Shape parameter analysis using cloud spectra and gamma functions in the numerical modeling RAMS during LBA Project at Amazonian region, Brazil

Abstract

The microphysical variable shape parameters have been investigated considering a numerical simulation based on RAMS (Regional Atmospheric Modeling System), with focus on southwest Amazon Basin during the transition from dry to wet seasons within the scope of LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia). All data were obtained during the dry-to-wet campaign from September to October 2003. Three precipitation events were chosen during the campaign, classified as polluted, intermediary and clean events. The dataset, for the description of the observed convective systems, includes airborne observations with a microphysics airplane. The variables were cloud droplet contents and spectra, in several flights; radiosonde launches; a 10 cm Doppler radar; visible and IR satellite maps and rain gauge network. The preliminary results have shown that over polluted areas, shape parameters with higher values, from 5 to 7 are better fitted to cloud spectra observations, than the usual shape parameter 2 used in RAMS microphysics default table. For its turn, shape parameter 2 is related to the cleaner areas, i.e., without biomass burning. On the other hand, the numerical simulations, reproducing as close as possible the microphysical variable spatial distributions in the observed dataset, have shown that the average total rainfall precipitation at the surface generally decreases as the CCN concentration and shape parameter increases. CCN vertical and temporal profiles support that result, emphasizing that the higher CCN concentrations are related to biomass burning, where higher shape parameters are better fitted. Additionally, it is possible to conclude that different shape parameters and CCN concentrations may influence directly the maximum and average amount of precipitation and cloud water, implying that different regions should have each the proper specification characteristics. Shape parameters and CCN concentrations are quite useful tools in order to evaluate the spatial and temporal cloud patterns as well as, the rainfall amount with regional implications.