Regional variation of convective structure at monsoon onset across South America inferred from TRMM observations

Abstract

The variation of precipitation in South America is characterized by different mechanisms that both initiate and sustain precipitation. Analysis to characterize regional differences is done utilizing datasets derived from the Tropical Rainfall Measuring Mission (TRMM) satellite as well as National Center for Environmental Prediction (NCEP) Reanalysis data. Four regions were studied. These included the Amazon Basin (West), northeast coast (East), location of the South Atlantic Convergence Zone (SACZ), and extreme southeastern Brazil (South). In these regions, variables such as rain rate, feature size, lighting flash counts, updraft strength, and thermodynamic fluxes were analyzed. Before onset in the Amazon Basin, sensible heat flux increases and is well correlated with increases in lighting flash counts that were observed using TRMM, implying thermodynamic forcing. After onset, the features become less electrified, however their frequency and rain rates increase. The East region experiences similar pre and post-onset behavior, however oceanic influence not only limits the vertical intensity of convection, but also acts to delay the timing of onset by two months, compared to the West. Fronts are the dominant driver of precipitation in the SACZ region, increasing in frequency as onset approaches. Time series analysis from TRMM illustrates high variability in precipitation and increases in frequency of features here that is consistent with an increase in frontal activity. Also, composite directional wind plots using NCEP 850 mb wind data, reveals interactions between the Tropics and sub-Tropics. Pre-onset MCCs dominate South time series and frontal intrusions drive precipitation in the post-onset period. Understanding distinct regional mechanisms of precipitation can increase the predictability of monsoon onset all over South America. This is important for those who live there and depend on seasonal rains for their livelihood. Also, knowing what mechanisms are working in different regions at different times of the year and incorporating their respective contributions to the global heat budget, we can increase the accuracy of global models. The next step would be to use these observations to model the observed regional variability and timing, and identify the contributions and long-term implications of heat release from the South American Monsoon System (SAMS) on the global scale.