The TRMM-LBA (Tropical Rainfall Measuring Mission - Large Scale Biosphere-Atmosphere) experiment was conducted over southwestern Amazonia during the 1999 wet season (Jan-Feb) in order to provide detailed information on precipitation characteristics in the interior of a tropical continent. During TRMM-LBA, two contrasting meteorological regimes were observed during the wet season. The regimes were associated with systematic changes in the direction of the low level wind. Westerly wind periods were characterized by modest CAPE, low CIN, significant moisture through a deep layer, and shallow westerly wind shear. Conversely, easterly wind periods were associated with significantly larger CAPE, drier lower and middle tropospheric humidity, a stronger and deeper wind shear layer, and a weak low-level capping temperature inversion (higher CIN). It was also noted that the mean CCN (cloud condensation nuclei) concentration was a factor of two higher in the easterly regime compared to the westerly regime.

Detailed case study and comprehensive statistical analyses of the NCAR S-POL (S-band, polarimetric) and NASA TOGA (C-band, Doppler) radar data were conducted in order to determine the kinematic, microphysical, precipitation, rain rate and DSD properties of convection in the two regimes. A deep and active mixed phase zone (4-8 km) characterized convection in the easterly regime. Convection in the easterly regime had stronger mean updrafts at low-to-mid levels, larger reflectivities, and larger precipitation ice and rain mass contents in the mixed phase zone. Consistent with the vertical mixed phase structure, the easterly regime also produced significantly more lightning.

The stronger low-level updrafts in the easterly regime lofted more mm-sized supercooled water into the mixed phase zone where the drops froze to produce hail. These frozen drops grew efficiently by accretion in the water rich updrafts. This process, which was enhanced in the easterly regime, produced rain near the surface characterized by larger Z, larger D₀, and higher mean and maximum rain rates in the easterly regime. The westerly (easterly) regime produced nearly 38% (only 25%) of the total rain depth at rain rates less than 5 mm h^-1. At moderate rain rates (5 < R < 50 mm h^-1), the two regimes had similar contributions to the total rain depth. At high rain rates (R > 50 mm h^-1), the easterly (westerly) regime produced nearly 28% (only 15%) of its rain depth. Interestingly, the easterly and westerly regimes produced about the same mean rain depth over the TRMM-LBA domain. These differences in vertical precipitation structure and rainfall properties have important implications for the vertical distribution of latent heating in each regime, parameterizations in numerical cloud models, and for the remote sensing of Amazonian rainfall.