Monday, September 19, 2016

Fooled By Ground Clutter

At the end of the previous post I referred to the radar data from Yuma and San Diego. I thought that storms had developed over the mountains of far southern California, based on a very quick perusal. I went back and took a second look and discovered that I'd been fooled by ground clutter from the Yuma radar (KYUX). The clutter may have been amplified by an inversion at the top of the moisture surge layer this morning. Regardless, I show some examples below from 1440 UTC to illustrate how caution is advised whenever one attempts to interpret radar data - all examples are from the CoD web page (link to right), which offers a nice variety of NEXRAD products to their users.

The KYUX VAD time series is shown above - it appears the top of the surge layer is about 2,000 ft ARL (above the radar elevation).

Above is KYUX base scan reflectivity, but when the 2.5 degree scan is examined (below), there is no echo over southern California. The area of high reflectivity at long range to the northwest is ground echo from the San Bernardino Mountains (these show up more frequently in the KYUX data).

The echo-top product is shown above (note that the high tops out by Palm Springs probably indicate that the radar's first side-lobe is also returning signal from the mountains). Although it is of little consolation, the ground echo is also fooling the dual-polarization precipitation type algorithm, which is indicating rain over southern California (below).

Finally, the storm-total precipitation product indicates that the precipitation algorithm has failed also in the clutter area, with totals as high as 3 to 4 inches indicated through 1440 UTC.

So it goes.

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