Hail Caps and Rain Rates

One of the bigger challenges with estimating precipitation is when hail is present. The presence of hail gives a different signal in many radar products, which impacts the ability to use radar to accurately provide a liquid accumulation value. In the operational version of MRMS, a hail cap is used in identified hail cores (i.e., where the Maximum Expected Size of Hail, or MESH, product is non-zero) by capping the precipitation rate a 2.12 inches per hour. While the hail cap was designed to prevent some overestimations of QPE, it has also created some significant underestimations as well.

In the upcoming build of MRMS, two changes will be made to mitigate the issues of hail contamination. The first is in the creation of the new MRMS dual-pol synthetic QPE. Instead of using reflectivity to estimate precipitation, hail cores will utilize specific differential phase (KDP) to better estimate rainfall in these convective storms. The image below shows how the hourly rainfall estimation is improved using this technique. The second is to reduce the impacts of the hail cap for MESH with the reflectivity-based approach by linearly increasing the influence of the hail cap from MESH = 0.01 inch per hour to MESH = 1.00 inch per hour. This would be applied in areas where radar coverage is degraded or if there are technical issues impacting the dual-polarization products from a radar.

One-hour QPE ending at 1100 UTC 14 June 2018 for the operational Z-only MRMS radar QPE (left) and the experimental MRMS dual-pol synthetic radar QPE (right). The gridded MRMS QPE values are compared against gauge E7617.
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