Effects of the Interaction of Penetrating Cumulus Clouds on the Microphysics of Stratocumulus

Decoupling of the boundary layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of the surface layer have frequently been observed and modelled. When such cumulus clouds penetrate the overlying stratocumulus layer they directly affect the cloud microphysics and hence the radiative properties of the cloud locally. Observations from the MRF C-130 aircraft during ASTEX showed many occurrences of this type of situation; one particular case on 19th June 1992 will be presented. The cloud layer thickness and cloud top height were strongly related to the presence of cumulus beneath, with the cloud sloping downwards and thinning away from each centre of convective activity. Thermodynamic properties such as the equivalent potential temperature and the total water content in the stratocumulus layer were observed to change in the regions of cumulus penetration to values similar to those found in the surface layer. Further mixing of the intruded air into the stratocumulus smoothed out such changes. The increase in available moisture within the penetrating cumulus resulted in increased liquid water content and hence changes in the droplet size spectra. The cumulus and stratocumulus each exhibited characteristic spectral shapes, so that regions where they had interacted could be identified. The occurrence of drizzle appeared to be correlated with this interaction. The increase in liquid water content resulted in increased albedo, so that regions where cumulus were interacting with the stratocumulus layer could be observed when flying above the cloud. Therefore, the overall amount of short wave radiation reaching the earth's surface must be significantly altered by this process.