A detailed LES Study of Transport and Mixing near Cloud Edges

In recent studies (Jonker et al 2008, Heus et al 2009) it was found that most of the upward and downward mass transport in shallow cumulus cloud fields occurs near the edge of clouds. This appears to contradict the idea that most mass transport occurs in the buoyant cores of large clouds. To address these and other issues, in this study we make a detailed analysis of Large Eddy Simulations of cumulus clouds with a focus on the cloud edge region. Apart from diagnosing the mass flux as a function of distance to the nearest cloud edge as done previously, we now also simultaneously measure the size of the cloud in order to learn whether the mass transport is dominated by the cores of many small clouds, or by the wide perimeter region of a few large clouds, or by intermediate sized clouds. As the edge region features strong mixing and is responsible for the generation of the descending 'shell' of air surrounding the cloud, we also study the lateral size of cloud shells and test whether or not shell-sizes scale with cloud-sizes. Next we diagnose cloud entrainment properties as a function of distance to cloud edge and verify whether the inner core regions of large clouds entrain relatively less as would be expected from the fact that this region is fenced off from the dry environment. This information relates directly to the notion of 'undiluted cores'. Finally we focus on the local mixing fractions X of conserved variables such as total specific humidity and liquid potential temperature, and investigate whether X can be expressed at arbitrary locations as a linear mixture of cloud(-core) properties and environmental properties. Since we know the distance (r) to cloud edge as well, we are able to establish a straightforward relation between geometric properties (r) and mixing properties (X).

Jonker, Heus and Sullivan. 2008. A refined view of vertical mass transport by cumulus convection. GRL 35 L07810. Heus et al 2009. Observational validation of the compensating mass flux through the shell around cumulus clouds. QJRMS 135, 101-112