Dynamics of cumulus rising into stratocumulus as observed during the first ‘Lagrangian’ experiment of ASTEX

Abstract

AbstractThe Atlantic Stratocumulus Transition Experiment took place during June 1992 in the eastern Atlantic Ocean in an area extending south‐east of the Azores to Madeira. One of the goals of this experiment was to study the physical processes responsible for the transition from stratocumulus to shallow cumulus clouds. The cloud type most frequently observed was cumulus rising into stratocumulus. Two ‘Lagrangian’ experiments were carried out to study the evolution of the boundary layer and to observe any transition in the cloud characteristics downstream.Here we concentrate on the last flight of the first ‘Lagrangian’ experiment during which cumulus cloud was penetrating thin and broken stratocumulus from below. The immediate cause of the cumulus convection was condensation at the top of the well‐mixed surface‐based boundary layer. The lifting condensation level was observed to be at about 500 m and above this height there was a conditionally unstable layer up to 1600 m. In the upper part of the conditionally unstable layer the horizontally averaged thermodynamical variables showed that the air was very close to saturation, indicating that the upper part of the cloud deck consisted of broken clouds. The vertical structure of mean variables, variances, fluxes and turbulent kinetic energy budget is discussed. One of the findings is that the cumulus convection is an important factor in counteracting the gradual dissipation of the stratocumulus deck in that it supplies moisture to the latter. Furthermore, the net longwave radiative loss at cloud top was observed to be about 40 ± 15 W m−2, which tended to destabilize the cloud layer from above. The skewness factor S showed that the upward convection was quite intense owing to the rising cumuli, the factor S reaching a maximum of about 2 ± 0.7 in the middle of the cloud layer. The skewness was found to be slightly negative near the cloud top.