An Improved Beta Method for Ice Cloud Retrievals Using Spaceborne Thermal Infrared Observations

The beta method is a daytime–nighttime consistent approach for ice cloud property retrievals based on spaceborne passive thermal infrared observations and is widely used to investigate the global coverage of optically thin ice clouds and global distributions of optical and microphysical properties. Several operational cloud products are based on the beta method, and some further improvements have been made. However, the assumptions used in the beta ratio have been kept the same for nearly 30 years, including the assumptions of 1) an isothermal cloud layer and 2) a non-scattering cloud and atmosphere. These assumptions are not generally applicable to actual ice clouds, which suggests that the beta method might lead to biases in ice cloud property retrievals.

We present significant improvements in the beta method that take into account a non-isothermal cloud layer and contributions of the multiple scattering effects. The retrieval biases due to ignoring these two factors are small for optically thin cold ice clouds (i.e., T_top< 203 K; τ< 0.3), but become large with optically thicker clouds with warmer cloud temperatures. Furthermore, we apply the improved beta method to MODIS measurements and compare ice cloud retrievals with MODIS operational cloud products.