The Influences of Thermodynamics and Aerosols on Deep Convection and Lightning in the Tropics

Areas of convective precipitation observed by the Tropical Rainfall Measuring Mission (TRMM) satellite's Precipitation Radar and Lightning Imaging Sensor between the years 2004 and 2011 in the TRMM domain (36⁰S – 36⁰N) were studied in order to investigate the relative roles of thermodynamics and aerosols in modulating convective intensity and lightning in the Tropics. Estimates of normalized convective available potential energy (NCAPE) and warm-cloud depth (WCD) in close proximity to convective features (CF's) were derived from the Interim Reanalysis (European Center for Medium-Range Weather Forecasts). A global atmospheric aerosol transport model, GEOS-Chem, was used to estimate boundary layer cloud condensation nuclei concentrations (D ≥ 40 nm; N40) in the vicinity of each CF. Following previous work, we hypothesize that aerosols may serve to regulate the amount of available potential energy which is released during convective development. Furthermore, for a given NCAPE and N40, deep convection and lightning are sensitive to the WCD and these sensitivities will be manifest by variability in 30 dBZ echo-top heights and total lightning density.

Consistent with previous studies, 30 dBZ echo-top heights (within lightning-producing CF's) and total lightning density exhibit strong dependence on NCAPE. When CF's are simultaneously stratified by N40 and WCD, 30 dBZ echo-top height is insensitive to aerosol concentrations and WCD. This finding occurs over both land and ocean across the global Tropics. In contrast, total lightning density increases approximately by a factor of two between pristine and polluted environments (again for both land and ocean). Importantly, the results suggest that a synergy between thermodynamic and aerosol forcings exists as lightning activity is most enhanced in polluted environments where NCAPE is above the global median value and WCD is near an intermediate value (~3000-3500 m). Vertical Profiles of Radar Reflectivity (VPRR) constrained by the three chosen forcing parameters are shown to be significantly different from a statistical standpoint in line with the aforementioned results; however, it is difficult to discern if the observed differences in VPRR structure are due to one, two, or all three parameters. A statistical analysis aimed at deconvoluting these simultaneous influences in select regions in the Tropics is on-going. Finally, we offer potential explanations for these findings and place them in context of the relevant literature from the past two decades.