall. This difference in location suggests a seasonal difference in the surface heat flux as reflected in the buoy observations discussed earlier. In these case studies, cold-air outbreak clouds formed just downwind of the point where the air mass encountered its first significant surface heat fluxes. Whether this location is the coast or the Gulf Stream North Wall depends, to a large extent, on whether there is a significant air–sea temperature difference in the inshore region.

Because the annual cycle of surface air temperature is different for the Gulf Stream, the inshore waters, and the upwind landmass, there is a distinct seasonality to the temperature contrast among these three regions, and thus in the possibility of a cold-air outbreak creating unstable conditions over the inshore waters. Figure 13 depicts the mean annual cycles of surface air temperature for three points on the southeastward trajectory of a typical cold-air outbreak: the coastal climatological division of Maine, an offshore point at 44°N, 68°W, and a Gulf Stream point at 38°N, 62°W. While air over the Gulf Stream is warmer than that in coastal Maine during all months of the year, the air over the coastal waters becomes cooler than that over the adjacent landmass during the months of April through August. Thus, the odds of cold-air outbreak clouds initiating at the Gulf Stream instead of the coast increase as spring progresses. Of course, strong cold-air outbreaks are cooler than the climatological mean, but these become less common as spring progresses. Thus, particularly in March and April, a polar air mass must penetrate southeast to the Gulf Stream before encountering water warm enough to drive convective development in the boundary layer. The rarity of such air masses after April results in a narrow season for Gulf Stream meander mesoscale cloud bands. The season is bounded on one side by winter's large land–sea temperature contrast and on the other by summer's lack of significant cold-air outbreaks. This window exists only because there is a lag between the spring warm-up of land and ocean.

4. Conclusions

Exceptionally broad wind-parallel stratocumulus cloud bands are sometimes seen in satellite images of the western North Atlantic during March/April cold-air outbreaks. The lateral scale (the upper-half of meso β scale) of these bands indicates that they are not just the result of wide-mode boundary layer roll vortices. Examination of high-resolution infrared satellite imagery reveals that the bands typically originate at the upwind end of high-amplitude, high-intensity warm meso-β- to -α-scale meanders in the Gulf Stream. Climatological data suggests that the bands form only during those months when the coast provides no thermal contrast but cold-air outbreaks still penetrate to the Gulf Stream. These observations suggest that the mesoscale cloud bands result from solenoidal circulations similar in dynamics and structure to those associated with midlake cloud bands and bay-effect cloud bands.

Based on the 2 yr of observations, it appears that this phenomenon is less common than are the cloud streets caused by wide-mode boundary layer rolls. All three of the cases observed occurred in two calendar months (March and April). Thus, the phenomenon may not be particularly rare “in season.”

The existence of normal-looking wide-mode cloud streets apparently linked to SST hot spots of meso γ scale suggests that wavelength alone is not an adequate discriminator between wide-mode convective rolls and solenoidal-circulation cloud bands. The band versus roll dynamical distinction (i.e., surface flux variation gradient production of buoyancy) is of greater value. While the SST variations may at times control where the rolls form (as in the 10 December 2000 case) these variations always control the very existence of the solenoidal-circulation bands (as in the 8 and 24 April 2002 cases).

Because these mesoscale cloud bands have not been studied as extensively as the more common wide-mode boundary layer rolls, much work remains to be done before a complete understanding of their structure and dynamics is achieved. Researchers with access to a long-term archive of infrared high-resolution satellite imagery could construct a more extensive climatology, verifying or refuting the seasonal dependence hypothesized above. Likewise, mesoscale modelers confident of their abilities to simulate a phenomenon that strongly modulates shallow convection could test our hypothesis concerning the role of surface-forced solenoidal circulations and provide more detail on their structure. Given the slow timescale on which Gulf Stream meanders evolve, operational mesoscale modeling of the atmosphere over the open ocean would permit forecasting of the resulting mesoscale circulations and cloud patterns. Likewise, simulation is probably the only means of determining whether free tropospheric gravity waves interact with Gulf Stream meander cloud bands as they do with the much narrower cloud streets (Hauf 1993). Further investigation with satellite observations and mesoscale model simulations might reveal interesting solenoidally driven mesoscale circulations associated with other high-thermal-contrast ocean current features such as warm-core rings, cold-core rings, etc. Sublette and Young (1996) provide a hint of that with their study of sea-breeze-like solenoidal circulation occurring when the synoptic-scale wind blows parallel to the straighter sections of the Gulf Stream North Wall.

