Estimations of precipitation rates from CloudSat measurements

Traditional non-polarimetric radar approaches typically use reflectivity factor data to estimate rainfall rates. At traditional precipitation radar frequencies used for precipitation measurements, the effects of attenuation of radar signals in rain are negligible for most practical cases. At mm-wavelength frequencies, attenuation often becomes the dominant factor responsible for apparent reflectivity changes in the vertical direction. At the same time, the variability of non-attenuated reflectivity of rainfall diminishes at these frequencies due to Mie scattering. These factors make the use of traditional rainfall measurements at W-band impractical. An attenuation-based method to retrieve vertical profiles of rain rate from nadir-pointing W-band (94 GHz) radars is proposed and illustrated with CloudSat radar measurements. Under this approaches the range derivatives of apparent (i.e., attenuated) reflectivity factors in rain are used as proxies for the attenuation coefficient. This attenuation coefficient is than related to rainfall rate using a linear model. The variability of non-attenuated reflectivities and the uncertainty of the attenuation coefficient – rain rate relation present the main source of uncertainties of this method. The suggested method is immune to the radar calibration errors and can be applied to observations above the water as well as above the land surfaces. CloudSat retrieval illustrations and comparisons with weather surveillance radars will be presented. It should be mentioned, however, that large vertical range and horizontal footprint resolutions of the CloudSat radar can result noticeable multiple scattering contributions which are typically are ignored when interpreting radar echoes in rain. These contributions can present additional rain rate retrieval errors for CloudSat based retrievals. This presentation will also show results of snowfall retrievals using the CloudSat data. Unlike for rainfall, attenuation of radar signals in snowfall is relatively small even at W-band frequencies, so traditional radar approaches that are based on absolute reflectivity measurements were applied for snowfall estimates.