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NEXRAD Weather Radar Expert Speaks Out






NEXRAD Weather Radar Expert Speaks Out
An Analysis of the So-called "Anomalous" Radar Images on the Enterprise Mission Website

RE: Radar Images at (The Truth is Out There - and We Found It.)
See also:
Turret Peak Event – Radar Analysis

One of the reasons I was reluctant at first to go into great detail about this matter is the complexity of the WSR-88D radar and the various weather and internal systems phenomena that can cause weird looking things to appear. I knew that if I came out with only partial explanations that the things I couldn't fully explain would be considered "beyond the explanation of scientific experts" and used to bolster the points that the above web page tries to make. The risk for that is still present, however, as I am skilled at weather radar interpretation only, which does not necessarily make me any kind of expert on the exact nature of patterns which could be non-meteorological. Still, I should be able to eliminate many of those patterns from the "unexplainable" column. Suffice it to say that I've outright seen almost all of those radar patterns before. In cases where I haven't, I believe I have seen iterations of them and can therefore provide some plausible insight.

I want to make it clear that I am not trying to debunk that grander theory behind what the producers of the web page are getting at. I have listened to Art Bell's show on numerous occasions, and have to say that some of the findings there are too eerily coincidental to be pure bunk. I am not an expert on any of the subjects and phenomena discussed on the show and so can't comment on them with any scientific plausibility. However, I am an expert at weather radar interpretation and so will speak my piece accordingly. I will also add, for those of you who might think I'm another government plant to play spoiler, that I myself have seen unexplained phenomena in the clear night skies of New Mexico. I do not comment to debunk, merely to keep the conversation within the realm of the unbiased scientific process. I suppose you could say I'd like to play Scully to the above web page's apparent Mulder!

Complexities of Meteorological Radar Systems

Prior to about 1990, the vast majority of radar data in North America came from simple analogue radar systems that were developed in the mid 1950s and updated in the mid 1970s. Simple blasts of EM energy were sent out, and the strength and "rebound time" of the return pulses were analyzed to determine target strength and range. "Anomalous" patterns were frequent with these systems, owing usually to military chaff drops, nearby mountain ranges, and atmospheric density discontinuities which caused abnormal refraction of the radar beam. NWS Meteorologists at the time played 100 % of the role in differentiating between "anomalous" and real returns. (By the way, "anomalous" in weather radar speak simply means that the returns are not caused by precipitation or clouds. Thus, returns from mountain ranges, though completely valid and scientifically explainable, would be considered anomalous.)

With the implementation of the WSR-88D NEXRAD system in the early and mid 1990s, this changed completely. The new systems are much more powerful, much more sensitive and almost completely automatic. The new systems attempt to keep anomalous returns (called AP for Anomalous Propagation of the radar beam) to a minimum by using complex algorithms and filtering techniques. These same algorithms, however, can also block out or impair detection of meteorological targets if not applied properly, causing some very unnatural looking patterns. The radars can also measure the velocity of radar returns using the Doppler theory. Because of this, full color images of target velocity (measured inbound and outbound along each radial) are available for every elevation slice. This is what allows us to actually see and measure the "spin" within a tornado, as well as see it's shape! Cool, huh!?

So now we have a huge, new nationwide radar network with incredible capability. Things began to be seen on radar that had never been seen before. With time, they have almost all been fully explained. Swarms of bees, clouds of dust, migrating birds, airliners, highway traffic, subtle atmospheric boundaries (fronts), inversions, hail "spikes", cloud layers, virga (evaporating rainfall) and sunrise/sunset returns are just a few examples of phenomena that were initially unexplained. Those are just the things that have made it through the signal processing intact. There is an array of other AP phenomena that occur purely due to the automatic processing of the return radar signal. These can occur due to noise/feedback within the system and improper application of clutter suppression and attenuation applications/algorithms, among other things.

So now we have two confirmed sources of AP to start with: actual non-meteorological physical phenomena and errors inserted by the radar system itself. Then you have to consider what you're looking at on these Internet images. Every network radar operates independently on a 5 to 10 minute cycle in which in an entire volumetric sample of the atmosphere is obtained within 248 nautical miles of the radar site. This is a called a "volume scan". A series of providers (WSI/Intellicast is one) take all these individual pictures and run their own algorithms on them to make a composite picture. Since a lot of radars overlap, I can only wonder at how the providers handle conflicting data between radars. It gets even more complex when you attempt to produce a "composite loop" of radar images. Imagine over 150 independent, complex systems meshed together by a computer application to produce a regional radar loop. (If you ever see a loop on the Weather Channel, you will note that the picture seems to shimmy and move irregularly around each radar. That is the result.) Curiously, regional loops from Intellicast on the Internet don't seem to do that nearly as much. Perhaps that company has even further processed the signal to make it seem like all the radars are acting as one? As you can see, by the time you get your Internet or TV composite or loop, chaos has reigned supreme! You've taken a signal with inherent AP, processed it through an individual radar that may then add (or remove) more AP to the equation, and from there sent it into the hands of private companies who do god-knows-what to it to make it look prettier for us!!! So, on the one hand, this would make analysis of any of the images on the web page dubious at best. On the other hand, the distinct patterns in some of the images should be associated with something, as they either made it through several processing phases unscathed or were modified as a whole right before dissemination to the world. That last part is the unknown to me here. It seems necessary to examine further if any of the patterns shown could have come from final processing on the provider's end before final display of the product.

