Characteristics of water cloud optical property as simulated by a non-hydrostatic spectral microphysics cloud model

The numerical simulation of cloud particle size distribution was performed for low-level water clouds with a non-hydrostatic spectral microphysics cloud model. The simulated cloud optical properties are compared with those obtained from the satellite observations.

The result shows two different correlation patterns in the simulated optical thickness and effective particle radius, i.e., positive and negative correlations for non-drizzling and drizzling clouds, respectively, similar to the observed features reported by past remote sensing works. The simulated correlation pattern was also found to systematically depend on aerosol amount. The correlation pattern in a pristine condition has a large portion of negative correlation with small fragment of positive correlation, similar to the satellite-observed characteristics over the FIRE region off California. The simulation for a polluted condition, on the contrary, reproduced the correlation pattern composed by only a positive correlation part, similar to the satellite observation over the ASTEX region of North Atlantic Ocean. Satellite-observed contrast of correlation patterns between FIRE and ASTEX regions can be explained by the difference in aerosol amount over these two regions.

The column-averaged effective radius re(column) was also simulated in a same manner as satellite observation with simultaneous use of VIRS and TMI aboard the TRMM satellite. Because the re(column) contains the signal of drizzle particle, the combination of re(column) and the effective radius near the cloud top re(top) provides us with useful insight into the vertical stratification of cloud microphysical structure. The simulated re(column) and re(top) are correlated differently for small and large cloud particles. The column-averaged effective radius re(column) increases with re(top) slowly over the size range of re(top)<15um, in which the condensational growth process is dominant. Over the size range of re(top)>15um, on the contrary, re(column) increases with re(top) so rapidly because the collision-coagulation growth mechanisms becomes active for such large particles and thus the abundant drizzle particles are produced. The simulated correlation pattern between re(column) and re(top) closely resembles the global statistics obtained from the TRMM satellite observation, in which the transition of correlation curve is also found at around re(top)=15um, implying that the satellite-derived global statistics of re(column) and re(top) follows a microphysical growth curve.