Gulfstream Cloud Formation Due to Sea Breeze Circulation

Gulfstream Cloud Formation Due to Sea Breeze Circulation

GULFSTREAM CLOUD FORMATION DUE TO SEA BREEZE CIRCULATION

Andrew Condon

Department of Marine and Environmental Systems

Florida Institute of Technology

Melbourne, FL32901

Abstract:

Satellite imagery, surface analysis, and cross sectional analysis are used to determine the presence of an organized cloud line along the sea surface temperature gradient that defines the Gulf Stream. This line features similar characteristics to the observed and well documented sea breeze front. The Gulf Stream cloud line (GSCL) results from a thermal circulation cell identical in nature to the sea breeze circulation cell that forms due to the temperature contrast of the ocean waters. Over a three day period in May 2004, the waters offshore of Cape Canaveral,Florida were monitored for the presence of the GSCL.

On the first day, May 25, the front was present. The cloud feature is easily seen on visible satellite images. The second and third day did not show any signs of an organized GSCL. The existence of the front is backed up with cross sectional analysis of winds, omega, and relative humidity. The cross sections for day one show the circulation cell associated with the Gulf Stream as well as the traditional sea breeze cell.

The large scale flow on day one was easterly. Over the course of the three day period an upper level ridge slid down to the south of the area. With this motion the upper level wind direction switched to the west and the cloud line was no longer seen. Although a longer observation period is needed, easterly flow and an abundance of low and mid level moisture appear to be necessary ingredients for the formation of the GSCL.

A comparison of the synoptic situation on May 25 and June 25 was made. Both days featured the development of a cloud line along the Gulf Stream sea surface temperature gradient. The key variable that both days had in common was very low convective inhibition. With little convective inhibition, slight convergence of surface winds was enough to generate clouds.
Introduction:

The sea breeze is a mesoscale phenomenon that occurs over coastal regions during the warm season. As the sun heats the land, a thermal low develops over the hot land surface, while a thermal high develops over the cooler ocean surface. A sea breeze develops which blows from the high over the ocean to the low over the land. The cool air that is advected over the land surface forms the sea breeze front. The thermal circulation is completed by rising air over the land diverging out back over the ocean aloft (Ahrens, 2003). This general circulation pattern is displayed in figure 1. A similar thermal circulation can develop over the ocean itself. Off the coast of east central Florida the Gulf Stream is separated from the peninsula by a pool of colder water. In the same manner as the sea breeze circulation, a thermal low develops over the warmer Gulf Stream waters and a thermal high develops over the colder waters of the cold pool. A thermal circulation develops and a front can be formed along the Gulf Stream (figure 2).

Figure 1 : Typical Sea Breeze Circulation Cell

Figure 2 : Possible Sea Breeze and Gulf Stream Circulation Cells

Cloud formation over the Gulf Stream has been found to vary diurnally during the summer and winter (Alliss and Raman, 1995). The time of cloud formation varies with the height of the cloud and the time of year. Convergence has been found to play a role in the formation of many low clouds, while advection of high clouds from over the land surface contributes to the presence of high clouds over the Gulf Stream. During the winter due to a large gradient in sea surface temperatures (SST), large gradients in cloudiness have been observed (Alliss and Raman, 1995). During the summer when the SST gradient is less pronounced, the cloudiness gradient is also lessened. However, during the summer mesoscale convergence has been found to be an important factor in cloud formation. The effects of the air-sea interaction on surface and low-level wind speeds and low-level temperature and moisture distribution over the Gulf Stream have been documented (Sweet et. al. 1981). The air over the colder regions was found to be much more stable, while increased sea state and strong winds were found over the warmer waters of the Gulf Stream. This could be due to a thermal circulation cell as discussed above.

Lericos et. al. (2002) conducted an extensive study into lightning data over the Florida region. Their study reveals that although lightning activity over the peninsula peaks in the afternoon, activity over the ocean remains fairly constant during the day. In fact nocturnal lightning is very frequent over the waters of the Gulf Stream due to the effects of the land breeze. It may be possible that a thermal circulation cell over the ocean contributes to Gulf Stream convection. Since sea surface temperatures do not vary as much as land surface temperatures, the circulation cell can be much longer lived. Their study also looked at the differences in lightning distribution with regards to the large scale synoptic flow. They found that during southwesterly flow the lightning flash densities are enhanced, possibly as a result of advection of clouds off the peninsula or from enhanced front development associated with the Gulf Stream.

