Rain Climatology in the Central Region of the State of São Paulo Using Radar Echoes During the Wet Period

This work presents a climatology of rain in the central region of the State of Sao Paulo, based on observations within a range of 240 km from a weather radar located in Bauru. This radar forms part of IPMet's (Instituto de Pesquisas Meteorológicas) network of two S-band Doppler radars, being operated continuously with volume scans every 7.5 – 15 min, if rain occurs within the 240 km range, comprising 11 PPIs between 0.3º and 34.9º elevation. The beam width is 2º and the resolution is 1km in range and 1º in azimuth. Since the majority of rain falling in this region is of convective nature, it was decided to concentrate on the wet-period months: November, December, January, February and March from 1994 to 2004. About 70% of the average rainfall is being recorded during these months.

In the first step, the available data had been processed with a resolution of 1 km², yielding a matrix of 480x480 points, for every 7.5 min volume scan. Average fields show the distribution of the precipitation in terms of predominant areas of echoes exceeding 40 and 50 dBZ, respectively, as well as the diurnal variation, anomalies and the frequencies of occurrence. The mean frequency of areas exceeding 40 dBZ during January shows a concentration of storms located in a region north to north-west at about 100km range from the radar, while the frequencies in other regions are significantly lower. The areas exceeding 50 dBZ during January correspond to 33% of all rain echoes >15 dBZ. During the February months the intensity of rain and the number of days with rain reached a maximum.

Rainfall records from an automatic weather station, located in Botucatu, approximately 100 km south-east of the radar are being used for the validation of the rainfall estimated from radar data. The accumulated rainfall was derived by using three different Z-R relationships, viz., Marshall Palmer, Calheiros & Zawadski and Antonio. It is proposed to deploy statistical techniques to evaluate these three Z-R relationships under local conditions.