Heat Island, Sea Breeze and HP Thunderstorms in Metropolitan Area of São Paulo

The Metropolitan area of São Paulo (MASP) constitutes a heat island with intense thermal circulations interacting with the sea breeze circulation (SB) throughout the year, but in summer, dew point temperatures can reach above 20 ºC and so triggering very deep HP thunderstorms associated to downbursts, hail and lighting. A total of 125 such events were analyzed between 2005 and 2008. The study included surface and upper air measurements, weather radar, satellite and numerical modeling with ARPS. Results indicate the SB front reaches MASP between middle and late afternoon. The advection of cooler and moist air mixed with relatively warmer and drier urban air increases instability and very deeper thunderstorms are formed with heavy rainfall and flash floods, high gusty winds. Three weather stations used to analyze patterns indicate air temperature drop from 29 ºC to 25 ºC and dew point temperature jump from 18 ºC to 21 ºC after the inset of the SB over MASP. Most intense events had wind direction shift from NW to SE at about 1820 UTC in Congonhas International Airport in 74% of cases analyzed. The yearly total rainfall related to SB events estimated with the São Paulo Weather Radar (SPWR) was up 600 mm downstream of the maximum surface temperature in MASP. This rainfall maximum is about four times higher than around MASP. The 11-JAN-2010 SB episode was simulated with the ARPS and its results agree with observations. For instance, sensitivity simulations of ground cover effects with and without urban features suggest temperature differences in excess of 5 o C over MASP. Also, the SB front depth may or not be a condition to trigger HP thunderstorms. In many instances, the SB front features were well characterized with the mobile X-POL weather radar (MXPOL) measurements and so constitutes a useful nowcasting tool at very high spatial-temporal resolution.