Investigating the Potential for Rimed Mass Retrieval Using Polarimetric Cloud Radar Observations

Ice-related microphysical processes play a central role in global precipitation formation, but remain complex and difficult to quantify. Among these, riming, a mechanism by which supercooled liquid droplets freeze onto falling ice particles, significantly influences precipitation properties (essentially particle’s mass and fall speed). However, accurate measurements of riming remain difficult to obtain due to the limitations of remote sensing.

Current radar-based riming retrievals mostly rely on the higher fall velocity of rimed particles of the vertically-pointing radar Doppler spectrum. However, these retrievals are easily disturbed by vertical air motions and turbulence limiting their applicability in e.g. complex terrain.

Here, we explore the potential of slanted polarimetric W-band cloud radar observations for quantifying riming. Slanted view gives the opportunity to extend the retrieval capability over larger areas and more importantly enables the potential of polarimetry in case of horizontally aligned hydrometeros. As a reference, we use a normalized rime mass retrieval approach combining a 94 GHz Doppler cloud radar and in situ snowfall measurements from the Video In Situ Snowfall Sensor (VISSS). The instrument was deployed during the Surface Atmosphere Integrated Field Laboratory (SAIL) campaign.

Our analysis shows a strong relation between the normalized rime mass, column-integrated radar reflectivity (Ze), and the differential phase shift (ΦDP) from slanted polarimetric W-band radar observations. This suggests that polarimetric radar measurements, particularly the combination of Ze and ΦDP, can be used to estimate rimed mass with comparable performances than the joint use of radar and direct in-situ measurements.

This finding is particularly relevant in view of the forthcoming selection of ESA's Earth Explorer 11 mission, with WIVERN (WInd VElocity Radar Nephoscope) as one of the final candidates. WIVERN includes a conically scanning 94 GHz Doppler radar with polarimetric capabilities. Our results indicate that the correlation between ΦDP and Ze could potentially be used to retrieve rimed mass from spaceborne observations, opening a new way to study ice-related microphysical processes on a global scale.