An Analysis of the South Florida Sea Breeze Circulation: An Idealized Study John Cangialosi University of Miami/RSMAS Abstract This experiment is an idealized study (removal of mean large scale flow) to gain further knowledge of the South Florida sea-breeze circulation. The non-hydrostatic PSU/MM5 model is employed, with two simulations only differing in their soil diffusivity of heat value. This analysis compares the two model runs and depicts the changes related to the diurnal cycle of temperature and the sea breeze circulation. Introduction Many studies have explored the dynamics of the sea breeze circulation, using different meteorological models and techniques to understand this phenomenon better. Previous sea breeze studies have used several different types of instruments to achieve results and increased knowledge of this local circulation. The use of aircraft and gliders in studying sea breezes offers the possibility of more comprehensive measurements (Simpson et al 1994). LIDAR studies (Nakane & Sasano 1986; Banta et al 1993) and Doppler radar studies (Laird et al 1995) can reveal the vertical structure of the sea breeze front. Aircraft surveys may, in addition, simultaneously measure local fluid and thermodynamical variables as well as chemical tracers. Aircraft surveys are, however, constrained by a finite transit speed and, in ubran areas, by air traffic control. Tongue (1997) showed how the WSR-88D is becoming an important diagnostic tool that augmented by modeling, can enhance understanding and improve seabreeze forecasts.
The sea breeze circulation is a quasi-regular circulation that develops due to differential heating of the air over land and over sea. As the sun heats the boundary layer over land, the resulting pressure gradient causes the movement of low-level air from sea to land (sea-breeze) with a return flow aloft (return current) to close the circulation. Figure 1 displays a model of the sea breeze circulation. Figure 1: Schematic of a sea-breeze circulation The existence and intensity of the sea breeze depends strongly on seasonal and latitudinal factors as well as on the time of day. In South Florida the sea breeze is a regular phenomena throughout the year, however it has an enhancement in the summer months due to instability over land and the existence of a temperature gradient between land and sea. The sea breeze convergence over the Florida peninsula produces the highest number of days with thunderstorms in the United States. Previous works explain the daily life cycle of the sea breeze. Pielke (1981) uses a simplified model that depicts the sea breeze daily life cycle. A sea breeze normally starts as a gentle breeze in the morning a few hours after sunrise, when the solar radiation starts heating the boundary layer over land. The return flow carries the excess of air to the sea. Cloud development frequently occurs in the ascending part of the circulation, while clouds tend to dissipate 2
over sea, where the air is sinking. In the afternoon, when the boundary layer heating over land is at its maximum, the sea breeze is most intensive. If the large-scale flow is weak, the direction of the sea-breeze often veers with time. In South Florida, due to its shape and coastline propagation and the fact of being a peninsula, the merging sea breeze originating from both sides of the peninsula enhances convection. The divergent breeze off of Lake Okeechobee will also enhance convection. The onset of a sea breeze is marked by changes in wind speed, temperature, and humidity at a specific location as the sea breeze front passes through. As the leading edge of the circulation passes through, there is an abrupt shift in the wind direction towards the onshore direction. There is an increase in wind speed and humidity, and a decrease in temperature as the air parcels moistened above the sea surface blown onto the land (Atkinson 1981). While the local wind speeds may increase abruptly, temperature and dew point changes are comparatively gradual with the passage of the front. One of the many major advantages using a numerical weather model is the capability of isolated a meteorological event. In this experiment we attempt to explain the sea breeze circulation with removal of the mean flow. The objective of this study, by creating an idealized experiment over the South Florida domain, permits us to vividly analyze the local sea breeze circulation and its depth and intensity without effects from the mean flow. 3
