The minor importance of giant aerosol to precipitation development within small trade wind cumuli observed during RICO

Abstract

The present study examines the importance of giant aerosol to precipitation development in marine trade wind cumuli with a new analysis method of a new extensive dataset collected during the Rain in Cumulus over the Ocean (RICO) field campaign. Giant aerosol particles were quantified from clear-air aircraft data, and the precipitation development in the clouds was tracked with dual-polarization radar data. For six different days and multiple clouds on each day, the evolution of the maximum reflectivity, and its associated differential reflectivity, was analyzed. These data were compared with the results of microphysical calculations run within an adiabatic parcel model initialized with the environmental conditions and giant aerosol concentrations on each day of interest. Despite all clouds having formed in a similar maritime environment, this study found substantial variability in the radar echo development both in clouds observed during the same day, and across other days. The maximum reflectivity attained by the clouds appeared to be largely a function of their depth. The radar echo evolution could be represented fairly well by the microphysical parcel model initialized with the observed giant aerosol in some cases but not others. The importance of the giant aerosol to precipitation formation in the small trade wind cumuli appears to be minor: the early radar echoes appear to be more consistent with the development of precipitation by collision and coalescence among droplets formed on the more numerous and smaller cloud condensation nuclei, although the number of giant aerosol appears to have some effect on the actual radar reflectivity values.