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July 9, 2010  

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Satellite-delivered weather - does it show clouds?   XM-delivered satellite weather radar image

     The satellite-delivered radar image that you view in the cockpit on your portable or panel mounted display is derived from the network of NWS WSR-88D NEXRAD Doppler radars. This is the source of the image, but keep in mind that the product you see in the cockpit is not the same as what comes out of these radars.  But does it show the presence of clouds not producing precipitation?

     Ask yourself this question.  Do you want a radar that shows clouds that are not producing precipitation-sized drops or ice crystals?  Your answer should be no.  The visible and infrared satellite images do a fine job telling you the whereabouts of clouds.  Instead, we want the radar to tell us which clouds are actually producing precipitation in the form of rain, snow, ice pellets, hail and virga.  Clouds would only be a nuisance.

MAF radar image of convective rolls     But from time to time the WSR-88D will indeed show the presence of non-precipitating clouds.  As shown on the left (click here to view a larger image), the radar image from the NWS radar site from the  Midland/Odessa weather forecast office (WFO) shows both precipitation and non-precipitation returns.

     This image clearly shows areas of convective precipitation in extreme western Texas and southeastern New Mexico.  However, the returns shown in the center of the image (around the radar site) include both ground clutter and returns from shallow boundary layer clouds
called horizontal convective rolls. Horizontal convective rolls such as this can produce a ripple-effect you see in the NEXRAD image.   They are produced by counter-rotating vortices that commonly occur in the boundary layer.  Highly capped vertically-developed clouds are produced in the updraft portion between the vortices and are frequently referred to as cloud streets.  So will these convective roll returns be shown on your satellite-delivered weather display?  Not likely.

     The WSR-88D will see anything that intersects the radar's pencil beam and backscatters the energy.  The reflected energy (called a radar return or radar echo) is measured.  Returns that pass a certain backscattered energy threshold are recorded by the NWS radar.  However, as mentioned in the
WxWorx Facility in Huntsville, Ala. beginning, the data that comes from the WSR-88D is not the same as what you will see in the cockpit.  It goes through a process that endeavors to remove all returns that are not associated with precipitation before it is broadcast.

     The XM-delivered satellite weather broadcast is produced by WxWorx located in Huntsville, Alabama (shown to the right).  They run the data from the NWS through a process that includes some filtering.  It is this process that will likely filter out the returns produced
Reflectivity scaleby the convective rolls.

     Initially WxWorx applies a manual gross filter to areas that could not possibly produce precipitation.  An area with clear skies (or nearly so) is not a likely candidate to produce precipitation.  This filter alone will reduce most of the non-precipitation radar returns caused by ground clutter and anomalous propagation (AP) that ordinarily occurs around each radar site when skies are clear (especially at night and during the early morning).  
For the remaining returns, the process is a bit more selective and proprietary.

     WxWorx applies an algorithm that will use both the reflectivity data as well as the velocity data (Doppler portion) from the radar. At a minimum, WxWorx filters all low power returns that fall below 10 dBZ.  This will reduce a good portion of the remaining clutter and AP.  The returns produced by the convective rolls in west-central Texas are all below 15 dBZ based on the reflectivity legend on the left.  In fact, most the returns around the radar site are indeed below 10 dBZ and will be filtered.  The few returns remaining that are 10 dBZ and higher around the radar site will most likely get filtered as well, but in some circumstances this may leak through the algorithm and produce an occasional area of light precipitation that may show up on your XM-delivered satellite display.

     It is also important to note that the hardware/software vendor that receives the broadcast from the satellite (i.e., Garmin, Avidyne, etc.) has the ability to take the filtered data and display it in any fashion they choose.  This means they can filter the data further or even reduce the resolution.         
     What about echo tops?  Echo tops are only generated by the NWS WSR-88D radars for returns that are greater than or equal to 18 dBZ.  The data is also at a 5,000-ft resolution.  Once again, these returns will generally fall below the threshold produced by non-precipitating clouds.

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