Toward More Accurate Retrievals of Ice Water Content from Radar Measurements of Clouds

Abstract

There has been considerable discussion concerning the accuracy of values of ice water content (IWC) in ice clouds derived from measurements of radar reflectivity (Z). In this paper, the various published relationships that are based on ice particle size spectra recorded from aircraft are analyzed, and it is shown that a relationship between ice water content and reflectivity can be derived (IWC = 0.137Z0.64 at 94 GHz and IWC = 0.097Z0.59 at 35 GHz), which only varies by 20%?30% for different climatological areas, providing the same ice density as a function of particle size is assumed. Uncertainty as to the true variation of density of ice particles with size may reduce the average IWC for a given Z by up to 30% for an IWC of ≈0.1 g m?3 and 20% for an IWC of ≈0.01 g m?3. Individual values of IWC derived from a single measurement of Z are likely to have an error of about +100% and ?50%, but if some characteristic size estimate is available, this is reduced to about +50% and ?30%. The remaining errors are due to deviations of the size spectra from exponentiality, so there is no advantage in measuring the characteristic size more precisely than this limit. Remote sensing of ice particle size is not trivial, and it is shown that if instead of size, an estimate of the temperature of the ice cloud to within 6 K is available, then, rather surprisingly, the reduction in the error of IWC is almost as good as that achieved using size. Essentially this result is reflecting the well-known correlation of crystal size with temperature. When the mean values of IWC for a given Z and T are compared for a tropical and midlatitude dataset using a common ice density variation with size, then the difference is usually less than 25%. A spaceborne instrument may need to integrate over horizontal distances of 10 km to achieve sufficient sensitivity; this necessity may introduce a bias into the retrieved IWC because the relationship between IWC and Z is not linear, but analysis shows that any bias should be less than 10%.