Riming-dependent Snowfall Rate and Ice Water Content Retrievals for W-band cloud radar

Accurate measurements of snowfall in mid- 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 (SR) are hard to measure due to their high spatial variability and the remoteness of polar regions.

Here, we present novel ice water content - equivalent radar reflectivity (IWC-Z_e) and snowfall rate - equivalent radar reflectivity (SR-Z_e) relations for 40° slanted and vertically pointing W-band cloud radar. The relations are derived from joint in situ snowfall and remote sensing (radar and radiometer) data from the SAIL site (Colorado, USA) and validated for sites in Hyytiälä (Finland), Ny-Ålesund (Svalbard, Norway), 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-Z_e and SR-Z_e on riming, which we utilize to reduce the spread in the IWC-Z_e and SR-Z_e spaces. Normalized root mean square errors (NRMSE) 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 (LWP) 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, which consists of a conical scanning 94 GHz radar and a passive 94 GHz radiometer. With this approach, NRMSE are below 75% for IWC>0.1 gm⁻³ and below 80% for SR>0.2 mmhr⁻¹.

The proposed IWC and SR relations provide a novel way to reduce uncertainties of IWC and SR estimates for W-band radar by accounting for particle riming. Advantages to current literature relations are the flexibility in terms of viewing angle and the inclusion of LWP, allowing the application to ground-based and space-borne radar-radiometer combinations like EarthCARE or the proposed WIVERN mission.