Application of the features in double-moment microphysics into single-moment microphysics scheme in KIAPS-GM

The Korea Institute of Atmospheric Prediction System-Global Model (KIAPS-GM) is a global numerical weather prediction (NWP) system with the element-based dynamical core on cubed-sphere grids. The physics package as a part of the KIAPS-GM has been developed, including the radiation, cloud macrophysics/microphysics, convection, planetary boundary layer, orographic/non-orographic gravity wave drag, and land surface. As one of external modules, the aerosol parameterization is being currently implemented into KIAPS-GM, based on WRF/Chem GOCART module. For the cloud microphysics parameterization, we implemented the following schemes in the 3-dimensional framework of KIAPS-GM; Weather Research and Forecasting model (WRF) Single-Moment 6-class microphysics scheme (WSM6, Hong and Lim 2006), and WRF Double-Moment 6-class microphysics scheme (WDM6, Lim and Hong 2010). Compared with WSM6, WDM6 not only predicts additionally the number concentration of cloud droplet and rain drop (only in warm-rain microphysics), but also includes CCN prediction. The number of 3-dimensional microphysical prognostic variables should be increased in WDM6, and it eventually increases the computational burden. In this study, some of microphysical processes in WSM6 scheme of KIAPS-GM have been modified to adopt the features of double-moment microphysics, without substantially increasing computational time. The cloud-to-rain autoconversion rate and the accretion rate were modified so that the varying cloud droplet number concentration could be used. The varying cloud droplet number concentration is assumed to be equal to an activated CCN number concentration without being prescribed globally as a fixed value. The activated CCN number concentration is calculated from the aerosol-CCN activation scheme developed for KIAPS-GM (see the details of aerosol-CCN activation scheme in Soo Ya Bae's presentation). Both CCN and cloud droplet number concentrations were treated diagnostically in WSM6 scheme to emulate the double-moment microphysics, so they are not advected in dynamic core of KIAPS-GM. A suite of global simulations with KIAPS-GM was designed to investigate the differences between the original WSM6 (i.e., with the prescribed cloud droplet number concentration as a fixed value) and the modified WSM6. More activated CCN/cloud droplet number concentration over the region with high aerosol loading tends to suppress warm-rain processes (e.g., lower autoconversion rate from cloud water to rain water), increasing the supercooled water and ice crystals.