Initial radar echo development in different regimes of tropical contintental convection

The NASA TOGA Doppler radar, a diffusion chamber-based counter of boundary layer cloud condensation nuclei and a series of thermodynamic soundings are being used in tropical Brazil to understand the role of aerosol particles in the initial formation of precipitation and subsequent cloud electrification. Two distinct meteorological regimes have been investagated: the synoptically-controlled 'monsoon' regime in which the CCN concentration was typically 100-300 per cc and the Convective Available Potential Energy (CAPE) was modest, and the 'break period' regime in which the particle concentration reached 700-1200 per cc and the CAPE values are much larger. (A third strongly polluted regime is planned for investigation in October 1999 when the CCN concentration may be higher by an order of magnitude.) The initial radar echo heights were found to be about 2 km higher, on average, in the more polluted, more unstable break period regime. The initial echoes in the monsoon regime are almost exclusively in the 'warm rain' region, whereas about half of the break period first echoes are found above the freezing level. The surface mixing ratios and the cloud base heights are not noticeably different between the two regimes. Both the cloud droplet size and the effect of time are examined in interpreting the first echo observations.