Detecting Hail
Hail can be detected using radar. On Doppler radar, hail generally sends a return signal that looks like extremely heavy rainfall.
Techniques for getting popular WSR-88D cell-based hail information from the Hail Detection Algorithm (HDA) have been incorporated into high-resolution gridded products that allow a user to diagnose which portions of storms contain large hail. Hail size data are accumulated over time to provide precise hail swath maps, showing both maximum hail size by location, and hail damage potential (a combination of hail size and how long the hail has been falling). This lets the forecaster have a "quick look" at the direction the hail swath will likely take and how severe it will be. In addition, verification efforts can be better focused on areas where hail was most likely to be observed at the ground.
Polarimetric radar (radar that sends horizontal and vertical pulses) can help discriminate between hail and rain, and even determine hail size. Another important discovery is that polarimetric variables can help pinpoint the location of hail within a storm. A new algorithm, using polarimetric data, has been developed, called the HCA (hydrometeor classification algorithm).
HOW DOES NSSL CONTRIBUTE?
Past
NSSL developed a Hail Detection Algorithm used
by the NWS WSR-88D radars in the late 1990's. This algorithm indicated
whether or not a storm cell was producing hail. An enhanced Hail Detection
Algorithm was developed several years later, which estimates the probability
of hail (any size), probability of severe-size hail (diameter greater
or equal to 19mm), and maximum expected hail size for each detected
storm cell. A new parameter, called the severe hail index (SHI), was
developed as part of the suite.
Present
NSSL improved the original single-radar
based HDA into what is known as the Enhanced Hail Diagnosis Algorithm
(EHDA), which uses multiple radars. This improved hail diagnosis integrates
the traditional reflectivity radar information with velocity radar
information (for rotation and storm-top divergence) from several radars,
as well as near-storm environment data from a mesoscale model.
Future
NSSL engineers are working to develop polarimetric
technology to be
added to the current WSR-88D Doppler weather radars used by the NWS
This technology will be available for installation into the national
radar network in five to 10 years. Polarimetric radar can identify
the specific location of hail, rather than the probability of hail
at any particular location within the storm.
Dual-polarized radar not only supplies information for hydrometeor
classification, but also proves useful for gauging hail size within
the storm. NSSL continues to investigate the relation of polarimetric
variables to hail size, which will be beneficial in improving warnings
for hail- producing storms, understanding the physical processes that
lead to hail formation, and determining possible early signatures associated
with hail formation and growth.
WDSS-II Hail Detection Algorithm – NSSL's cell-based Hail Detection Algorithm (HDA) helps users diagnose the probability of severe hail and hail size using combined multiple radar data and near-storm environment data from mesoscale models. The user can also determine which portions of storms contain large hail. Hail size data are accumulated over time to provide precise hail swath maps, showing both maximum hail size by location, and hail damage potential (combination of hail size and duration of hail).
SHAVE – The Severe hail Verification Experiment was conducted over the summer of 2006 to evaluate the performance of a multi-sensor, multi-radar hail detection algorithm (CONUS WDSSII) and to use the high-resolution verification data in the development of techniques for probabilistic warnings of severe thunderstorms.