An odd thing has been happening over the past several years: property insurance hail claims in many parts of the country have been increasing, but official National Oceanic and Atmospheric Administration (NOAA) hail reports have not. If no more hail has been falling, why are more people filing claims for hail damage?
One prominent reason for the increase in hail claims is the increased prevalence of automated hail track products. You've probably seen them or used them yourself: digital maps with hail swaths color-coded to indicate hail size, available online for instant download. These products are offered by a number of different companies and are heavily marketed to contractors as well as to insurance adjusters. It's an equal opportunity sales approach: pitch the roofer who will encourage the homeowner to file a claim, as well as the insurance adjuster who will investigate it.
The marketing is not subtle. One prominent hail map provider advertises, "Learn how contractors are using [our product] for explosive sales growth!" As you can imagine, that explosive sales growth is not being funded out-of-pocket by property owners; rather, it's being funded by a parallel growth in insurance hail claims.
All modern, automated hail track products are built upon the same fundamental data: output from National Weather Service (NWS) dual-polarization weather radar processed through a hail detection algorithm. Since these products all rely upon dual-polarization radar's ability to distinguish different precipitation types, they are all subject to the same data-based uncertainty and limitations.
This is not to say that such hail track products aren't valuable. They are: they provide a convenient and generally accurate account of where hail is being produced within a storm. Meteorologists, both operational and forensic, often use hydrometeor classification algorithms (techno-speak for algorithms that offer a best-guess as to the type of precipitation occupying a given section of sky) as a first step in identifying likely regions of hail production, although we take the algorithm output with a healthy grain (or two) of salt when conditions warrant.
Dual-polarization radar (bottom) collects data about both the horizontal and vertical characteristics of precipitation. Since different precipitation types have characteristic shapes and movement, this two-dimensional information can be used to distinguish between different precipitation types. Source: National Weather Service
|Dual-Polarization Radar: The basics
By collecting information from two dimensions instead of one (hence the "dual" in dual-polarization), dual-pol radars can provide sophisticated information about the shape, density and variety of precipitation particles within a storm.
Such detailed information wasn't always available. Before the National Weather Service upgraded its network of NEXRAD radars to dual-polarization operation during 2011-2013, those radars were single-polarization and collected only one-dimensional information that did not explicitly reveal hydrometeor properties. Radar algorithms had to infer precipitation type based on characteristics like storm structure and the atmospheric freezing level.
Dual-pol changed that. The upgraded radars and their likewise upgraded hydrometeor classification algorithms don't have to rely on such inferences. Instead, they can directly discern and discriminate between areas of heavy rain, hail, sleet and snow based on collected data about hydrometeor shape, density and variety.
So dual-pol radar is a powerful tool, but like any tool, it can be abused when its limitations and inherent uncertainty are not well understood (or simply ignored). Which brings us to the dark side of dual-pol.
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