Thursday, 12 June 2008
The existence of the subsiding shell around cumulus clouds has been observed before by e.g. Jonas (AR, 1990) and Rodts et al. (JAS, 2003). Results from large-eddy simulations (LES), extensively discussed in Heus et al. (JAS, in press) and Jonker et al. (submitted to GRL), pointed out that the downward mass flux through the shell compensates a significant fraction of the upward mass flux through the cloud; this was not observed before, possibly because these previous studies usually focused on the one-dimensional velocity profile across the cloud and the shell without considering the shell area versus the cloud area. Since the mass flux is equal to the vertical velocity integerated over the area within the shell, and the shell area is much larger than the the cloud area, the mass flux in the shell has previously been underestimated. In this study, air-plane measurements from the Rain in Cumulus over the Ocean (RICO) field campaign where used to verify this hypothesis. Similar to the LES results, the in-shell mass flux was found to be significant. However, a few differences could be found in comparison with the LES results; most of them could be explained by taking the difference between the 2-dimensional slabs in LES and the 1-dimensional lines from observations into account.
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