THE REALATIONSHIP BETWEEN THE SATELLITE CLOUD EDGE, THE RADAR THIN LINE, AND THE SURFACE INDICATED SEA BREEZE FRONT ALONG THE EAST COAST OF FLORIDA
Jennifer Bewley
Department of Marine and Environmental Sciences
Florida Institute of Technology
Melbourne, FL32901
June 2004
Abstract
The sea breeze circulation system is a mesoscale phenomenon caused by the differential heating between land and sea, and has a very big impact on the meteorological conditions of Florida during the spring and summertime. This study was designed to compare the location of the cloud edge indicated on satellite images to the surface sea breeze front observed at different locations for 25 and 26 May 2004, and to compare the location of the radar thin line to the satellite cloud edge and to the surface sea breeze front observed for 27 May 2004. Differences in the locations of the indicated fronts are expected because of the nature of the vertical profile of a sea breeze front. The second objective is to gain a better understanding of the vertical structure of the East Coast Florida sea breeze by creatingpossible vertical profiles of the sea breeze fronts.
The sea breeze data used in this experiment was collected from 25-27 May 2004 by Florida Institute of Technology, and several other stations located across Central Florida. Once all the data was collected and mapped on the same scale, it was evident that the radar thin line precedes the surface front which precedes the satellite cloud edge. On 25 May 2004, the sea breeze front was moving at an estimated 4.6 ms-1, or 276 m min-1, with the satellite cloud edge located approximately 69 km behind the surface front. The sea breeze front on 26 May 2004 was observed to be moving at an estimated 1.8 m s-1, or 108 m min-1, with the satellite indicated cloud edge approximately 8.1 km behind the surface front. May 27, 2004 proved to be the most interesting day with the indication of the sea breeze front on the radar. With the front moving at an estimated 2.8 m s-1, or 168 m min-1, the cloud edge was approximately 1.7km behind the surface front, while the radar thin line was approximately 8.4 km ahead of the surface front, or 10.1 km ahead of the cloud edge. Overall, this experiment was able to quantify the relationship between the satellite cloud edge, the radar thin line, and the surface observed sea breeze front, and the vertical structures of the sea breeze fronts were analyzed.
Introduction
Wind is described as the horizontal, and sometimes vertical, fluid motion of air resulting from differential pressures in the atmosphere, e.g. Ahrens (2003). The pressure gradient force, a result of differential pressure distribution in the atmosphere, is the force that causes the wind to blow from higher to lower pressure and at right angles to the isobars, e.g. Ahrens (2003).
Thermal circulations are brought on by changes in air temperature, such as when warmer air rises and colder air sinks, e.g. Ahrens (2003). An example of a thermal circulation is the sea breeze. The seabreeze circulation is driven by daytime heating contrasts between land and water surfaces, e.g.Dailey and Fovell (1999). Because of a difference in the specific heats of the land and water, during the day the land heats faster than the adjacent water, and causes the formation of a thermal low. The air over the water however, still remains cooler than the air over the land, and as a result, a thermal high is formed over the water, e.g. Ahrens (2003). This set up creates a pressure gradient force which results in the sea breeze, or a wind that blows from the water towards the shore. The sea breeze is strongest during the afternoon hours when there are the strongest temperature and pressure gradients.
Figure 1- Simple depiction of a sea breeze circulation.
Source: <
The leading edge of the sea breeze is called the sea breeze front, and as it moves inland, a drop in temperature, a wind shift, and an increase in relative humidity occurs behind it, e.g. Ahrens (2003). “Sea breeze frontogenesis is a balance between the convergent horizontal winds, which act to generate a front, and turbulent convective mixing over land, which tends to prevent its formation,” e.g. Atkins and Wakimoto (1997). The movement of a sea breeze front can be detected by radar, satellite imagery, and surface observations.
Although radar was originally developed for the radio detection and ranging of aircraft, after World War 2 radars began to be used for studying the atmosphere, e.g.Simpson (1994). Sensitive Doppler radar can detect a sea breeze front, seen as a region of enhanced radar reflectivity in the optically clear boundary layer, e.g. Atlas (1960); Atkins and Wakimoto (1994), and is referred to as a “thin line”. The cause of clear air echoes along the thin line is still debated in literature. It has been attributed to backscattering from bugs and insects and Bragg scattering from refractive index gradients on a scale of half the radar wavelength, e.g. Atkins and Wakimoto (1997).
