Liquid water path variability in unbroken marine stratocumulus cloud

Liquid water path variability in unbroken marine stratocumulus clouds is ex amined using a simple adiabatic model and both ground-based and airborne ob servations. Observations from FIRE suggest that fluctuations in liquid wate r 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 mode l of cloud liquid water content with a normally distributed cloud base and constant inversion height results in liquid water path distributions that c ompare favourably with observations. Further, it is shown how cloud-base he ight fluctuations can be predicted from the sub-cloud turbulent fluctuation s of temperature, specific humidity and their covariance. The relative impo rtance of these three contributing terms is assessed. Aircraft observations of these fluctuating variables are used to predict the widths of liquid wa ter path distributions observed radiometrically, The observations also sugg est 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 alb edo biases and offer a possible framework for future parametrizations of su bgrid-scale variability in general circulation models.