Dual-Polarization Radar Detects Severe Weather in the South

The severe weather that tore through the Southern United States during the week of January 22, 2012, provided us one of our first opportunities to see the advantages of dual-polarization weather radar in ESP:LIVE™.

Dual-polarization technology will not change standard radar products like reflectivity and velocity, but new products available with dual-pol will provide unprecedented insight into high-impact weather. Detection of giant hail, debris from damaging tornadoes, better flash flood warnings, and even identification of some winter precipitation is now possible.

A storm produced golf ball size hail just north of Colt, AR – about 40 miles west of Memphis. The Memphis radar is one of only a handful presently capable of displaying dual-polarization data.

Correlation Coefficient

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Correlation Coefficient

Correlation Coefficient (abbreviated “CC”) – detects the diversity of objects that the radar detects. Lower values (highlighted in orange and yellow) that are co-located with thunderstorms can indicate the potential of hail, or hail mixed with raindrops, as in the example shown above. Correlation Coefficient can also be used to detect debris tossed about by a destructive tornado or even a flock of birds flying high above the radar.

Since the radar often detects objects such as hail well above the ground, it’s important for the meteorologist to know if the hail is melting in the sky or slamming into crops and cars before it melts.

Differential Reflectivity

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Differential Reflectivity

Differential Reflectivity (abbreviated “ZDR”) – detects whether the object is shaped vertically, horizontally, or spherically. Gray and blue colors indicate that the radar is observing objects that are shaped like a sphere, while oranges and reds mean that the object has a horizontal shape. Meteorologists already know liquid water droplets have a greater horizontal shape, meaning that the hail is melting in the sky or mixed with raindrops, as seen on the right-hand side of the spotlight. However, on the left hand side, dry hail is falling because it appears to be “tumbling” to the radar. This tumbling suggests a spherical shape, meaning the dry hail is likely to make it to the ground without melting. Spotters reported hailstones of golf balls to the north and west of Colt, confirming what we see in the dual-polarization data!

You may ask if an easy-to-use product has been developed that can look at these seemingly complex variables to tell us the most likely type of precipitation falling from the sky. Indeed, there is.

Hydrometeor Classification Algorithm

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Hydrometeor Classification Algorithm

Hydrometeor Classification Algorithm (abbreviated “HCA”) – detects the most likely type of object falling from the sky. It’s not always precipitation (but it usually is). Our analysis of the other dual-polarization products is backed up with what we see in the HCA. Areas of red show where the radar is detecting hail, while dark green indicates heavy rain. Hard-core meteorologists will also examine other dual-polarization radar moments, standard radar products (such as reflectivity and velocity), along with their experience and understanding of the environment that day to provide the best possible estimate of what’s actually happening. With dual-polarization capability comes another set of tools that will take months, if not years, of practice to fully utilize.

Specific Differential Phase

Specific Differential Phase

Standard Reflectivity

Specific Differential Phase (abbreviated “KDP”) – left image on the following page – detects a heavy rain shaft associated with thunderstorms moving through Eastern North Carolina. Standard reflectivity (on the right) shows lots of yellows and oranges, which might indicate heavy rain, but sometimes can indicate hail or even melting snowflakes. KDP can only detect the concentration of water droplets, meaning that hail contamination and bright-banding won’t fool meteorologists into believing there’s heavy rain falling, when there really isn’t. In this case, high KDP is detected – giving meteorologists absolute certainty that heavy rain is occurring. KDP will be a powerful tool for detecting the potential for flash flooding.

Your Quick Guide to Dual-Polarization Products

Product Level 2 or Level 3 Level 2 Resolution Level 3 Resolution
Correlation Coefficient Both 0.5 deg x 250 m 1.0 deg x 250 m
Differential Reflectivity Both 0.5 deg x 250 m 1.0 deg x 250 m
Differential Phase Level 2 Only 0.5 deg x 250 m N/A
Specific Differential Phase Level 3 Only N/A 1.0 deg x 250 m
Hydrometeor Classification Level 3 Only N/A 1.0 deg x 250 m

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