What determines the distribution of shallow convective mass flux through cloud base?

摘要

The distribution of cloud-base mass flux is studied using large-eddysimulations (LES) of two reference cases, one representing conditions over thetropical ocean, and another one representing mid-latitude conditions over land.To examine what sets the difference between the two distributions, nineadditional LES cases are set up as variations of the two reference cases. Wefind that the total surface heat flux and its changes over the diurnal cycle donot influence the distribution shape. The latter is also not determined by thelevel of organization in the cloud field. It is instead determined by the ratioof the surface sensible heat flux to the latent heat flux, the Bowen ratio $B$.$B$ sets the thermodynamic efficiency of the moist convective heat cycle, whichdetermines the portion of the total surface heat flux that can be transformedinto mechanical work of convection against mechanical dissipation. Thethermodynamic moist heat cycle sets the average mass flux per cloud $\langle m\rangle$, and through $\langle m \rangle$ it also controls the shape of thedistribution. An expression for $\langle m \rangle$ is derived based on themoist convective heat cycle and is evaluated against LES. This expression canbe used in shallow cumulus parameterizations as a physical constraint on themass flux distribution. The similarity between the mass flux and the cloud areadistributions indicate that $B$ also has a role in shaping the cloud areadistribution, which could explain its different shapes and slopes observed inprevious studies.