Improvements on Understanding Rainfall in the Amazon Basin from LBA Field Campaigns and How Models Benefited From Enhanced Observations (Invited Presentation)

Rainfall in the Amazon Basin is certainly one of the most significant features of the region. However, the details of cloud – rainfall evolution under different large-scale settings and their relationship to surface and boundary layer processes over the forest and deforested areas were superficially known until the 1980s when a series of international field campaigns was initiated. The previous view of tropical convection indicated the need of “hot towers” essential for the Hadley circulation, as proposed by Herbert Riehl in the early 1950s. The view of how rainfall forms and evolves has been constructed over the last few decades, not only in the Amazon but in general, and has indicated a more complex picture where different cloud types co-exist. The research on convection features in the Amazon evolved during the last ~30 years but mainly as consequence of the Large-Scale Biosphere Atmosphere Experiment in the Amazon – LBA.LBA started from the insights provided by previous campaigns such as ABLE and ABRACOS. The rainfall variability due to soil and vegetation processes, the deforestation, the presence of large rivers, the impact of biomass burning aerosol, and combinations within these processes were addressed by the several projects within LBA, starting in 1998 up to now. The simple picture of the “hot towers” meaning rainfall dominated by totally random cumulonimbus clouds has been replaced by a much more complex view, certainly not complete, but more accurate. Large scale features such as the link of Amazon convection to the South American Monsoon System and the Madden and Julian Oscillation have been highlighted by several authors. The global impact of Amazon convection as a tropical heat source has been shown through its interaction with tropical waves and teleconnection patterns with higher latitudes. The role of large-scale deforestation as well as regional deforestation in changing rainfall features as a result of different surface energy budgets has been extensively studied. A potential increase in rainfall due to small scale deforestation has been suggested by model results and satellite rainfall products. The microphysics of rainclouds and the impact of biomass burning aerosols into the character of rainfall is one of the major challenges that emerged in the early days of LBA and continues to produce results and raise new scientific questions. One of these questions is the different clouds and rainfall regimes in sub-regions of the Amazon, extreme differences between the NW and the SE halves of the Amazon, for example. The observations related to the physical processes highlighted weaknesses in numerical modeling of the atmospheric evolution over the Amazon and on the cloud and rainfall simulation. The Brazilian developments in the CSU Regional Atmospheric Modeling Systems – BRAMS was a focus of the model development since the very beginning of LBA. To name a few points that made an impact on how well rainfall is simulated: (1) root depth and soil moisture are essential to get the mixed layer temperature, moisture and height in agreement with observations over forest and deforested regions. Previous version of the soil model used very shallow soil while LBA observations indicated that water was pumped from quite deep layers in forest areas and was thus able to maintain tree evapotranspiration during the length of the dry season. (2) Aerosols from biomass burning were incorporated into BRAMS from actual burning spots and new scalar conservation equations introduced for several biomass burning products. Smoke injection heights associated to convective clouds and impact into the radiation parameterization were introduced in BRAMS. The results showed radiative surface cooling under the smoke plumes of up to 4o C, with significant impacts on the rainfall fields. Long range transport of biomass burning aerosols in a continental scale was demonstrated in several papers by comparing model output to satellite derived products. This indicated potential impacts over the South Atlantic Ocean, the Tropical Andes and the Eastern Tropical Pacific. (3) Introducing biomass burning aerosols into the cloud microphysics parameterization showed changes in convective intensity and surface rainfall and partially explained differences in cloud evolution under polluted and clean environments. The use of LBA data for modeling purposes had a focus on BRAMS but the data and the model developments have been used to improve other models such as WRF regional model and the ECMWF global model.