Marine stratus clouds: Changing liquid-water and temperature structure

Abstract

Aircraft measurements with the Wyoming King Air investigated the response of marine stratus clouds off the north California coast, advected to regions with changed sea surface temperature in coastal upwelling. Cloud and associated clear-air observations were made before and near sunrise to eliminate any role of solar heating. Repeated vertical soundings followed temporal advected air. Subcloud structure was observed for the presence of moist convection, reported below clouds in some conditions. The atmospheric structure in the clear air above the marine stratus cloud layer showed shallow layers having large temperature gradients with height, apparently remaining from an earlier decoupled overlying cloud layer. Such gradients appear to be associated with thermal radiative-heat transfer in the clear air. Cloud tops were observed to be mostly warmer than the cloud just below. It is suggested that a large temperature increase over a small height difference at the cloud-top inversion may serve to neutralize radiative cooling of the cloud drops below, occurring through the atmospheric thermal radiative window. Observations show that subcloud air is often stable to dry convection. This is not evidence that there is decoupling from the sea surface, since there is no wind shear in the layer and moisture continually moves upwards. Subcloud convection from the sea appears to continue everywhere, demonstrated by the rise of intermittent patches of condensation. Such clouds are found below the stratus deck after passage over cooler water, and are clearly responsible for the upward transport of moisture. These transfer processes have been shown to influence cloud-drop spectra and liquid-water content in the clouds as well as the growth and dissipation of stratus clouds. It is concluded that high spatial-resolution (less than a few metres) measurements of vertical radiative-flux divergence are required to confirm the importance of the overlying warmer region in the overall heat balance of a stratiform cloud.