Sensitivity of the Estimated Monthly Convective Rain Fraction to the Choice of Z–R Relation

This study investigates the sensitivity of the estimated monthly convective rain fraction— that is, the percentage of the areal rain accumulation contributed by precipitation identified as convective—to variations of the Z–R parameters used in radar-based rainfall estimation. Accurate knowledge of the fractions of precipitation that are convective and stratiform is important for climatological studies estimating the heating of the atmosphere. Extensive datasets from two climatologically different precipitation regimes, Darwin, Australia, and Melbourne, Florida, are used. The potential uncertainty of using (i) an arbitrary choice of the power factor b and (ii) either single or multiple Z–R relations (stratified by precipitation type) for converting radar reflectivity to rain rate is investigated quantitatively. The analyses reveal that estimates of the monthly convective rain fraction are sensitive to the choice of Z– R parameters. A maximum sensitivity is found for precipitation regimes with an approximately equal mix of rainfall from convective and stratiform precipitation systems. For example, estimates of the convective rain fraction for monsoonal rainfall at Darwin may range from 30% to 80%, solely depending on the choice of Z– R parameters, even though all of these Z–R relations are tuned to produce the same total rainfall. In contrast, for the highly convective, sea-breeze-triggered, multicellular storms around Melbourne, the estimates of the convective rain fraction may range from 80% to 100%. Different approaches to how the appropriate parameters of the Z–R relation(s) may be obtained are discussed. Varying the Z–R parameters to maximize the correlation of the radar-estimated monthly rainfall at the gauge sites and the rain gauge accumulations does not reveal enough sensitivity to make any choice significantly better than a single Z–R relation for both convective and stratiform rain. Multiple Z–R relations may be justified, but apparently not on the basis of a convective–stratiform separation.