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Radar Tutorial
Chapter 1, Page 1: Base Reflectivity
NEXRAD radar, model WSR-88D, is a monumental improvement over previous generations of radar the National Weather Service (NWS) has been using. The greatest advantage of NEXRAD is its ability to see mesocyclone development (rotation) in the middle levels of a thunderstorm. See Chapter 2.
This tutorial is designed to give the general public a little more knowledge than they currently have about radar images they may see on TV or the Internet. Specifically, it is hoped that someone interested in severe weather spotting for the NWS in the Skywarn Program will come away with enough information about Base Reflectivity Radar to enable use of the images in support of weather spotting.
The reader is cautioned that this discussion will not constitute anything more than a general introduction. There is no replacement for the official reports from the NWS via watches and warnings. Nor should one assume that this will in anyway approximate the kind of training one can receive from university meteorology programs or from your local NWS Skywarn trainer, the Warning Coordination Meteorologist. This tutorial is for non-commercial use.
Radar images are available free with a 30 minute delay at www.wsi.com. You can subscribe to a NIDs service like www.weathertap.com for about $10 a month and receive both reflectivity and velocity images virtually live time. There are many services on the Internet. The one's I've mentioned are the one's that I use.
Introduction
Radar images are color coded to indicate the strength of the reflected radar signal. A strong reflected signal will be
colored differently than a lower one. The amount of reflective signal is a relative indicator of rain or hail intensity.
The greater the precipitation intensity, the greater the downdraft and the greater will be the updraft of the
thunderstorm to support it. Intense downdraft can be manifest in "downbursts" which cause severe and damaging straightline
winds. Intense updraft can lead to the development of a wall cloud, funnel, or tornado. Thus radar reflectivity gives a
relative indication of the intensity of the a thunderstorm and the potential for severe weather.
To the left you will see the reference key for WSR-88D or NEXRAD radar. You'll find this same key on radar images at WSI Corporation. The colors indicate radar signal reflectivity in dbz. The higher dbz the greater the reflected signal. Intense precipitation often associated with severe weather will produce returns in the red part of the scale or more. At the top of the key image you'll see a three letter abbreviation for the particular site. A list of sites by state and city with the three letter abbreviation is available here. Also listed is mode, in this case precipitation. The other mode you are likely to see is clear air mode. This is a much more sensitive mode used when there is no precipitation to see cloud formations. Be sure you don't confuse the two modes. See image below for an example of a clear air mode NEXRAD radar image. There are some obvious differences because in clear air mode, even group clutter around the center of the image is in the yellow and red range.
All of the following images in this chapter are in the precipitation mode. See this page for several links with more detailed information about NEXRAD radar.
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