Aerosol-deep convection interaction based on joint cell-thermal tracking in Large Eddy Simulations during the TRACER campaign

Abstract. In cumulus clouds, aerosol concentrations control cloud droplet concentrations, modifying cloud radiative properties, precipitation processes, and cloud electrification. However, mechanisms of aerosol-deep convection interactions are not well understood due to complex cloud dynamics and microphysics. We investigate the interaction of aerosols with isolated deep convection using Large Eddy Simulations during the TRacking Aerosol Convection interactions ExpeRiment (TRACER) in the Houston area, using a joint cell-thermal tracking algorithm. Cumulus thermals are droplet generators, since supersaturation and droplet nucleation coincide with thermal centers, where the strongest updrafts occur. Primary ice crystal formation does not take place inside thermals, but at layers where previous thermals detrained moisture. As subsequent thermals containing supercooled droplets penetrate these layers, hail and graupel form at or near these thermals. Higher aerosol concentrations result in higher droplet concentrations that suppress drizzle, delay warm rain processes, and transport more moisture aloft. This increases snow and ice amount, as well as graupel and hail, also associated with more lightning. Also, thermals initiate slightly higher, are slightly larger and faster, suggesting a weak invigoration. We also find more thermals per cell, albeit fewer isolated cells. Convection aggregates more, explaining the lower isolated cell count, and enhancing convection, especially near the end of the 24-hour integration. This non-linear mesoscale feedback is likely triggered by temperature and moisture responses due to aerosol-thermal interactions. Additional time-lagged aerosol-reinialization experiments show that the mesoscale response is the predominant forcing for the invigoration. These changes happen within one day, on a smaller scale than previously suggested.