The NWS has just completed a year long project to upgrade all of their radars to what is known as “Dual-Pole.” What this means is that the radar pulses that are transmitted have electrical polarity properties in both the horizontal and vertical axis. Before the upgrade the pulse just had the electromagnetic polarity in the horizontal axis.
This dual polarity allows for an improved examination of the returning particle’s signal to better ascertain its shape and size. The benefit of this additional information is a more accurate radar picture. A better estimation of rainfall rates and amounts, the rain/snow line, the type of precipitation, hail cores, and tornado debris aloft among other items.
One of the new radar products is called the Correlation Coefficient product. What this product is showing is a measure of how similarly the horizontally and vertically polarized pulses are behaving within a pulse volume. In other words, how much pulse change there is from pulse to pulse. The more change means a larger mixture of shapes and sizes in the particles that are scattering back the radar signal.
One of the practical use for meteorologists is to see any possible tornado debris aloft. Any debris being thrown aloft by a very destructive tornado is going to be quite varied in size and shape. The CC values will be quite low compared to say just a uniform rain. The low value will also show in a fairly small circular area, and will match in time and location to the rotational couplet. This match is critical to increase the confidence of the radar interpretation process.
The graphic shows the CC product from the Norman Oklahoma radar. The product time is 3:04 p.m. cdt. The Moore tornado has now been on the ground since about 2:45 p.m. cdt and has intensified rapidly since formation. Debris is now being thrown aloft. This shows in the bright blue circle with the mouse cursor. The radar beam is roughly 900 feet above the radar level. The value is extremely low, at .345. It takes time for the debris to be thrown aloft, and of course it takes some time for the tornado to be strong enough to destroy property and produce debris.
Based on this and other radar data, the meteorologists knew with time a very destructive tornado was on the ground. New radar products like the CC can help meteorologists identify very destructive tornadoes. Weak tornadoes have no debris to throw aloft. The tracking of the storm can be a real help in rural areas where there might not be spotters, or at night when spotters can not see the funnel. It greatly helps in issuing a very emphatic tornado warning that people must seek immediately seek sturdy or underground shelter, their life is at risk.
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