Radar and satellite data from the TRMM/LBA project have been examined to determine causes for convective storm initiation in the southwest Amazon region. An automated computer program was used to register the locations and times of storm initiation as observed by the NCAR S-pol radar. Storm initiation was defined when a convective cell first reached 30 dBZ covering an area of 16 km2. Both the radar and GOES 9 visible data were used to identify gust fronts and cold pools from storm outflows. This data along with high resolution topographic data were used to determine possible convective storm triggering mechanisms. The terrain elevation varied from 150 m to 500 m. The area is covered by tropical forests with numerous clear cut areas used for cattle raising and small scale farming.

This paper presents the results from 5 February 1999. This day was classified as a weak monsoon regime where convection developed in response to the diurnal cycle of solar heating. Scattered shallow cumulus during the morning developed into deep convection by early afternoon. A total of 303 storms initiated within 130 km of S-pol on this day. Initiation began about 1100 LT and the initiation of cells peaked around 1500-1600 LT.

Storms tended to first form over the higher terrain. Many of these storms generated cold pools with associated gust fronts. New storms were then initiated by the gust fronts typically on the downwind shear side. With time gust fronts would collide and initiate new storms which tended to be the more intense and longer lived storms.

The causes of storm initiation were classified into 7 categories that will be described in the paper. The most common initiation mechanisms were along a gust front (27%), over high terrain (>300 m) without any other triggering mechanism (20%) and colliding gust fronts (16%). An initiation cause could not be determined for 21% of the 303 cases.

Comments will be provided on 1) implications these results may have on convective storm nowcasting systems, and 2) the effect deforestation may have on storm initiation.