Analysis of Images at as of 1/8/99...

Note: The following analyses were made without distinct knowledge of local weather conditions, topography, internal radar systems calibrations and settings or even simple "knowns" such as the exact nature of the image displayed. The WSR-88D produces at least a dozen separate products for as many as 14 separate elevation slices every 5 to 10 minutes. That's on the order of 1,000 distinct radar generated products every hour, but we're seeing only one per hour with the images on this page. Looking at just one unknown image for a detailed analysis is like having a mechanic listen to your car idle to assess engine trouble, without ever opening the hood. The composite maps shown on this web page and by most media outlets are usually either Base Reflectivity (0.5 degree beam elevation) or Composite Reflectivity (maximum reflectivity at each data point in entire volume scan). I will use the information provided with the image, as well as my personal knowledge of the area, to provide the most detailed scientific analysis that I possibly can given those resources.

*First Image - US Composite Image (no clue on specifics)

Date/Time: Dec. 7, 1998 / Time Unknown

Caption: "Note small partial ring over central Arizona"
Analysis: When I clicked on the picture to blow it up I got the 404 error message. No conclusion.

*Second and Third Images - US Composite Image with blowup over northwest Washington
(Looks like Base Reflectivity)
Date/Time: Nov. 12, 1998 / 0906 GMT (106 AM PST)

Caption: "Note partial radial emanating from Bangor, Washington"
Analysis: To tell the truth, I'm not quite sure what the caption here is referring to. (It would probably be better if I sat down with someone to have them explain to me in detail what it was they thought was unusual.) I'm not sure exactly where the radar is here either, but it appears to be somewhere northwest of Seattle and Everett, maybe towards Port Townsend? (See:
NEXRAD Map) Considering the location, time of day, time of year and the other radar returns from the west coast, I'd guess that there was layer of marine air seeping eastward into the west coast states. This happens quite frequently, especially in northwest Washington. What I'm seeing here is a typical pattern depicted on a WSR-88D when there is a marine inversion present. The radar beam, which normally slopes upward logarithmically from its origin due to atmospheric refraction, is likely being "reflected" off the base of the inversion/moisture discontinuity (and hitting the ground) when it should be shooting off into space. Since the internal settings of the radar assume normal refraction of the radar beam, these returns are AP since the beam refraction is not normal. What I believe you're seeing is a combination of returns from low clouds, fog, drizzle and topography. The solid, red returns to the east of the apparent radar site are most likely the west slopes of the Cascade range. Under normal beam refraction, most of the beam would probably pass over the Cascades and they probably wouldn't make such a dramatic appearance. Now, of course, the question is: Why then is there a distinct lack of returns to the southwest of the radar site? The most likely explanation for this is that the Olympic Mountains are simply blocking the radar beam. Also, since the Olympics are obviously a constant factor in radar images from this radar, it's likely that the clutter suppression algorithm has been enabled to remove false radar returns that are created by them. This might also explain why the Cascades show up much more than the Olympics in this image. The Olympics are considered more heavily in the clutter suppression scheme under normal beam refraction conditions than the Cascades.

Short and Sweet: This image shows conditions know to be associated with a marine inversion, that is cooler and more moist air at the surface with warmer and drier air aloft. Under these conditions, AP is always encountered in the form of spurious ground returns and enhanced topographic features. A combination of beam blockage and clutter suppression algorithms are most likely responsible for the lack of returns to the southwest of the radar site.

