Use of WRF-HAILCAST to Produce a Dynamically Downscaled Hail Climatology

Identifying frequency of severe hail events, specifically those greater than or equal to 25 mm, is key to producing hail climatology records across the contiguous US (CONUS). However, a climate record for severe hail is difficult to generate. Severe weather reports are naturally biased by population and only within the last decade have radar-based estimates of hail size become available. A key to increasing forecasting skill and mitigating claims against property and agricultural damage that total billions of dollars is understanding the progression and distribution of hail events during the spring months. A potential method to address this issue is the use of a convective allowing model, the Advance Research-Weather Research and Forecasting (WRF-ARW) model, to dynamically downscale a coarser-resolution climate model. For this study, a hail size field is produced by using WRF-HAILCAST to downscale ERA-Interim data over CONUS for the 1 April – 30 June period over the years 2011-2018.

Hail event days, where an event day is defined as a day where hail was forecast or observed of at least 25 mm in size, were totaled over 1°x1° latitude-longitude boxes for monthly (April, May, June) and seasonal (April – June) periods. WRF-HAILCAST’s output was evaluated from 2011-2015 using the Verisk Insurance Solutions Respond radar-estimated hail size product, and from 2016-2018 using the Multi-Radar Multi-Sensor (MRMS) Maximum Estimated Size of Hail (MESH) product. Preliminary results show that progression and distribution of hail event days in radar-estimated products and WRF-HAILCAST during both monthly and seasonal periods are generally in agreement. For each of the monthly averages, radar-based estimates and WRF-HAILCAST can track the progression and distribution of hail events days as large hail moves from the Southwestern states in April to the shift over the Midwest in May and June and the progression of large hail back towards the Southeastern states in June.