Representation of the cloud optical properties of a tropical convective system using a double moment cloud microphysics scheme

In this study, we developed a new double moment bulk cloud microphysics scheme to be implemented to Non-hydrostatic ICosahedral Atmospheric Model (NICAM). NICAM is one of Global Cloud Resolving Models, which are designed so as to reduce the uncertainties related with the cloud radiative forcing (CRF) to climate systems by simulating with fine horizontal grids up to a few kilometers around the globe. In the past, NICAM has been achieved the representation of the organized moist convective systems by using a single moment bulk cloud microphysics scheme. However, the cloud optical properties such as the cloud optical thickness and the effective radius are not well determined by using only the mass concentration of hydrometeors. For the purpose of the evaluation of CRF, we need a cloud microphysics scheme which predicts both the mass concentration and the number concentration of hydrometeors at least. The new cloud microphysics scheme, NDW6 (NICAM Double-moment scheme of 6-Water categories), is based on Seifert and Beheng (2006) and Seifert (2008). We elaborately investigated its coupling procedure with the radiation process in order to evaluate CRF more consistently. The optical properties of hydrometeors used in the radiative transfer process are prepared in the range of effective radius from 1 micrometer to 1 millimeter and non-sphericity of ice particles are treated on the basis of Fu (1996) and Fu et al. (1998). In this study, we conducted a set of control experiments in the case of a tropical squall line to investigate the representation of the optical properties of precipitating particles such as rain droplets, aggregated and rimed ice particles and their radiative forcing. Results show that the cloud optical thickness of precipitating particles are comparable to that of cloud droplets or ice crystals around the convective region. In addition, we also demonstrate that the contribution of each hydrometeors to the radar echo signals recognized in the observation by the CPR aboard the CLOUDSAT. Through the insights drawn here, we will realize the appropriate calculation of cloud radiative forcing based on the cloud microphysics in global cloud resolving experiments.