Weather hazards – Hail detection and mapping by modern radars
by Daniel Joita P.E. – Senior Forensic Engineer
Applied Technical Services, Inc.
Traditionally, the National Weather Service received hail events reports through the usage of World War II adapted radar technology. Each hail record from NWS contained the time, coordinates, diameter, county, and the number of fatalities and injuries caused by each event. Hail reports typically came from specific observers, so no information on the path and extent of the hail swath was available.
Modern hail mapping technology based on dual polarization radars improved the amount and the quality of hail-related information making “hail mapping” possible through algorithms and extrapolation. The radar can look at the heart of the storm providing valuable information, but one should be cautious when interpreting data. Automatic hail tracking products assemble data into reports based on algorithms and software without human intervention and are subject to data-based uncertainty and limitations.
How is hail produced
Updrafts carry raindrops into freezing high areas of the atmosphere where they freeze into balls of ice forming hailstones. Frozen droplets fall and are also carried down by cold downdrafts being picked up again by warm updrafts and carried back to cold air and re-frozen. With each trip from lower to higher regions from warm to cold air, water droplets add another layer of ice. When hailstones become heavy enough, updrafts can no longer lift the multilayer frozen droplets, and hailstones are pulled by gravity and finally fall on the ground. The upper regions contribute to the milky white colored rings of lesser density, while the lower regions create transparent rings of ice where precipitation in liquid form is present. Hail falls in paths known as hail swaths. Per NWS, hail swaths can range in size from a few acres to an area 10 miles wide and 100 miles long. Hail swaths average 1 mile wide and 6 miles long.
Typically, the larger hailstones fall out closest to the updraft and within a small region of the hail swath. The smaller hail falls throughout the hail swath and accounts for the most significant volume. Comparison with a known object can estimate hailstone size. Most hailstorms are made up of a mix of sizes, and only the largest size in the mixture poses severe risks of damage.
Hail Size Chart
Pea < 1/4″ Marble = 1/2″ Dime / Penny = 3/4″ Quarter = 1″ (quarter size hail or larger is considered severe) Ping-pong ball = 1-1/2″ Golf ball = 1-3/4″ Tennis Ball = 2-1/2″ Baseball = 2-3/4″ Softball /Grapefruit = 4″
How is hail detected
Ever since World War II, radar operators noted precipitation feedback while searching for enemy aircraft. Scientists studied the phenomenon since and adapted the technology to detect hydrometeors. Traditionally weather radars have used horizontal scanning techniques to identify the location and intensity of hydrometeors in the atmosphere. The radio wave traveled in the horizontal dimension and received only horizontal measurements of an object.
Modern Dual-Pol technology radars can simultaneously transmit and receive both horizontal and vertical polarizations. The advantage is to improve the accuracy of the precipitation estimates and the ability to distinguish between heavy rain, hail, snow or sleet at a site. Dual polarization gives meteorologists a better sense for the size and shape of the atmospheric targets detected by the radar.
The dual-pol radar algorithms were developed by NWS to categorize radar data into different types of liquid or frozen precipitation, and to estimate the amount of precipitation. The estimates were improved by approximately 20%. However, the current technology will not provide exact precipitation type at ground level. Algorithms can be used to estimate the precipitation type and size at the ground level. Dual-pol radars can: – detect multiple forms of precipitation – discern debris inside tornadoes (studs, shingles, insulation), – discern flying biological targets (birds, bats, insects, etc.) What automated hail tracks disclaimer should read (the fine print)
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.
The upgraded radars can directly discern and discriminate between areas of heavy rain, hail, sleet and snow based on collected data about hydrometeor shape, density, and variety. 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 just ignored).
Hydrometeor (Any product of condensation or deposition of atmospheric water vapor)
Discrepancies between hail detection algorithm output and ground conditions:
– hail track maps can indicate an artificial degree of certainty and detail
– hail melts as it falls out of a storm
Precise hail swath maps will show smooth lines with accurate hail zones. However those “precise” lines are the result of smoothing and interpolation, so the algorithm’s attempt to “fill in the blanks.” The exact location and hail size zones are not absolutes but best guesses.
Hail aloft is different than hailstones at the ground level. Hail is produced in the cold upper regions of the storm, but as soon as it falls beneath the freezing level, it starts to melt.
The higher the ground level (closer to the higher freezing altitude), the less melting will occur, since hail won’t have time and distance to melt before reaching the ground. This is why in Colorado large hailstone size can occur at the ground level.
The lower the ground altitude (closer to the warmer sea level) hailstones have plenty of time and distance to melt before reaching the ground. That’s why in Florida small hail size is usually expected and encountered.
The initial size of the hailstones – smaller hailstones melts faster. The smaller the hailstone, the faster will melt before reaching the ground. Hailstones that are initially 0.5-inch diameter or less often melts entirely before reaching the ground.
Each storm is unique
No two storms are alike, and the algorithms are used to describe most but not all conditions. A single algorithm cannot capture the interpretation of the complex data. An algorithm that provides an accurate estimate of hail size in one type of storm is not guaranteed to deliver similar performance with all storms.
Generally, hail detection and hail size algorithms are useful first-alerts, are reliable tools for the hail aloft, and can distinguish between large vs. small hail. However, algorithms are less precise at estimating exact hail size on the ground or providing street-level detail of the location of hail swaths.
In conclusion, an automated hail track report should be taken with a grain of salt. A forensic meteorologist can be consulted as he can provide a detailed analysis of dual-polarization data, considering those factors that computerized hail track products typically don’t: distance to the radar, melting considerations, comparison with on-the-ground hail reports and data reliability.
If you have any questions or would like to further discuss this topic, please contact Daniel Joita P.E. – Senior Forensic Engineer at firstname.lastname@example.org or call him at (714) 588-1012.
NSSL Hail Basics
NSSLTools: Dual Polarized Radar
Megan D. Walker-Radtke,Automated hail tracks