Insects that may be transported into the convergence zone do not want to be carried upward toward lower temperatures in the updraft, so they concentrate in the updraft, which creates the radar-detected thin line, e.g. Atkins and Wakimoto (1997). However, there have been recorded incidences where thin lines have been observed with no insects or other solid objects seen. In this sea breeze case, the simple theory of discontinuity between the moist, cool sea air and the dry, warm land air is considered, e.g. Simpson (1994). These diffuse radar echoes inclear air can be associated with the maximum refractive index fluctuations located at the sea breeze front where intense mixing occurs, e.g. Simpson (1994). Figure 2.0 (next page) shows a vertical cross-section of a typical sea breeze front. The area in green indicates the updrafts associated with the front. In this region there is intense mixing of land and marine air, which provides several interfaces capable of reflecting radio energy. Therefore, the radar could be indicating the mixing region present at the frontal boundary, which provides a way to locate the sea breeze front.
Figure 2- Vertical cross-section of a sea breeze front
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Visible satellite images can also be used to identify the inland spreading of the sea breeze front, e.g. Hadi et al. (2002). Near the sea breeze boundary, the convergence of the winds cause the air to rise, condense, and form clouds, e.g. Simpson (1994). A sea breeze front can be described as a line of clouds, cumulus in nature, parallel to the shore, with no more clouds on the seaward side of the line of development. Even though the return current aloft carries the excess of air and clouds towards the sea, the clouds tend to dissipate since the air in the return flow is sinking, e.g. FMI (2004). Most of the time, the sea breeze front is outlined with ragged veils or curtains of clouds, e.g. Simpson (1994). The detection of the boundary separating the cloud-free and cumuliform clouds is possible using either visibleor infrared satellite imagery, according to FMI (2004). If several satellite images are taken, then the evolution and movement of the sea breeze front over time can be seen. Hadi et al. (2002) found that the interpretation of the cloud patterns through satellite imagery made it possible for the horizontal extent and propagation speed of the sea breeze front to be analyzed.
Since the leading edge of a sea breeze front is marked by a decrease in temperature, an increase in relative humidity, and a wind shift, it is also possible to locate a sea breeze front based on surface observations. This tool is not as valuable as radar and satellite imagery because stations are fixed locations and sparsely distributed across the coast. For this experiment, a mobile station was created to make several transects through the frontal region, and to provide a way of following the front as it propagated inland. This mobile unit (seen in figure 3 next page) was equipped with a wind vane and cup anemometer.
Figure 3- The mobile unit, “Team La Grangian,” used to transect the sea breeze front.
The objective of this experiment is to compare the location of the cloud edge indicated on satellite images to the surface sea breeze front observed at different locations for 25 and 26 May 2004, and to compare the location of the radar thin line to the satellite cloud edge and to the surface sea breeze front observed for 27 May 2004. Differences in the locations of the indicated fronts are expected because of the nature of the vertical profile of a sea breeze front. The second objective is to take the differences in the location of the indicated fronts and create a possible vertical profile of the sea breeze front for that day.
Location of Study
There were several locations of study for this experiment, spanning all the way across Brevard County, FL. Two stationary sites were used throughout this period including the LinkBuilding on the campus of Florida Institute of Technology, and the beach station located at the MelbourneBeach. In addition to the two stationary sites, the mobile unit also added several sites across BrevardCounty.
The first location, the LinkBuilding is located on the campus of Florida Institute of Technology, at 150 West University Blvd. in Melbourne, FL, indicated in Figure 4 by the red star.
Figure 4- Station locations for observations
The second stationary location was MelbourneBeach, located on Ocean Avenue and A1A. The station ID FTS indicates the location of the Florida Tech Team Eularian in Figure 4. Here the team took wind and wave measurements.
The mobile unit, Team La Grangian, traveled to several locations west on 192 during the three days of data collection, attempting to transect the sea breeze front. The location name, latitude and longitude, along with corresponding number to Figure 4 are shown in Table 1 below.
Table 1- Mobile Station Locations
# / Location / LAT / LON1 / IndiatlanticBeach / 28.09 / -80.57
2 / Florida Ave / 28.06 / -80.63
3 / Golf Course / 28.08 / -80.63
4 / Sam’s Club / 28.08 / -80.65
5 / Texaco / 28.08 / -80.70
6 / Texaco / 28.08 / -80.71
7 / Texaco / 28.08 / -80.72
8 / Deseret Ranch / 28.10 / -80.90
9 / Holopaw / 28.14 / -81.08
Methods
Sampling for this experiment took place over three days, starting 25 May 2004 and ending 27 May 2004. Weather, wave and wind observations were collected at the beach station; weather, wind, and weather and satellite data were collected at the Florida Tech campus; and weather, wind, and solar observations were collected by the mobile unit as it transected the sea breeze front.
