Evaluation of Rain Microphysics Using a Radar Simulator and Numerical Models: Comparison of Two-Moment Bulk and Spectral Bin Cloud Microphysics Schemes

This study extended a method to evaluate rain microphysics in a shallow cumulus regime using space-borne radar observational data with a forward simulator of satellite measurements. We compared a two-moment bulk scheme with a two-moment bin scheme. A dynamic-kinematic model was used to isolate cloud microphysics processes from their interactions with dynamics. The relationship between horizontally averaged reflectivity Z(m) and optical depth from the cloud top tau(d), that is, the Z(m)-tau(d) relationship, was similar between the bulk and the bin schemes for clouds with large updraft velocity and clean cloud condensation nuclei (CCN). However, the differences in the Z(m)-tau(d) relationship between the two schemes became more apparent for clouds with smaller updraft velocity or polluted CCN. For these clouds, the differences resulted from differences in the autoconversion rate. We increased the autoconversion rate by decreasing the shape parameter of the cloud droplet size distribution in the bulk scheme. This reduced the differences in the Z(m)-tau(d) relationship between the two schemes, indicating that for these clouds, the autoconversion rate of a cloud microphysics scheme can be evaluated by the Z(m)-tau(d) relationship derived from satellite observations. Although the clouds with the smaller updraft velocity or the polluted CCN did not contribute to the rainfall amount significantly, the lifetime of these clouds greatly affected the radiation budget. Thus, the improvement in the autoconversion rate for such weak cumulus clouds provides a better representation of precipitation efficiency and is important for global climate evaluations. Plain language summary We investigated how space-borne radar observations can improve the bulk cloud microphysics schemes used in regional and global numerical models. We conducted numerical experiments using the bulk and bin schemes. The results allowed us to calculate the radar reflectivity and optical thickness of clouds using a forward simulator of satellite measurements. The two schemes were compared, and we tested the sensitivity of the radar reflectivity and optical thickness of clouds to parameters in the bulk scheme. We found that the parameters in the bulk scheme could be constrained by space-borne radar observations, provided that the observed data are classified by specific atmospheric conditions, such as cloud condensation nuclei, updraft velocity in clouds, and others.