A Sub-Grid Precipitation Generator Based on NICAM for Simulating Cloud Radar Signals With GCMs

Abstract

The forward simulation of radar reflectivity requires details of clouds and precipitation from general circulation models (GCMs). But such details are represented as sub-grid processes that involve parameterizations and assumptions about the spatial coverage and thus depend on the GCM. In this research, we propose the use of a statistical method to generate sub-grid precipitation for generic use. The sub-grid variability is obtained from simulation with a global storm-resolving model called NICAM (non-hydrostatic icosahedral atmospheric model). The proposed method first generates sub-grid precipitation masks based on probabilistic scenarios and then sub-grid precipitation rates are generated from the generalized gamma distribution for the given cloud fraction and grid-scale precipitation rates. Compared to the standard method (which neglects the probabilities) that overestimates the precipitation fraction, our method well reproduces the NICAM data set profiles of both the precipitation fraction and the radar-based cloud fraction. The in-cloud signal frequencies are also reproduced, although less accurately over a tropical region. Inclusion of sub-grid variability in precipitation rates was particularly important for the tropical region to obtain agreement of the precipitation fraction. Application of the two methods to a GCM shows it to have a robust bias for low-level liquid clouds. Furthermore, the sub-grid variability of precipitation led to more occurrences of the small signals, particularly for a range of high precipitation rates. The proposed method was designed to produce geographically dependent sub-grid variability in precipitation, indicating an effective way to use a global storm-resolving model to evaluate conventional GCMs.