Subsiding shells and vertical mass flux in warm cumulus clouds over land

Abstract. The mass flux of air lifted within the updrafts of shallow convection was thought to be compensated outside the cloud through either large scale subsidence or stronger downdrafts in a thin shell surrounding the cloud. Subsiding shells were postulated based on large eddy simulation and are experimentally tested in this study for shallow convection over land. Isolated cumulus clouds were probed with a small research aircraft over flat land, mountains, in different wind situations and at different levels of the clouds. The subsiding shell varies considerably between individual cloud transects. A shell-like narrow downdraft region was present on at least one edge in 105 out of 191 transects and on both edges in 29 transects. However, the average over all cloud transects shows a narrow downdraft region outside the cloud boundaries. The ensemble-mean subsiding shell is narrower on the upwind side of the cloud, while it is at least half a cloud diameter wide and more humid on the downwind side. At least half of the upward mass transport in the cloud is compensated within a distance of 20 % of the cloud diameter. A shell is not uniform. Distinct regions of downdrafts and updrafts with high variability of the vertical wind are frequent and randomly distributed in the vicinity and also within the cloud. Based on these findings, a subsiding shell is, however, a valid concept to describe an ensemble of shallow cumulus clouds over land.