Liquid water path variability in unbroken marine stratocumulus cloud

Liquid water path variability in unbroken marine stratocumulus clouds is examined using a simple adiabatic model and both ground-based and airborne observations. Observations from FIRE suggest that fluctuations in liquid water path at scales smaller than around 60 km can be modelled reasonably well as a function of fluctuations in cloud-base height. A simple adiabatic model of cloud liquid water content with a normally distributed cloud base and constant inversion height results in liquid water path distributions that compare favourably with observations. Further, it is shown how cloud-base height fluctuations can be predicted from the sub-cloud turbulent fluctuations of temperature, specific humidity and their covariance. The relative importance of these three contributing terms is assessed. Aircraft observations of these fluctuating variables are used to predict the widths of liquid water path distributions observed radiometrically. The observations also suggest that the standard deviation of cloud-base height and hence liquid water path variability increases with increasing boundary-layer depth. The model liquid water path distributions could be used to derive plane-parallel albedo biases and offer a possible framework for future parametrizations of subgrid-scale variability in general circulation models.