The beach station consisted of the renewable energy mobile unit andTeam Eularian. The renewable energy unit had a wind turbine, solar panel, anemometer, wind vane, and radiometer, all of which were used to measure several meteorological parameters associated with the sea breeze. This unit was equipped with its own data logger, which could store all of the meteorological data that was collected throughout the study. While the renewable energy unit collected data, Team Eularian took weather observations. Every fifteen minutes air temperature, relative humidity, and wind speed were measured using a handheld Kestral. Temperature readings were taken in the shade to prevent erroneous temperature readings. Wind speed measurements were taken facing the wind and away from any obstructions, and averaged over 5 minutes. The wind direction was determined by holding a compass in the direction that the wind was coming from. Other observations such as percent cloud cover, sky observations, water temperature, wave height, wave period, salinity, and dissolved oxygen were taken every half hour.
The mobile unit, known as Team La Grangian, moved with the sea breeze each day in order to complete several transects of the sea breeze front. The first two days the team started at Florida Avenue and then traveled west on 192. The last day of the study, the team started at the beach before moving west on 192. At each location a wind vane, cup anemometer, and radiometer took measurements which were recorded in a laptop by a data logger. The team members also used a Kestral to record duplicate wind speeds, relative humidity, and air temperature. Also taken were sky observations, percent cloud cover and wind direction. Once the wind direction switched to an onshore flow, the team moved to the next location, west of the last position.
At every location, several pictures were taken with digital cameras. These pictures included a 360 degree shot of the surrounding objects, as well as any interesting cloud features and cloud fields. These pictures could then be used to analyze the cloud fields or possible obstructions from the wind.
Other data used in this analysis includes satellite imagery and radar. These data were collected by the team at the Florida Tech station, and include GHCC satellite images. GEMPAK, a meteorological program, was also used to view GOES 4 km visible satellite imagery, the Melbourne base reflectivity radar, and METAR surface observations from nearby stations. The surface observations were viewed using the SFLIST command and were analyzed for wind changes which would indicate the passage of the sea breeze front. The time series of these images were extremely important in analyzing the differences in the sea breeze fronts.
Results
25 May 2004
The GHCC visible satellite imagery for 25 May 2004 is shown in Figures 5-8 below. The cloud field shows an indication of a sea breeze front, especially in south Florida. In BrevardCounty, the cloud field, and hence sea breeze front, is hardly visible from this view. The radar from this day shows little indication of the sea breeze front.
Figure 5- GHCC satellite image of cloud field at 15:31 UTC
F
igure 6-GHCC satellite image of cloud field at 16:31 UTC
Figure 7-GHCC satellite image of cloud field at 17:31 UTC
Figure 8- GHCC satellite image 18:15 UTC
Figure 9, a separate page, is a map analysis of the progression of the cloud edge when compared to the observed sea breeze passage at the surface. The time marked in purple is the time (in UTC) at which the sea breeze front passed that station according to a shift in wind direction,and the blue lines represent the cloud edge indicated by satellite.
From Figure 9, an estimation of the average velocity of the sea breeze front can be determined by the distance traveled in a certain amount of time. On this day the front was moving approximately 4.6 ms-1. (See Appendix for calculations)
26 May 2004
The GHCC visible satellite imagery for 26 May 2004 is shown in Figures 10-14 below. The cloud field shows an indication of a sea breeze front, especially in northern Florida. In BrevardCounty, the cloud field, and hence sea breeze front, is hardly visible, and then develops as the cloud edge becomes more noticeable to the south. The radar from this day shows little indication of the sea breeze front.
Figure 10- GHCC satellite image 17:31 UTC
Figure 11- GHCC satellite image 17:45 UTC
Figure 12- GHCC satellite image 18:15 UTC
Figure 13- GHCC satellite image 18:45 UTC
Figure 14- GHCC satellite image 19:02 UTC
Figure 15, a separate page, is a map analysis of the progression of the cloud edge when compared to the observed sea breeze passage at the surface. The time marked in purple is the time (in UTC) at which the sea breeze front passed that station according to a shift in wind direction, and the blue lines represent the cloud edge indicated by satellite.
From Figure 15, an estimation of the average velocity of the sea breeze front can be determined by the distance traveled in a certain amount of time. On this day the front was moving approximately 1.8 ms-1. (See Appendix for calculations)
27 May 2004
The GHCC visible satellite imagery for 27 May 2004 is shown in Figures 16-18 below. The cloud field shows an indication of a sea breeze front, but it is not really noticeable until Figure 18. In BrevardCounty, the cloud field, and hence sea breeze front, is hardly visible, and then develops nicely by Figure 18. The radar from this day shows a very good indication of a sea breeze front. Figures 19-23 show the Melbourne radar reflectivity along with some surface observations. The thin line, an indication of the sea breeze front, can be seen clearly.
Figure 16- GHCC satellite image 16:15 UTC
Figure 17- GHCC satellite image 16:45 UTC
Figure 18- GHCC satellite image 17:45 UTC
Figure 19- Melbourne radar and surface observations 17:55 UTC