The variable role of giant aerosol in precipitation development in shallow trade wind cumuli

Colleen Henry Reiche, Purdue University

Abstract

Accurate weather forecasts of precipitation events hinge on both a complete understanding and an accurate representation of rain formation processes. A major uncertainty in precipitation development is how the large drops necessary to initiate coalescence growth are created in a realistic time. One explanation for accelerated coalescence growth onset in real clouds is the presence of giant aerosol, having large sizes that permit earlier onset of coalescence growth and thus precipitation formation. Numerical modeling studies have shown these giant aerosol can be important in continental clouds, but past studies have disagreed about their importance in maritime clouds. The present study examines the role of giant aerosol in marine trade wind cumuli with a new analysis method using a new extensive dataset collected during the Rain in Cumulus over the Ocean (RICO) field campaign. Giant aerosol particles are quantified from clear-air aircraft data, and the precipitation development in the clouds is tracked with dual-polarization radar data. These data are used and compared with the results from an adiabatic parcel model run for typical environmental conditions and aerosol concentrations on each day of interest. Despite all clouds being pristine and maritime, this study reveals a varying role of giant aerosol on precipitation formation, governed by the observed CCN and resulting drop concentrations in the clouds. The giant aerosol appear to be important for precipitation development on days with high CCN concentrations where droplet growth is slower and relatively unimportant on days with low CCN concentrations where coalescence is able to initiate effectively in the absence of giant aerosol. The overall development of precipitation is found to be strongly influenced, however, by environmental factors limiting cloud development on many of the days. Differences in the observed radar echo evolution and the modeled precipitation development also suggest that the clouds may "linger" at low radar reflectivities for an extended period of time before the onset of higher reflectivities associated with precipitation.

Degree

Ph.D.

Advisors

Lasher-Trapp, Purdue University.

Subject Area

Atmospheric sciences

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