Impact of Ice Microphysical Assumptions on RTTOV Simulations of MSG/SEVIRI Visible and Infrared observations Using W-Band radar Retrieved IWC

Abstract. This study investigates the impact of ice crystal optical properties on the simulation of deep convective cloud radiances (in visible and infrared) observed by geostationary satellites. We performed several simulations for MSG2/SEVIRI channels with the radiative transfer model RTTOV. In these simulations, we used different assumptions in order to test the sensitivity of our simulations to the parameters that are usually used to define the ice cloud properties in models, i.e., size distributions, mass-size relations of ice crystals, and ice crystal shape distributions, where for all these simulations, the ice water content profiles are the same. The ice water content profiles were retrieved using a cloud radar at 94 GHz during an airborne campaign dedicated to the observation of deep convective clouds: Megha-Tropiques in 2010. Simulated radiance for one flight during this campaign are compared to the ones observed by SEVIRI onboard the geostationary satellite MSG2. These simulations allowed us to observed the effect of the sensitivity of the radar reflectivity that can lead to miss information at the top in deep convective clouds. However, the missing information seems mandatory to explain and reproduce the observed radiance. With the help of additional simulations carried with various amount of ice water content on the top of the cloud and their related ice water path, we are concluding that the most important microphysical parameters to simulate deep convective cloud radiances are the size distributions (especially concentration of small ice hydrometeors) and the distributions of ice water content at their tops. These conclusions are valid for most of the SEVIRI infrared channels and two of its visible channels. However, this study failed to simulate accurately the 1.6 µm and 3.9 µm channels. Hence, the necessary knowledge on condition to perform simulations with these two channels stay an open question. One explanation could be it is not about cloud’s definition.