A definitive observational study of the atmospheric response to Gulf Stream meanders will require in situ observations. To fully resolve this phenomenon will require, at the least, cross sections extending horizontally across two or more bands and vertically through the depth of the boundary layer. Long-range multiengine research aircraft offer the best opportunity to obtain such data. A flight leg flown perpendicular to the cloud bands at an altitude well above their top could be used for creation of an atmospheric and oceanic cross section via a “picket fence” of dropwindsondes and air-dropped expendable bathythermographs. The individual drops would have to be spaced at roughly 1/8 of the band wavelength in order to adequately resolve the horizontal gradients. Because of the sampling error inherent in instantaneous point measurements of a turbulent flow, the mesoscale variations in the boundary layer structure would need to be sampled directly by the aircraft. This task would require at least two more flight legs perpendicular to the bands, one near the surface for flux measurement and one flown in a saw-toothed pattern across the top of the boundary layer to document its mesoscale variation. Documentation of the downstream evolution of the band structure and dynamics would require either a second set of cross-band flight legs or a set of legs along the axis of a single band. The latter approach would require onboard access to real-time satellite imagery and exquisite navigation. The appropriate aircraft instrumentation suite would include fast-response measurements of temperature, humidity, and vector wind for turbulence flux calculations, microphysical measurements for cloud-feedback studies, and a downward-looking radiometer capable of accurate SST measurements. All of these sensors would need to work well in an environment with supercooled cloud water and possibly precipitation. Observational verification of the hypotheses presented herein will, therefore, require careful consideration of some challenging flight-planning and instrumentation issues.

Acknowledgments

We greatly appreciate the provision of AVHRR imagery by Ray Sterner of The Johns Hopkins University Applied Physics Laboratory and of GOES-8 imagery by Ken Eis of the Cooperative Institute for Research in the Atmosphere at Colorado State University. The critical suggestions of three anonymous reviewers were of great assistance in furthering our analysis.

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Fig. 1.
Fig. 1.

Sea surface temperature analysis for the Gulf Stream region based on an average of the JHU/APL composite of AVHRR channels 3B, 4, and 5. Based on a composite for the (a) 5.71 days prior to 2319 UTC 9 Apr, (b) 5.64 days prior to 2218 UTC 24 Apr 2002, (c) 5.57 days prior to 2159 UTC 10 Dec 2000, and (d) 5.57 days prior to 2243 UTC 18 Mar 2001

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 2.
Fig. 2.

AVHRR channel-4 infrared image from 0713 UTC 8 Apr 2002

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 3.
Fig. 3.

Locations and station identifiers for the meteorological buoys and Coastal-Marine Automated Network (C-MAN) stations within the study area. Locations of the coastal ME and offshore climatological data (black crosses). The Gulf Stream climatological point at 38°N, 62°W is approx 1° lon east and 1° lat north of the lower right corner of this map

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 4.
Fig. 4.

GOES-8 channel-5 infrared image from 1345 UTC 8 Apr 2002. Gulf Stream meanders are noted (black dots)

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 5.
Fig. 5.

A portion of the National Weather Service Northern Hemisphere 850-hPa analysis for 1200 UTC 8 Apr 2002. Satellite-derived winds are shown as stars and rawinsonde observations as circles

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 6.
Fig. 6.

A portion of the National Weather Service Northern Hemisphere surface analysis for 1200 UTC 8 Apr 2002. The National Weather Service, Federal Aviation Administration, and volunteer ship observations upon which this analysis is basesd are not shown

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 7.
Fig. 7.

As in Fig. 4 but from 1415 UTC 24 Apr 2002

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 8.
Fig. 8.

As in Fig. 2 but from 0918 UTC 24 Apr 2002

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 9.
Fig. 9.

As in Fig. 4 but from 1215 UTC 10 Dec 2000

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 10.
Fig. 10.

As in Fig. 2 but from 1039 UTC 10 Dec 2000

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 11.
Fig. 11.

As in Fig. 4 but from 0015 UTC 20 Mar 2001

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 12.
Fig. 12.

As in Fig. 2 but from 2019 UTC 20 Mar 2001

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

Fig. 13.
Fig. 13.

Graph of the annual cycle of mean surface air temperature for coastal ME, the sea surface temperature off the coast of ME, and the Gulf Stream temperature southeast of ME

Citation: Monthly Weather Review 131, 9; 10.1175/1520-0493(2003)131<2177:MSBCBG>2.0.CO;2

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