Methodology The South Florida sea breeze circulation is an important meteorological phenomenon in this region. Sea breezes play a significant role in precipitation and temperature forecasting in South Florida. In this study we employ the PSU/NCAR MM5, which is a non-hydrostatic, primitive equation model in sigma coordinates and a staggered B-grid. The mean flow is ideally removed from the model. The model is initialized at June 1, 1999 at 0Z and its 60 hour simulation extends to June 3, 1999 at 12Z. Two simulations are conducted, the first has a DIFSL value of 5.0 E - 7, and this is a typical value used as a constant in numerical weather models. The second has a DIFSL value of 1.5 E-7. DIFSL is the soil diffusion constant. All of the other parameters and variables are identical in both model runs. The center latitude and longitude position is over interior South Florida at (28.0 N, 83.0 W). The approximate expansion is 225 km, with a grid of 85 x 94 and 28 vertical levels. Initial and boundary conditions are from the NCEP ETA data for the full integration. There are three domains created in the model. The outer domain has a 36 km resolution and a one way nesting mode. The second domain has a 12 km resolution with a 2 way nesting node, and the third domain has a 4 km resolution with a one way nesting mode and a one second time step. In this model parameterizations are made for the planetary boundary layer and radiation schemes. The MRF PBL scheme is used for the planetary boundary layer and the simple ice scheme is used for the microphysics parameterization 4
concerning radiation. Since the resolution is fairly high (< 10km), there is no need for a cumulus parameterization scheme. The DIFSL value, only variable adjusted between the simulations, determines how much heat will be diffused into (and out of) the substrate from the surface. The smaller the DIFSL value, the less heat will be penetrating into the substrate during the heating of the day, and reverse during the cooling at night. Therefore, the diurnal cycle of the land surface temperature is expected to be larger using smaller value of DIFSL. Because there is not a significant amount of observations of soil diffusion value, the DIFSL value in numerical models are not necessarily correct. In this study we want to analyze how the smaller DIFSL will change the diurnal cycle of the ground temperature and whether this change will effect the sea breeze circulation in South Florida. Below is a figure displaying graphically the MM5 domain used in this analysis. Figure 2: The three domains used in the MM5 simulation. 5
Results We will first examine the wind flow in South Florida throughout the entire simulation for both model runs. The model runs are initialized at 0000 UTC (8 pm EDT), once the model is able to spin up it is 0300 UTC and there is no sea breeze circulation observed at this time at night. The first model verification is to study the model output and to identify if the model simulations are depicting the correct features. In these simulations, the mean flow was successfully removed, and the sea breeze circulation dominates. The sea breeze circulation clearly develops by the midday hours in both simulations. The sea breeze circulation relaxes and turns into a land breeze flow after sunset as the earth cools. This cycle repast throughout the 60 hour simulations. The next feature studied is to determine the difference between the two model simulations and how they vary. A time series plot of the ground temperature was conducted to show the diurnal cycle. This is a time series plot for Fort Lauderdale and Fort Myers, from Jun 1, 1999 at 0000 Z until Jun 3, 1999 at 12 Z. In the time series figure, both model runs are plotted. The red curve indicates the 5.0 E -7 soil diffusivity value and the blue curve represents the 1.5 E -7 soil diffusivity value. The x-axis is the time in the simulation, extending from 00 to 60 hours and the y-axis is the ground temperature in degrees C. From the Fort Myers time series it can be observed that the low soil diffusivity run increased the maximum temperature slightly, on 6
the order of 1/4 of a degree C, and decreased the minimum temperature more substantially, 2 degrees C. The Fort Lauderdale time series of ground temperature produced very similar results. It can be concluded that the low soil diffusivity run increases the diurnal cycle of ground temperature. Figure 3: On the left is a time series of ground temperature through the entire simulation for Fort Myers, and on the right for Fort Lauderdale. The different simulations are distinguished in the legend. Since there is a clear indication that the DIFSL variable has a direct effect on the diurnal cycle of temperature, the main research question is will the change in the diurnal cycle of ground temperature affect the sea breeze circulation? The diagrams below display the sea breeze circulations over South Florida at 2100 UTC June 1 (21 st hour in the simulations) for both model simulations. The color shading is sea level pressure with higher pressure indicated in orange and lower pressure in red. In both model runs lower pressure is located in much of interior South Florida, with higher pressure offshore. Since higher pressure is located over the ocean with lower pressure over land, the sea breeze circulation can prevail. A distinct sea-breeze can be noted on the Gulf Coast of Southwest Florida in both simulations. A sea-breeze can also be identified on the east 7
