Effects of airmass type on the interaction between warm stratocumulus and underlying cumulus clouds in the marine boundary-layer

Decoupling of the marine boundary-layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface-based mixed layer (SML) have frequently been observed and modelled. Observations of the cumulus-stratocumulus interaction during a Lagrangian study as part of the Atlantic Stratocumulus Transition Experiment (ASTEX), detailed in Martin et al. (1995), were made in an airmass which, although it was over the sea, was highly polluted, since it had recently come from industrial Europe. These observations suggested that the interaction was associated with significant changes in the stratocumulus thickness, microphysics and radiative properties. However, it was suggested that such changes may vary according to the type of airmass in which the cumulus-stratocumulus interaction was taking place. In this paper, two further case studies of the interaction in airmasses from different locations, exhibiting different thermodynamic properties and different levels of pollution, but being generally cleaner than that in the ASTEX Lagrangian study, are analysed to try to assess the influence of airmass type on the interaction. Although the cloud liquid-water content in the stratocumulus generally increases locally in a region of cumulus cloud penetration, the changes in the droplet spectrum which result from mixing between the cumulus and stratocumulus droplets depend on the individual droplet spectra in the two cloud types. This, in turn, is influenced by the aerosol characteristics in the boundary layer, the updraught velocities associated particularly with the cumulus clouds, and the actual and relative cloud thickness of the cumulus and stratocumulus. The effects of penetrating cumulus clouds on the droplet-effective-radius in the stratocumulus, and hence on the cloud radiative properties, may therefore differ significantly between boundary layers in which the interaction is occurring.