Improved parameterization of ice particle size distributions using uncorrelated mass spectrum parameters; results from GCPEx


AbstractSatellite retrieval algorithms and model microphysical parameterizations require guidance from observations to improve the representation of ice-phase microphysical quantities and processes. Here, a parametrization for ice-phase particle size distributions (PSDs) is developed using in-situ measurements of cloud microphysical properties collected during the Global Precipitation Measurement (GPM) Cold-season Precipitation Experiment (GCPEx). This parametrization takes advantage of the relation between the Gamma shape parameter (?) and the mass-weighted mean diameter (Dm) of the ice-phase PSD sampled during GCPEx. The retrieval of effective reflectivity (Ze) and ice water content (IWC) from the reconstructed PSD using the ?-Dm relationship was tested with independent measurements of Ze and IWC and overall leads to a mean error of 8% in both variables. This represents an improvement when compared to errors using the Field et al. (2007) parametrization of 10% in IWC and 37% in Ze. Current radar precipitation retrieval algorithms from GPM assume the PSD follows a Gamma distribution with ? = 3. This assumption leads to a mean overestimation of 5% in the retrieved Ze, whereas applying the ?-Dm relationship found here reduces this bias to an overestimation smaller than 1%. Proper selection of the a and b coefficient in the mass-dimension relationship is also of crucial importance for retrievals. An inappropriate selection of a and b, even from values observed in previous studies in similar environments and cloud types, can lead to more than 100% bias in IWC and Ze for the ice-phase particles analyzed here.

Journal of Applied Meteorology and Climatology