Monte Carlo-based subgrid parameterization of vertical velocity and stratiform cloud microphysics in ECHAM5.5-HAM2
A new method for parameterizing the subgrid variations of vertical\nvelocity and cloud droplet number concentration (CDNC) is presented for\ngeneral circulation models (GCMs). These parameterizations build on top\nof existing parameterizations that create stochastic subgrid cloud\ncolumns inside the GCM grid cells, which can be employed by the Monte\nCarlo independent column approximation approach for radiative transfer.\nThe new model version adds a description for vertical velocity in\nindividual subgrid columns, which can be used to compute cloud\nactivation and the subgrid distribution of the number of cloud droplets\nexplicitly. Autoconversion is also treated explicitly in the subcolumn\nspace. This provides a consistent way of simulating the cloud radiative\neffects with two-moment cloud microphysical properties defined at\nsubgrid scale. The primary impact of the new parameterizations is to\ndecrease the CDNC over polluted continents, while over the oceans the\nimpact is smaller. Moreover, the lower CDNC induces a stronger\nautoconversion of cloud water to rain. The strongest reduction in CDNC\nand cloud water content over the continental areas promotes weaker\nshortwave cloud radiative effects (SW CREs) even after retuning the\nmodel. However, compared to the reference simulation, a slightly\nstronger SW CRE is seen e. g. over mid-latitude oceans, where CDNC\nremains similar to the reference simulation, and the in-cloud liquid\nwater content is slightly increased after retuning the model.