Evaluation of Radar Reflectivity–Based Estimates of Water Content in Stratiform Marine Clouds

Abstract

The performance of radar reflectivity (Ze)?based relations for retrievals of marine stratiform cloud liquid water content (LWC) is evaluated by comparing liquid water path (LWP) estimates from microwave radiometers with vertically integrated LWC values retrieved from radar measurements. Based on a measurement dataset from a research vessel in the tropical eastern Pacific Ocean, it is shown that reflectivity thresholding allows minimizing of the influence of drizzle drops present in marine stratiform clouds to the extent that LWP estimates from a ground-/shipborne radar can have uncertainties that might be acceptable for different applications. The accuracies of Ze-based retrievals depend on the thresholding level Zet, and they are generally better than a factor of 2 for Zet ? ?15 dBZ. These accuracies typically improve when Zet is lowered; however, the amount of cloud profiles that pass thresholding diminishes as Zet is decreased from about 50% for a ?15-dbZ threshold to only about 10% for a ?25-dBZ threshold. Different thresholding strategies are considered. Ancillary information on cloud-base heights can improve LWP estimates from reflectivities. The ship-based dataset was used to simulate measurements from prospective 94-GHz spaceborne cloud radar (CloudSat). CloudSat measurements would, on average, detect about 75% of warm marine stratiform clouds, though many clouds with negligible presence of drizzle will be missed. Because of sensitivity and resolution issues for the spaceborne radar, reflectivity-based estimates of LWP are generally biased toward high values and have higher uncertainties when compared with the ground-based radar, for the same Zet.