The aspects of the ABL during the onset of the rainy season in southeast Brazil

The region of the Paraiba do Sul Valley is located in the border of the tropics (23 S) between the 2 major cities from Brazil (São Paulo and Rio de Janeiro) and compass about 2 million of inhabitants. This region has a particular feature of strong convection associated with rainfall and lightning. It is considered an atmosphere discharge hotspot, comparable with tropical convection in Amazonia. Also, the convective clouds redistribute energy amongst the surface, the mixed layer and top of the Boundary Layer (BL), so it is an important process to couple the surface and BL. The topography is relatively complex due to the proximity of the ocean and the mountains. During November-December 2011, a field campaign (Project CHUVA) was conducted in order to collected meteorological data to study the formation and evolution of the BL in association with convective clouds. Radiosondes were released twice (00 and 12 UTC) per day during the whole experiment and additional sondes were also launched (at 06 and 18 UTC) during the intensive observation period (7 days). Surface measurements (automatic weather station, radiation budget and turbulent heat fluxes) and weather radar products (X Band) were also collected during the whole experiment. Values of CAPE and CIN were computed for the thermodynamic profiles obtained by sondes in order to characterize the time evolution of the convection during the experiment. As a case study of the coupling between the surface and the BL, the occurrence of a typical convective rainfall (Nov 30, 2011) and its modification of the BL were analyzed. At early morning (at 12 UTC), the height of the BL was around 300 m with an average potential temperature of 301.1 K and specific humidity of 11.8 g/kg. The CAPES and CIN values were 222 and 159 J/kg, respectively, showing the moderate/weak unstable atmosphere. At 18 UTC, after a rainfall of 4.5 mm, the BL was completed destroyed and it presents a structure of a classical stable BL, although the middle and upper part of the atmosphere still presents and unstable structure: CAPES is very high (1119 J/kg) associated with a reduced CIN (119 J/kg). Associated with the rain, there was a cooling up to 1000 m ranging from 3-4 °C and a moistening of 2-3 g/kg. These results indicate that the short time between the occurrence of the rain and the release of the radiosonde only affects the BL structure.