A Mixed Scheme for Subgrid-Scale Fluxes in Cloud-Resolving Models

A large domain large eddy simulation of a tropical deep convection system is used as a benchmark to derive and test a mixed subgrid scale (SGS) scheme for scalar and momentum fluxes in cloud resolving models (CRMs) The benchmark simulation resolves a broad range of scales ranging from mesoscale organisations through gravity waves and individual clouds down to energy containing turbulent eddies A spectral analysis shows that the vertical velocity kinetic energy peaks at scales from hundreds of meters in the lower cloud layer to several kilometers higher up these scales ire typical grid sizes of today s CRMs The analysis also shows that a significant portion of the scalar and momentum fluxes in the benchmark simulation are carried by motions smaller than several kilometers (i e smaller than a typical grid resolution of CRMs) The broad range of scales of the benchmark simulation is split into two components filter scale (mimicking CRM re solvable scale) and subfilter scale (mimicking CRM SOS) using filter widths characteristic of a typical CRM grid spacing The local relationship of the subfilter scale fluxes to the filter scale variables is examined This leads to a mixed SGS scheme to represent the SOS fluxes of scalars and momentum in CRMs A priori tests show that the mixed SOS scheme yields spatial distributions of subfilter scale fluxes that correlate much better with those retrieved from the benchmark when compared with an eddy viscosity/diffusivity scheme that is commonly used in today s CRMs