A critical view of the warm rain process in marine stratocumulus observations and numerical models

The results of LES bin microphysics models have previously been used to parameterize the warm rain process in bulk microphysics schemes (e.g. Khairotdinov and Kogan, 2000; KK2000). As such models develop warm rain in marine stratocumulus, and as they typically only treat drops grown on small aerosol particles, it is perhaps not surprising that almost all climate models parameterize the formation of warm rain based on drop parameters that are primarily sensitive to small aerosol particles (e.g. mean cloud droplet radius and cloud droplet concentration, or combinations thereof).

This study examines in some detail selected drop growth processes leading to initial drizzle formation (the so-called autoconversion rate). This is done in three ways: (i) By examining the underlying equations used in bin microphysics models, and by demonstrating how drop mixing, that is treated as a linear mixing processes, should be treated as a non-linear process; (ii) by calculating the autoconversion rate based on observations of drop spectra from the VOCALS field deployment and comparing it to the KK2000 parameterization for the same situation; and (iii) by calculating the autoconversion rate for VOCALS marine stratocumulus using a simple box model of drop growth on giant sea-salt aerosol particles.

The calculations cast serious doubt on the commonly used method of parameterizing autoconversion based on droplet growth on small aerosol particles.