Observational Studies of the Planetary Boundary Layer at Subtropical Regions of Brazil

Abstract

A 12m micrometeorological tower has been used to measure fluctuation of vertical velocity, temperature and water vapor density at three levels, 3, 5, and 9.40m, and horizontal z and u ðonrponerl,s at 11.5m with a sample rate i-10 IIz. Simultaneously, measurements of net radiation 2m above the surface and heat flux in the soil are also carried out. The area, aroun<l the measurement site is fl¿it and occupied by a corn field during the summer and short grass during the winter. The measurement site is located aL 22.5o- S, 120 km westward from the Atlantic Ocean, approximately 600 m above the mean sea-level, in the State of São Paulo, Brazil, The vertical evolution of the local Planetary Boundary Layer (PBL) was also followed by a tethered balloon system. Four campaigns were carried out, three during summer conditions (March 1991, 92 and 93) and one during winter conditions (July, 1992). The main objective o[ these campaigns was to describe the PB[, in typical areas of the Subtropical regions in order to investigate its impact in the local dispersion conditions of-pollutants. During the summer (rainy season), the Ptlt is strongly dependent from the cloud activity that inhibits its vertical evolution to 1300 m. In the absence of cloud activity the vertical extent of the PBL was observed to reach 2000 m above the surface. During the nighttime, the PBL height was characterized by stable conditions; its height was around 300 m coincident with low level.jet maximum wind speed in both seasons.