Assessment of the precipitation susceptibility of stratocumulus clouds

Description

Precipitation susceptibility So has been proposed as a means of quantifying the potential influence of changes in precipitation resulting from changes in drop (or aerosol) concentration. Various methods of assessing its magnitude in warm clouds have yielded sometimes conflicting trends in So with increasing liquid water path. For example, cloud parcel models, large-eddy simulations (LES), and some satellite analyses show a characteristic increase in So from very low values at low liquid water path (LWP) to some maximum at an intermediate LWP, followed by a steady decrease thereafter. On the other hand, heuristic models and aircraft observations show So decreasing monotonically with increasing LWP. The argument is that as LWP increases, the balance between (drop concentration-dependent) autoconversion and (drop concentration-independent) accretion shifts steadily in favor of accretion, and therefore So decreases monotonically.

In this presentation we will attempt to resolve the discrepancies between these results using a number of modeling techniques ranging from box models of stochastic collision-coalescence to ensembles of parcel models run along trajectories derived from LES. We will show that although the shift in balance from autoconversion to accretion does indeed decrease So, the time available for collision-coalescence is a key controlling parameter that can explain the non-monotonic trend in So.