The role of vertically varying cloud fraction in the parametrization of microphysical processes in the ECMWF model

Abstract

SummaryGlobal Circulation Models (GCMs) have generally treated only the radiative impacts of vertically varying cloud fraction by use of a cloud overlap assumption. In this study, the microphysical impacts of vertically varying cloud fraction are addressed by developing a sub-grid scale precipitation model which resolves the vertical variation of cloud fraction. This sub-grid model subdivides the grid boxes into homogeneous columns which are either completely clear or cloudy. By comparing the columnaveraged microphysical quantities from the sub-grid scale precipitation model to the parametrization in the ECMWF model, the ability of the ECMWF model to account for the sub-grid nature of cloud and precipitation microphysics is assessed. It is found that the ECMWF model overestimates precipitation evaporation in the tropical mid-troposphere. This results from (a) an incorrect parametrization of the area of the grid box covered by precipitation, and (b) the inadequacy of assuming a single value for the precipitation rate in the grid box.In addition to assessing the ability of the ECMWF model to parametrize the sub-grid nature of cloud microphysics, the subgrid precipitation model is used to show that the cloud overlap assumption has a large impact on the evaporation of precipitation. In light of the current trend towards more sophisticated cloud and precipitation parametrization in GCMs, more attention should be paid to the impact of vertical cloud fraction variations on the parametrized microphysics.

The Cloudsat Mission and the a-Train

“…For example, assumptions about the cloud vertical structure directly influence the seeder-feeder precipitation mechanism in large-scale models (Jakob and Klein 1999). Figure 4b illustrates the substantial sensitivity of the forecast precipitation to the assumption of cloud overlap.…”

Impact of an improved longwave radiation model, RRTM, on the energy budget and thermodynamic properties of the NCAR community climate model, CCM3

“…The CCM3_LW model and RRTMG were developed using a random cloud overlap assumption, and this is used in the CCM and ECMWF simulations described in sections 4 and 5. However, several recent studies [Jakob and Klein, 1999;Liang and Wang, 1997] have demonstrated the importance of the cloud overlap treatment to GCM simulations. Also, many of the GCMs contributing to AMIP use a mixed cloud overlap method in place of the simpler random method [Phillips, 1994].…”

Toward a more physical representation of precipitation scavenging in global chemistry models: cloud overlap and ice physics and their impact on tropospheric ozone

“…Collins, 2001), cloud and precipitation microphysics Klein, 1999 and, and photolysis rates (Neu et al, 2007). With scavenging, as with microphysics, the ambiguity of interpreting the overlap of fractional clouds in successive layers is complicated by the fact that the precipitating volume of the grid box is not necessarily the same as the local cloud volume.…”