The large scale synoptic flow has been determined to have a large effect on the sea breeze front. In 1962 Estoque found that an offshore wind intensified the sea breeze front, while an onshore wind weakened the circulation. The offshore winds help to concentrate the horizontal temperature gradient, intensifying the front. Since that study others have found that an onshore wind of just a few meters per second can thwart sea breeze development, while an offshore wind of up to 11 m/s has little effect on the circulation (Arritt, 1993). The effects of the synoptic flow on the Gulf Stream circulation cell have not been reported in the literature.

The goal of this project is to determine what is necessary for a front and associated cloud line to develop along the Gulf Stream. The existence and whereabouts of the GSCL is to be determined for a three day period in May 2004. The large scale synoptic flow as well as the mesoscale conditions will be examined. Conclusions will be drawn as to what conditions lead to the development of the GSCL and which conditions stunt its development.

Methods:

Data from May 25, 26, and 27, 2004 were examined in depth along with a brief review of the GSCL on June 25. The first objective was to identify whether a front developed over the Gulf Stream waters. To accomplish this, visible satellite data from the Global Hydrology and Climate Center (GHCC) was used. Using the GOES East satellite, visible images up to 1 km resolution were obtained over the state of Florida and the surrounding waters. On these images the sea breeze front shows up very distinctly as a line of clouds parallel to the shore. The Gulf Stream Front shows up as a line of clouds along the Gulf Stream. To determine the location of the Gulf Stream MODIS satellite information was used. MODIS stands for The Moderate Resolution Imaging Spectroradiometer. It is an optical scanner aboard the NASA satellite Aqua which has a spatial resolution between 250 meters and one kilometer. The images give a clear view of the location of the Gulf Stream.

After the satellite data was examined, a preliminary determination of the existence of the GSCL was made. To further support the conclusions regarding the front, data from the MelbourneFL office of the National Weather Service (NWS) was used. The Advanced Regional Prediction System (ARPS) Data Analysis System or ADAS, was used for hourly analysis of temperature, pressure, winds, cloud parameters, and soil moisture flux divergence to help identify the front. These plots are the result of observations from a variety of sources including metar reports, the KSC network, the Florida Automated Weather Network, and aircraft reports among many others. The data has a 4 km resolution and is valuable in determining the frontal location, although the observations over the Gulf Stream are sparse (NWS, 2004). Radar reflectivity and wind data was also used. This data was from the NWS Melbourne office NEXRAD WSR-88D radar. These data were compared for days when formation of a GSCL was observed versus days were there was no visible front formation. Another tool used to determine the existence of the GSCL, where cross section analysis using the Rapid Update Cycle model. The parameters analyzed included pressure vertical velocity (omega), wind vectors, and relative humidity values in a cross section from Orlando to 120 nautical miles (nm) offshore.

To determine what conditions favored the formation of a GSCL and what conditions deterred its formation, surface, 850 mb, and 700 mb data was studied. The RUC model was used for an analysis of the overall synoptic weather pattern over the southeastern United States. In addition the RAOB soundings launched from Cape Canaveral were used to aid in determination of available moisture and convective potential. Some of this data was obtained from the University of Wyoming meteorology website.

Results and Discussion:

Day 1: May 25, 2004Gulf Stream Front Identification

The sea surface temperatures off the FloridaCoast are displayed in figure 3. From this figure the Gulf Stream current is fairly obvious. It is the area of darker red, signifying the warmer surface waters of the Gulf Stream. The Gulf Stream flows parallel to the Florida coastline close to shore until the Jupiter/Palm Beach area. Here it tends to move offshore a little further as it progresses northward. Off of Cape Canaveral the Gulf Stream is located well offshore with a pool of colder water between the Cape and the Gulf Stream.

The surface winds over the peninsula start out blowing out to sea, as is expected with a land breeze. Over the Atlantic there is a small area of convergence in the vicinity of the Gulf Stream just east of the Cape as seen in figure 4. Winds in this area are fairly light and converging. An hour later at 1330 UTC there is still a predominately offshore flow and convergence in the Gulf Stream region. Figure 5 is a satellite image of the region. A cloud feature in the vicinity of the Gulf Stream is easily seen at this time. By 1515 UTC the cloud feature is much more distinct as a narrow line of clouds stretching from the Jupiter area along the Gulf Stream to offshore of Daytona Beach(figure 6). The ADAS analysis no longer displays as high of values for surface convergence and the confluence in the wind field is not as obvious but the cloud feature persists.

