Hydrometeor Classification (HC)

Hydrometeor Classification (HC) is an algorithm to identify the predominant hydrometeor in the radar beam. The pre-defined categories recognized under this classification are as follows:

• BI- Biological (birds, insects)
• GC – Ground clutter (buildings, trees)
• IC – Ice crystals
• DS – Dry snow
• WS – Wet snow
• RA – Light/moderate rain
• HR – Heavy rain
• BD – Big drops
• GR – Graupel (soft ice, snow pellets)
• HA – Hail-rain
• UK – Unknown
• RF – Range folded

The Hydrometeor Classification product should be used in conjunction with other data for proper interpretation, as it is merely an algorithm and not an absolute indicator of what is occurring at a particular location. As currently implemented, the algorithm determines only the most likely type of hydrometeor, omitting information pertaining to the likelihood of other categories.

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Hydrometeor Classification Algorithm (HCA)

The HCA is a complex algorithm performed by the NEXRAD that attempts to determine the most likely type of the echoes in the radar beam. It combines several base moments (reflectivity, velocity, ZDR, CC, KDP) with the environmental data (the Melting Layer levels) to produce 10 hydrometeor types:

HA	Hail
GR	Graupel
BD	Big Drops
HR	Heavy Rain
RA	Rain
WS	Wet Snow
DS	Dry Snow
IC	Ice Crystals
GC	Ground Clutter
BI	Biological

The most important thing to remember about the HCA is that it is an estimate of the type of hydrometeors at the height of the radar beam, not the type of precipitation you would see on the ground. Since the radar beam is at an angle to the ground, its height increases with distance from the radar site. Eventually, even the lowest tilt will enter the melting layer. This results in a ring-like structure of type classes centered on the radar site.

I strongly recommend watching the WDTB presentations on Dual Pol so that you can better understand the HCA, how its types are determined, and its limitations:

http://wdtb.noaa.gov/courses/dualpol/index.html