Multiple-wavelength radar measurements for rain profile retrievals at ARM SGP Central Facility

Description

The U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility sites now host continuous multiple-wavelength radar measurements with matched beams. Thanks to the recent reconfiguration and improved signal processing of the UHF wind profiler in a precipitation mode, triple wavelength profiles of rain events are now available at the Southern Great Plains (SGP) site.

Rain attenuation at the millimetre wavelength of cloud radars does represent a useful signal for the retrieval of precipitation characteristics. In particular, at Ka-band, the reflectivity gradient can be used to retrieve rain-rate profiles, assuming that the non-attenuated reflectivity profiles are constant within the vertical layer where the retrieval is applied. In order to improve or extend this method, a non-attenuated Rayleigh reference profile allows considering the more general case of vertical variability of hydrometeor profiles, while an additional attenuated cloud radar (e.g., W-band) permits extension of the retrievals to very low rain rates. The key step in this new methodology is to disentangle the contributions to the measured dual-wavelength ratios provided by rain attenuation and by non-Rayleigh effects that can be of the same order of magnitude for cloud radars. To achieve this goal, the retrieval of full drop-size distribution characteristics is mandatory.

This theoretical study aims at showing the feasibility of such retrievals. The improvement brought by the third wavelength will be evaluated, and the optimal combination of radar wavelengths will be determined depending on the rain-rate regimes. Finally, this methodology will be illustrated through different case studies using the UHF wind profiler and the Ka- and W-band cloud radars of the ARM SGP Central Facility.