Impacts of the autoconversion scheme on the clouds as simulated by a global cloud system-resolving model: regional variability of the impacts

Abstract:

This study investigates the regional variability of the impacts of the autoconversion rate on the warm clouds simulated by a global cloud system-resolving model (GCSRM). To investigate the impact, two types of autoconversion schemes i.e., Berry [1968] and Khairoutdinov and Kogan [2000] were tested for a single moment bulk microphysical scheme in GCSRM. The former scheme, which has been used in General Circulation Model (GCM), has larger autoconversion rate. The latter, originally developed for Large Eddy Simulation (LES) model, has smaller autoconversion rate. Results of the sensitivity experiments indicate that liquid water path (LWP) simulated by the Khairoutdinov and Kogan [2000] scheme is larger than that simulated by the Berry [1968] scheme, and the slow autoconversion of Khairoutdinov and Kogan [2000] improves the underestimation of the LWP simulated by the Berry [1968]. The detailed analyses of the results clarify that the improvement is mainly shown over the regions where the lower tropospheric static stability (LTSS) is higher than 12 K and the cloud droplet effective radius is smaller than other regions. This indicates that the scheme of Berry [1968] overestimates the autoconversion rate of small cloud droplets, which constitute shallow clouds existing below sharp inversion of potential temperature. This is due to the smaller dependency of the autoconversion rate of Berry [1968] on the cloud droplet size than inferred from satellite observations that tend to be better represented by the Khairoutdinov and Kogan [2000] scheme. These results demonstrate that such a process-oriented type of information is useful to improve the autoconversion and accretion parameterizations in GCMs.