That’s Not Rain

Oklahoma has gotten a lot of rain this year, and it looks like it is raining again this afternoon along the Red River near Frederick.

kfdr ref kfdr vis

However, there is hardly a cloud in the sky, as shown by the visible satellite image (right).

So it’s not raining there. What is the radar seeing, then? The answer is lots of grasshoppers, beetles, other bugs, and probably some birds, collectively referred to as “biological scatterers”. This type of radar return is very common during the warm months of the year, though today’s echoes are considerably stronger than normal. The yellow areas in the reflectivity image represent values between 30 and 40 dBZ, about the same as moderate rain.

We can use the dual-polarization variables to differentiate these biological echoes from actual rain. Return from bugs and birds is characterized by very high differential reflectivity (indicating objects that are wide but not very tall) and low correlation coefficient (objects of different size/shape/material mixed together) relative to typical values for rain or snow. The following images confirm that the radar echoes are the result of biological scatterers.

kfdr zdr kfdr cc

The differential reflectivity values (left) are near the top of the color scale, with most greater than 7 dB. Typical values for rain are 0 – 5 dB. The correlation coefficient values range from 0.7 (blue) to 0.95 (red). Typical values for rain are 0.97 – 1.0.

For the most common purpose of weather radar, knowing where and how hard it’s raining, it is often desirable to remove these biological echoes because they give a false indication of rain. The standard approach to this using dual-polarization data is through a Hydrometeor Classification Algorithm (HCA), which decides the most likely type of object present (rain, hail, biological, etc.) by comparing the observed values of the dual-pol variables to typical values for each classification. Regions that are identified as biological scatter or ground clutter are thenĀ removed from the final radar images. Below are the outputs of the National Weather Service HCA (left) and WDT’s POLARIS algorithm (right).

kfdr hca (nws) kfdr hca (wdt)2

The gray areas in both images are correctly identified as biological scatter. The green and yellow areas are misidentified as light rain and “big drops”, respectively. The “big drops” classification generally has the highest differential reflectivity and lowest correlation coefficient. It is most often observed in specific parts of thunderstorms. The misidentification by the NWS algorithm was likely a result of higher reflectivity values than are considered typical for biological echo. In these two images, it is clear that WDT’s algorithm did much better at correctly identifying the echoes as biological and filtering them out.


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