Tropical Convection in the Amazon: New Perspectives Gleaned from the TRMM Satellite and Field Campaign Data

Dr. Walter Petersen, Colorado State University

The TRMM-LBA field campaign was conducted in the southwest corner of the Amazon Basin in the Brazilian State of Rondonia during the wet-season months of January-February 1999. Aside from TRMM satellite ground validation, the goal of the field campaign was to provide a comprehensive physical view of convection that could be characterized as "typical" of a tropical interior continental regime. This was accomplished utilizing a variety of instrumentation including a dual-polarized S-band radar (NCAR SPOL), the NASA TOGA C-band radar, airborne Doppler radar (EDOP) and passive micro/millimeter wave sensors, an airborne microphysics platform (UND Citation), and numerous other instruments including a four station sounding sounding network, tethersondes and micrometeorology/flux towers. By combining the TRMM-LBA ground-based observations with those of the TRMM satellite Precipitation Radar (PR) and Lightning Imaging Sensor (LIS), we can begin to place Amazonian convective morphologies in the larger context of both regional and global tropical convective regimes.

For example, over seasonal/global tropical scales, analysis of TRMM-PR and LIS data suggest that wet-season convection in the Amazon is far from being "typical" of that observed in the interior of a tropical continent. Distributions of radar reflectivity in the vertical combined with LIS lightning flash densities place Amazonian convective systems somewhere in the spectrum of convection ranging from that observed over isolated regions of tropical ocean, to the typically more intense, vertically developed, highly electrified convection of the African Congo. Moving to regional spatial and sub-monthly temporal scales, convection observed in the Amazon during the six-week period of TRMM-LBA appeared to vary markedly in vertical structure as a function of the synoptic-scale wind regime (forced in part by transient extratropical frontal systems). For example, during extended periods of easterly wind at 850 mb, convection tended to take on more "continental" characteristics exhibiting deep vertical development, robust mixed phase microphysical characteristics, large updrafts and copious amounts of lightning. Conversely, during westerly periods in the 850 mb flow, the convection took on a much more "oceanic" characteristic, exhibiting a marked weakening in vertical structure, attendant weaker updrafts and mass fluxes, and an overall reduction in lightning.

To further examine the manifestation of these Amazonian convective regimes over extended temporal and spatial scales, an analysis of TRMM PR, LIS and Brazilian Lightning Detection Network (BLDN) data for three four-month wet-seasons was conducted over the Amazon basin and southern South America. These analyses reveal some interesting contrasts between the convection occurring in easterly and westerly regimes over the southern Amazon. In TRMM satellite observations, regime contrasts in convective structure over Rondonia are broadly consistent with, but not quite as pronounced as, those observed during the six-week period of TRMM-LBA in 1999. However, east-west regime variability in the three-year TRMM satellite data is quite pronounced just to the east of Rondonia in the southcentral Amazon. While the exact forcing mechanisms responsible for these regimes are still somewhat uncertain, these observations may have important implications for how cumulus parameterization schemes in numerical models represent convection over the Amazon.