Abstract.In

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An analytic cloud droplet effective radius expression is derived and validated by using field experiment microphysical data. This expression shows that the effective radius depends simultaneously upon the cloud liquid water content, droplet concentration and droplet spectral dispersion. It further suggests that the variability in these parameters present at all scales, due to turbulent mixing and secondary droplet activation, could limit the accuracy of the effective radius parameterizations used in climate models.

Two pertinent parameterizations representing respectively the cloud droplet effective radius of maritime and continental warm layer clouds are presented. They are obtained by assuming that warm layer clouds are adiabatic. In order to derive these parameterizations, a theoretical expression of the mean adiabatic cloud droplet effective radius is first developed. This expression depends upon the cloud droplet concentration, the liquid water path and a parameter A that relates linearly the liquid water content to the distance above cloud base. Secondly, an analytic expression of the adiabatic liquid water content is derived and reveals that A depends exclusively upon the cloud base characteristics. By using climatological cloud base values to determine the A value as well as two “standard” droplet concentrations representing respectively droplet concentrations in maritime and continental clouds, one obtains parameterized expressions of the droplet effective radius that depend exclusively upon the liquid water path. The maritime parameterization is validated by using observational data.