6.1 Providing a missing link: A more complete coupling of clouds to radiation and tests using both WRF-ARW and WRF-NMM (HWRF)

Thompson, Greg, Mukul Tewari, Shaowu Bao, and Sam Trahan, National Center for Atmospheric Research

The various radiation schemes in WRF have very weak linkages between the

assumptions within the cloud schemes and how the cloud optical depth, tau,

is computed inside the radiation parameterizations. Most simply use only the mixing ratio of cloud water and cloud ice and nothing further. The RRTMG scheme sums the snow into the cloud ice. None of these treatments exploits the possibility of linking explicitly computed water droplet and ice crystal sizes known within the microphysics schemes directly into the calculations of tau.

This missing link is now remedied in the combination of the RRTMG short and longwave radiation parameterization in combination with the Thompson microphysics scheme. A simple diagnostic routine is added within the microphysics code to calculate the proper radiation effective diameters of cloud droplets, cloud ice, and snow and these are communicated to the radiation codes where they are

individually applied into the calculation of tau. The newly connected physics was tested in a set of real experiments of a large winter storm using both ARW and NMM as well as Hurricane Earl (2010) using HWRF. Results of various sensitivity experiments will be presented.