Riming-dependent snowfall rate and ice water content retrievals for W-band cloud radar

Accurate measurements of snowfall in mid-latitudes and high latitudes are particularly important because snow provides a vital freshwater source and impacts glacier mass balances as well as surface albedo. However, ice water content (IWC) and snowfall rates (SRs) are hard to measure due to their high spatial variability and the remoteness of polar regions. In this study, we present novel ice water content-equivalent radar reflectivity (IWC-Ze) and snowfall rate-equivalent radar reflectivity (SR-Ze) relations for 40° slanted and vertically pointing W-band radar. The relations are derived from joint in situ snowfall and remote sensing (W-band radar and radiometer) data from the SAIL site (Colorado, USA) and validated for sites in Hyytiälä (Finland), Ny-Ålesund (Svalbard), and Eriswil (Switzerland). In addition, gauge measurements from SAIL and Hyytiälä are used as an independent reference for validation. We show the dependence of IWC-Ze and SR-Ze on riming, which we utilize to reduce the spread in the IWC-Ze and SR-Ze spaces. Normalized root mean square errors (NRMSEs) are below 25 % for IWC>0.1 gm-3. For SR, the NRMSE is below 70 % over the whole SR range. We also present relations using liquid water path as a proxy for the occurrence of riming, which can be applied to both ground-based and space-borne radar-radiometer instruments. The latter is demonstrated using the example of the proposed ESA Earth Explorer 11 candidate mission WIVERN. With this approach, NRMSEs are below 75 % for IWC>0.1 gm-3 and below 80 % for SR>0.2 mmh-1.