Building and Testing a GPM Passive-Microwave Hail Retrieval and Climatology

Severe hail, though a major contributor to global agricultural and infrastructural losses, presents a significant challenge to precipitation retrievals. The rarity of severe hail events and the difficulty in applying a uniform approach globally to countries with different reporting standards and radar networks drives the need to construct global hail climatologies using spaceborne platforms. The sensitivity of passive-microwave radiometers to scattering by hail, particularly in the channels near 37 GHz, make satellite-borne passive microwave datasets ideal candidates for hail retrievals, and this relationship has been used to develop hail proxies for the last several decades.

Using 17+ years of Tropical Rainfall Measuring Mission (TRMM) and 4.5 years of Global Precipitation Measurement (GPM) data, we aim to refine passive-microwave hail proxies further by constructing a multi-frequency passive microwave hail retrieval and training it on TRMM data matched with surface hail reports in the United States. We then, after careful consideration of differences in latitudinal extent and instrument footprint size, apply this hail retrieval to GPM data to create a near-global climatology.

We then test the effectiveness and regional variability of our passive-microwave hail retrieval using coincident GPM dual-frequency precipitation radar (DPR) Ku-band data. We aim to create a retrieval that, for a given estimated probability of hail, encompasses a narrow range of radar reflectivities at temperatures below freezing, has little geographical variability throughout the satellite domain, and does not erroneously retrieve hail over snow- and ice-covered surfaces.

Testing our retrieval against others in the literature, and against other available passive-microwave channels and metrics, we find that using the 19 and 37 GHz channels in combination, and normalizing by the height of the troposphere, provides the most effective estimation of hail probability, and leveraging the 10 and 85 (89 for GPM) GHz channels against each other provides an effective screen for snow- and ice-covered surfaces. A retrieval that employs these four channels simultaneously we believe provides our best and least geographically-biased passive-microwave hail climatology within the near-global GPM satellite domain.