coast of South Florida; however, it does not appear to be as intense. Both model runs appear to be very similar, the only slight difference is some changes in sea level pressure. In the first model, DIFSL = 5.0 E -7, the areas of lower pressure are in two areas of convergence, one due to the east coast sea breeze front, and the other due to the west coast. In the second model, DIFSL= 1.5 E-7, the two areas of convergence observed in the first model, mend into one large region of lower pressure. This model run may suggest enhanced lifting and sea-breeze convection from the sea breeze on each coast. This profile typically occurs when the sea breeze from both coasts converge. The sea breeze wind flow pattern itself appears the same in both model runs, just a bit more intense in the low soil diffusivity simulation. Figure 4: Sea breeze circulations at 2100 UTC June 1, 1999. On the left is the simulation with the 5.0 E-7 DIFSL value, and on the right is the 1.5 E-7 DIFSL run. The sea breeze pattern would not be a circulation unless the land breeze flow existed. Toward the late afternoon, sea breeze circulations slowly diminish, and then die away altogether one or two hours after sunset. The land cools, and the process reverses itself with the formation of a land breeze circulation. While 8
the land-breeze is significantly weaker, it is a vital part of the circulation. The land breeze flow in the two models is also analyzed. The diagrams below indicate the sea level pressure and wind flow at 1100 Z June 2, 1999. In this case, there appears to be slight sea level pressure differences in some locations. A great deal of higher surface pressure is denoted over the southern part of the state in the low DIFSL run. This causes a bit stronger land breeze in this run. One must consider that these surface pressure differences are very subtle, on the order of 0.5 hpa. In both simulations, higher pressure is located over land with lower pressure over the ocean. This is due to the cooler temperature over land and warmer temperature over the ocean. When this pattern exists the wind flow is offshore; this is the land breeze flow. Figure 5: Land breeze circulations at 1100 UTC Jun 2, 1999. On the left is the simulation with the 5.0 E-7 DIFSL value, and on the right is the 1.5 E-7 DIFSL run. Lake Okeechobee South Florida has its own uniqueness in its land-use. Lake Okeechobee has a local wind that effects the sea breeze circulation in this region. Two sets of figures are indicated 9
below displaying the lake breeze. The color shading is the ground temperature/ SST in degrees Celsius indicated on the color bar. During the day as the land heats, the lake s temperature remains constant. Since higher temperatures are located over land, lower pressure prevails, while cooler temperatures and higher pressures are located over the lake. The wind flows out of the lake towards the land. This divergence can be noted in the set of afternoon figures in both of the model simulations. At night as the earth cools, higher pressure forms over land, while warmer temperatures and lower pressure is located over the lake. The breeze flows across or through the lake. The boundary layer over land at night is quite stable, while over the lake the boundary layer is less stable producing this wind flow. This is noted in both the model simulations in the late night plots. In the afternoon figures, the model runs produce identical wind flow results over the lake. The only clear difference observed is some slight difference in ground temperature. The simulation with the lower soil diffusivity value indicates higher land temperatures than the model run with the higher soil diffusivity rate. This is expected from the time series plot displayed earlier and the analysis of the soil diffusivity effects. The divergent lake breeze in this model run is a bit stronger which may influence convection. Similar results can be observed with the late-night/early morning lake breeze. The low DIFSL run has considerable cooler land temperatures than the higher DIFSL model 10
run. This causes more of a temperature contrast, enhancing the flow through lake. Figure 6: The top figures display the afternoon lake breeze (1900 Z), and the bottom figure display the late night or morning lake breeze (1000 Z). The figures on the left are from the run with the soil diffusivity equal = 5.0 E -7, and on the right is the model run with the DIFSL value of 1.5 E-7. Depth of the Circulation Another factor involving the sea breeze circulation is its vertical extent. Low-level inversions play a very important role in the development of sea breezes. An inversion tends to limit the vertical extent of the heating to a shallow layer, which typically reduces the strength of the sea breeze. In the model simulations, a vertical cross section was created for the west coast of South Florida. The west coast appeared to have a stronger sea breeze flow than the east coast, so it was appropriate to further study this region. The cross sections displayed below slices through the atmosphere from 30 km offshore 11
