Evaluation of radar retrieval algorithms in stratiform clouds using large-eddy simulations

The performance of algorithms retrieving cloud liquid water content W from radar reflectivity factor Z is evaluated using data from a three-dimensiona l large-eddy simulation model with detailed size-resolving microphysics. On the basis of case studies of a marine stratocumulus cloud observed during the Atlantic Stratocumulus Transition Experiment in June of 1992 and a cont inental stratus cloud observed over north central Oklahoma on April 30, 199 4, it is shown that retrieval algorithms based only on Z are very sensitive to cloud microphysics and do not reproduce adequately the horizontally ave raged vertical profile of W. Additional independent measurements are needed to constrain the retrieved W profile. Surprisingly, algorithms show little improvement when the exact concentrations of cloud droplets obtained from the model are included in the calculations. Vertically integrated liquid wa ter path P is found to be a robust constraint that ensures a more accurate retrieval. In the two studied cases an algorithm based on the Z-W relation of the form Z = aW(b) gives an optimal performance when a is inversely prop ortional to P and b = 1.32. The horizontal averaging of P over a computatio nal domain of several kilometers does not affect the accuracy of the retrie val of the horizontally averaged profile of cloud liquid water content, whi ch adds to the versatility of the algorithm.