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

The goal of this paper is to study the changes in the vertical structure and horizontal organization of convective systems at the transition to the monsoon season in South America. We use data from the Tropical Rainfall Measuring Mission (TRMM) satellite to characterize convective system structure. In particular, we will contrast properties of convection for different regions where onset appears to be controlled by different large scale forcing mechanisms. This analysis will help to evaluate the roles of gradual thermodynamic priming of the atmosphere and rapid dynamical triggering (e.g. frontal systems) in establishing the monsoon in different regions of South America.

The timing of monsoon onset across South America is complex, as the southward-moving ITCZ interacts with baroclinic systems that intrude equatorward from higher latitudes. Onset pentads were determined (following Marengo et al. 2004) for 5 degree boxes from 5N to 30S latitude, 75W to 35W longitude, using Global Precipitation Climatology Project (GPCP) 1 degree pentad data for each year between 1979-2007. In general, onset begins by early October both in the northwestern Amazon region (ITCZ migration) and in southeastern subtropical Brazil (baroclinic system forcing). By December, large-scale rain “merges” across the South American Tropics as the South Atlantic Convergence Zone (SACZ) forms, marking the large-scale establishment of the monsoon. Only by late January does onset occur in the dry northeast region of Brazil, associated with the southernmost migration of the Atlantic ITCZ.

To examine regional changes in convective organization prior to and following onset, we analyzed the TRMM 3B42 0.25 degree 3-hourly merged precipitation maps for 1998-2007. This data set is based on geostationary infrared cloud top temperatures, adjusted to rainfall with the TRMM microwave imager, precipitation radar, and rain gauges. Our purpose is to characterize the organization of these precipitating cloud systems, specifically mesoscale convective systems, large squall line systems, and frontal cloud systems. We subjectively identified these systems and produced regional statistics of system type and organization in each 5 degree box.

Changes in the intensity of convective systems prior to and following onset are studied regionally via the metrics of the mean 30 dBZ radar reflectivity contour height, convective and stratiform rain rates, and lightning flash count using the TRMM Precipitation Feature (PF) database (Nesbitt et al. 2000) for 1998-2007. Based on passive microwave and precipitation radar signatures, PFs are characterized as mesoscale convective systems with and without an 85 GHz ice scattering signature (proxy for convective intensity). Past studies (Petersen et al. 2002 and Williams et al. 2002) suggest weakening in convective intensity in some regions after onset, for example more vertically developed reflectivity and higher lightning flash rate prior to onset in the Amazon Basin. Time series, cumulative distribution functions, and composite analysis of these metrics (pentad means for each 5 degree box) prior to and following onset for different regions will be presented.