of Cape Coral, FL to 40 km inland of Fort Myers. The entire distance is 90 km; the z-axis extends from the surface up to 2 km. The color shading is the wind speed in m/s. These cross sections were taken at 2300 Z June 1, 1999. From the plots, it can be observed that the sea breeze (westerly for the west coast of FL) propagates very far inland through the entire distance. Previous hours showed the sea breeze penetrating further and further inland until this time, and latter hours reduce the seabreeze flow and begin to turn the wind in the opposite sense. The vertical extent of the sea breeze during this time period at this location was about 250-300m. Generally from the simulations the depth of the sea breeze varied from 20m to 1km. The return current displayed flows at 1-1.5 km. The return current varied as well between 500m to 2 km throughout the simulations. Both the depth of the sea breeze and height of the return current are of typical observations, supporting that this idealized model is useful in studying the South Florida sea breeze circulation. The difference between the two model runs is very slight. However, there is one appreciable difference. The wind speed at the top of the sea breeze flow is 1 m/s stronger in the low DIFSL than in the higher DIFSL simulation. This difference only exists over land; the flow over water appears identical. One hypothesis that may explain this result is because of the more heating observed over land with the lower DIFSL value. This heating causes greater pressure gradient, enhancing the sea breeze intensity. 12
Figure 7: Vertical cross section near Ft. Myers at 2300 Z June 1, 1999. On the left is model run with the 5.0 E-7 soil diffusivity value, and on the right is the simulation with the 1.5 E-7 DIFSL value. Summary The fundamental question of this study is how can weather forecasters learn more about the sea breeze circulation in this idealized model study? The idealized model simulations successful removed the mean wind and all of the large-scale forcing. One problem arose in the boundary due to this idealization; however the problem at the boundary did not affect any of the results discussed. The model simulations, a few hours after initialization, displayed a clear sea breeze circulation. Seabreezes, land breezes and the Lake Okeechobee breezes were analyzed and observed ideally. All of these circulations showed a more classic text book flow, due to the idealization. The difference in the simulations was not very significant. However, some differences were observed. The low soil diffusivity value run (DIFSL) increased the maximum temperatures observed and decreased the minimum temperatures observed over land, while keeping the ocean s temperature consistent with the higher DIFSL run. This obviously increased the diurnal cycle of 13
land temperature. This increase in the diurnal cycle of temperature has had some effects on the sea breeze circulation. Higher pressures were observed during the nighttime hours in the low soil diffusivity values (compared with the higher DIFSL), and lower pressures were observed with the lower DIFSL value during the afternoon hours. The stronger diurnal cycle creates a slightly stronger sea breeze, land breeze and lake breezes. This makes physical sense, because there is a stronger temperature contrast, creating a stronger pressure gradient, which would enhance the flow. While the lower soil diffusivity value does not make a big impact on the sea-breeze circulation and local breezes, it does prove that the sea-breeze circulation in South Florida has a very strong dependence on the degree of the diurnal cycle of temperature. The concluding figure below displays a time series of wind speed in both the simulations proving the more intense flows caused by the lower soil diffusivity values. Figure 8: Time series of wind speed for each model run throughout the entire simulations. 14
In conclusion, if the DIFSL constant in numerical weather models is not correct, then the sea-breeze circulation will in fact be affected and slightly more intense than previously thought. 15
References Atkinson, B. W.(1981): Meso-Scale Atmospheric Circulations, Academic Press, London. Pielke, R.A. (1974): A comparison of three dimensional and two-dimensional numerical prediction of sea breezes. J. Atmos. Sci., 31, 1577-85 Pielke, R.A. (1981): An overview of our current understanding of the physical interactions between the sea- and land-breeze and the coastal waters. Amsterdam, Ocean Manage., Vol. 6, p. 87-100. Simpson, J.E. (1994): Sea Breeze and Local Wind. Cambridge University Press, Cambridge, UK. Tongue, J., G. J. Lehenbauer, M. A. Miller and P. Michael, (1996): Operational Observations of Atypical Meteorological Features Using the WSR-88D, Transactions of 15 th Conf. on Weather Analysis and Forecasting, Amer. Meteor. Soc. Meeting; pp. 336-339 16