Figure 3: Sea Surface Temperature map, showing location of the Gulf Stream in Dark Red

Figure 4: Surface Moisture Divergence and Surface Winds at 1230 UTC May 25, 2004

Figure 5: Visible Satellite Image at 1331 UTC May 25, 2004

Figure 6: Visible Satellite Image at 1515 UTC on May 25, 2004

As the day progresses the line of clouds along the Gulf Stream is still present although it diminishes in intensity. Figure 7 shows the visible satellite image from 1631 UTC. The line of clouds along the Gulf Stream is still visible and in the same location as the line at 1515. However this line has diminished in size and continues to do so throughout the rest of the day. By 1815 a faint line of clouds remains (figure 8). The southern edge of this line has moved to the west and is almost onshore while the northern edge has remained nearly stationary, along the Gulf Stream, throughout the day.

Figure 7: Visible Satellite Image at 1631 May 25, 2004

Figure 8: Visible Satellite Image at 1815 May 25, 2004

The ADAS analysis shows slightdivergence right along the immediate coastal area. The wind field also shows some diffluence along the western edge of the Gulf Stream, but the majority of the Gulf Stream is dominated by light northerly winds. As time progresses the size of the sea breeze cell increases as a larger fetch of wind off the ocean comes ashore. This increase in the sea breeze circulation pattern may explain the westward movement of the southern edge of the cloud line. By 1730 the cell reaches far offshore into the vicinity of the Gulf Stream (figure 9).

Figure 9: Surface Moisture Flux Divergence and Surface Winds at 1730 May 25, 2004

An analysis of a cross section from Orlando to the NDBC Buoy number 41010 located 120 nautical miles (nm)offshore shows the formation of the cloud feature very well. The total distance of this cross section is estimated to be around 150 nm. Each tick mark in the cross section therefore represents around 11.5 nm. The coastline is around 3 marks east of MCO and the Gulf Stream region is between 5 and 10 marks east of MCO. By 1000 UTC a large area of negative omega values are present in the offshore waters, about halfway between Orlando and the buoy (figure 10). As time progresses this area of rising motion tends to move very slightly to the east. The omega values also tend to increase throughout the day. By 2100 UTC there is a strong area of rising motion offshore (figure 11). This diminishes as the rest of the day progresses.

Figure 10: Cross section from Orlando to 120 Miles Offshore, showing Omega and Wind Vectors. 1000 UTC May 25, 2004

Figure 11: Cross section from Orlando to 120 Miles Offshore, showing Omega and Wind Vectors. 2100 UTC May 25, 2004

To further verify the presence of a front and clouds, a cross section of the relative humidity values was examined for the same area. The highest relative humidity values, indicating the air is close to saturation and clouds are likely to develop, are in red in figures 12 and 13. The first sign of high relative humidity values appears in the 1500 UTCcross section shown in figure 12. At this time a line of clouds was already visible on the satellite imagery. By 2100 UTC (figure 13), an area of high relative humidity values in the Gulf Stream vicinity is readily seen. This area corresponds well with the area of rising motion in figure 11.

Figure 12: Relative Humidity Cross Section from Orlando to 120 nm Buoy at 1500 UTC May 25, 2004

Figure 13: Relative Humidity Cross Section from Orlando to 120 nm Buoy at 2100 UTC May 25, 2004

There is conclusive evidence that there was formation of a front similar in characteristics to the sea breeze front along the Gulf Stream. The ADAS temperature analysis plots for this day support the scenario illustrated in figure 2. A temperature plot is shown in figure 14. The cold pocket of surface temperatures is clearly illustrated between the warmer land of the peninsula and the warm waters of the Gulf Stream.

Figure 14 : Surface Temperatures over Florida and Surrounding Coastal Waters. 1630 UTC May 25, 2004

Day 2: May 26, 2004Gulf Stream Front Identification

The second day of observations was a completely different day from the first. The visible satellite images from day two do not show any cloud feature over the Gulf Stream at any time during the day (figure 15). The winds offshore do not show any signs of convergence throughout the day. The overall wind speed is slightly greater and the wind direction remains westerly over most of the open water with the exception of a narrow stretch of near shore water (figure 16) due to a narrow sea breeze circulation cell. Unlike the previous day there is no area of reduced wind speed or area where the wind direction changes over the Atlantic.

Figure 15: Visible Satellite Image at 1815 UTC on May 26, 2004

Figure 16: Surface Moisture Flux Divergence and Winds at 1230 UTC May 26, 2004

The RUC cross sections for May 26 further support the absence of clouds in the offshore Gulf Stream region. For cross sections from 1000 UTC on the 26th to 0400 UTC on the 27th there is no consistent large scale region of negative omega values in the coastal waters. The previous day featured a large well defined region of rising motion, both over the peninsula and over the Gulf Stream region. On the second day there is a large area of positive omega, indicating sinking motion, over the Gulf Stream vicinity for much of the day as shown in figure 17.