Ryan T. Ellis and Shuyi S. Chen

Division of Meteorology and Physical Oceanography University of Miami/RSMAS (r.ellis1@umiami.edu) 11021 SW 88th Street #L105 Miami, FL 33176 (305) 968 - 5644 (305) 270 - 0828

Sea breeze circulation plays an important role in precipitation forecasting in South Florida. Current numerical weather prediction models are often deficient in resolving sea breeze circulations due to relatively low grid resolution and inadequate model physics. The objective of this study is to understand the physical processes associated with the sea breeze circulation that affects rainfall. We use a high-resolution nested-grid model to resolve the sea breeze, local wind patterns, convective clouds and precipitation. This will allow us to better understand the interaction between the local winds and convection and, finally, to improve the quantitative precipitation forecasts in South Florida. In this study, the impact of model grid resolution on precipitation forecasts in South Florida is evaluated. Using the nonhydrostatic, high resolution PSU/NCAR MM5 model, numerical simulations have been run at 2-km resolution for the month of June 1999 with a domain spanning the east and west coasts of South Florida including the Lake Okeechobee region. The model results are composed with observations from the NEXRAD radar derived rainfall, station observations of surface and upper level wind, temperature, and pressure fields. The NEXRAD ground based radar imagery and the MM5 both depict that precipitation associated with sea breeze occurred on nearly 50% of the days for the month. At 2-km resolution, model simulations indicate a clear sea breeze signal that propagates inland. The model simulation shows a lake breeze 15 to 20 km inland at a magnitude of close to 3-ms^-1 near the Lake Okeechobee. Vertical wind profiles show that the vertical extent of sea breeze circulation is typically from the surface to the 1500-m level. Modeled rainfall is compared with the NEXRAD ground based radar-derived rainfall totals and surface observations at various points along both coasts as well as the lake region. Preliminary results show that the higher model resolution improves model forecasts with respect to rainfall patterns associated with sea breeze and local winds. Quantitative precipitation forecasts are also very sensitive to model resolution, as the 12-km resolution run of the MM5 was unable to resolve sea breeze circulation patterns and convection. The diurnal composite of rainfall for the month of June 1999 showed that highest rain rates, on average of 5.4-mm hr^-1, occurred at about 2 p.m. The maximum diurnal wind speeds precede maximum precipitation by about one hour, showing a strong correlation between local winds and rainfall.