Super-Res Storm Relative Velocity

RadarScope 4.7 adds a long-requested requested radar product: super-res storm relative velocity. It’s available to all customers on all platforms in RadarScope 4.7. This post explains what it is, how it’s made, and how you can use it.

What is super-res storm-relative velocity? It is the super-res velocity data with the average storm motion (direction and speed) subtracted. It is also valid to describe it as the DTN-created super-res version of the classic storm-relative velocity provided by the NWS.

How is it calculated? The basic technique for calculating the storm-relative velocity is to determine the component of the average storm motion toward/away from the radar for each azimuth and subtract that value from the measured radial velocity for each pixel along the azimuth. The value of the average storm motion is provided as metadata in the classic storm-relative velocity product provided by the NWS. In this way, we get the same result as the NWS product, but with more detail. In a simple example, consider a measured velocity of 30 knots (outbound at 30 knots) in a pixel directly east of the radar. This indicates a wind blowing east at 30 knots. Let’s assume the average storm motion is due east at 20 knots. When we do the subtraction at this pixel, we end up with a storm relative velocity of 10 knots, because the wind is only 10 knots faster than the storm motion.

What are the advantages of this product? Relative to traditional super-res velocity, it makes it easier to see rotation in fast-moving storms because it subtracts out the storm motion. In the following image, the two circulations near Leon and southwest of Pleasanton are easier to identify (green/blue next to red/pink) in the storm-relative velocity image, even though it is lower-resolution.

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The classic storm-relative velocity product that has been in the app for a long time also has this advantage relative to the traditional velocity data. However, the new super-res version has a lot more detail (256 intensity levels vs 16, 250-meter range resolution vs 1 km, 0.5 degree azimuthal spacing vs 1 degree). Notice the difference in the following image.

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When should you use super-res storm-relative velocity? In short, any time you would have used the classic storm-relative velocity, you should use the super-res version to take advantage of the increased detail. The times it is particularly useful are when you are looking for rotation in storms that are moving rapidly toward or away from the radar. The same rules of which patterns of green and red correspond to cyclonic rotation, anticyclonic rotation, convergence, or divergence based on the relative position of the storm and the radar still apply as they do to all other radial velocity data.

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