*Fourth Image - South-central US Composite Loop (most likely .5 degree slice)

Date/Time: Dec. 20, 1998 / 2007-2307 GMT (207 - 507 PM CST)
Caption: "Dissipating radial pattern over
Brownsville, Texas. A "Radar anomaly?" Or ...?"
Analysis: Indeed, at first glance at this would appear to show a dispersion of EM energy or some type of reflector in a doughnut shaped area with a radius about 95 nautical miles out from the radar. At a .5 degree beam angle, that would place the phenomena somewhere between 10,000 and 15,000 feet. As to the apparent cause for the returns, it appears unlikely that is any kind of usual meteorological target. This is especially true because the adjacent radars at Corpus Cristi, Austin, and (I think) Laredo show no signs of it. The distinct circular appearance of the radar returns make it obvious that this image was detected and/or produced solely by the Brownsville radar. If there had been actual scattering bodies aloft over southern Texas at this time, some of them would have certainly been picked up by the adjacent radars, and the pattern would not nearly be so perfectly circular. Therefore, I must deduce that this was a glitch within the processing system of the Brownsville radar. Similar patterns have occurred before when there have been calibration errors, system noise, etc., and they are well documented. Either this was the case here, or every other radar system in southern Texas at this time was not operational!
Short and Sweet
: This image most likely shows an internal glitch or processing anomaly distinct to the Brownsville radar during that particular time frame. The absence of returns from any of the adjacent radars and the purely circular pattern further support this conclusion, assuming that the other south Texas radars were in operation at the time.

*Fifth Image - North-Central US Composite Loop (most likely .5 degree slice)

Date/Time: Dec. 21, 1998 / 0706-1106 GMT (106 - 506 PM CST)
Caption: Partial radial ring over South Dakota. Do "anomalies" only occur over 260 of a radar sweep?

Analysis: The answer is...YES! If that anomaly is being produced by the radar itself, then almost anything goes. You can get spikes, circles, partial radials or even a radar screen completely filled when there are clear skies. That leads to another of the unknowns I mentioned in my opening words. One of the things I see in this picture and the one from Brownsville is that there appeared to be fair weather at both locations when these loops were captured. What people unfamiliar with the system might not know is that this is when maintenance is routinely performed on these systems by staff electronics technicians. It is routine to have a bad or scan or two either at the beginning or end of the maintenance period, if not several. Of course, it is unlikely that this radar was being worked on at 2 am in the morning, so that was most likely not the case here! I have to say that I cannot in any way be sure about what exactly caused this image. I can say that I've seen this pattern many times before under different circumstances and in different locations. Possible reasons include: bad volume scan, change in detection mode (there are more sensitive and less sensitive modes for different applications), internal systems errors, etc.
Short and Sweet: Similar to the Brownsville image, this one most likely shows an internal glitch or processing anomaly distinct to the radar (Rapid City, SD?). The fact that it occurred in only one volume scan and that similar patterns have been proven to be attributed to internal processing problems further supports this conclusion.

*Sixth Image - National Composite Image (Can't tell if Base or Composite Reflectivity)

*Seventh and Eighth images - Midwest and Northwest Composite Loops at same time frame as sixth image (These appear to be Base Reflectivity)
Date/Time: Dec. 23, 1998 / 1306-1706 GMT (706 - 1106 AM CST / 506 - 906 AM PST)

6) Parallel streaks nationwide on the 23rd. Radar chaff or "the fleet" dropping by?

7) Midwest close-up of the images above.

8) And over the Pacific northwest.

Analysis: You're going to like this one! First off, I see nothing unusual at all with the National shot (sixth image), so that about wraps that up! None of the streaks exhibited in the following two pictures are on this one, and you'll soon know why. Notice in both pictures 7 and 8 how the streaks mysteriously appear from east to west across the country during the time frame 1306 - 1706 GMT. These times correspond directly to the hours just before and after sunrise from east to west. What you're seeing here is the glory of the rising sun as it blasts its low-angle EM energy down the pipes of the network radar system! This again is another frequent happening in the world of weather radar. During this time of year, it usually happens between 7 and 8 AM LST as the sun rises and corresponds directly to the angle of the rotating radar antennae for just an instant. The radar is pulsing out its power into clear air and getting few or no returns, when the sun suddenly slams it with a full dose of energy back down the radial. Seen on a single radar, it's just a single frame in the loop that flashes by each morning and is ignored. Put all together in these regional composite loops, it does look pretty freakish! If you check the times on the loops relative to when the flash occurs at each radar site, then do the math to change GMT to LST, you'll see that they ALL occurred at or between 7 and 8 AM LST. And yes, the same thing happens when the sun sets as well. Similar streaks developing from east to west on a radar loop just before sunset, but oriented westward from the radars, would indicate this. Oh, and the reason there were no streaks on the national image from 1 PM GMT (1306 GMT) is because that correlates to 7 AM CST, 6 AM MST and 5 AM PST...just at or before sunrise in the central time zone and well before sunrise in the mountain and pacific zones.
Short and Sweet: These images depict the rising of the sun as seen by the WSR-88D system when skies are mainly clear. This happens when the sun's energy is instantaneously detected as it rises to an angle coincident to that of the radar antennae. The fact that this phenomena was observed "flashing" from east to west across the country at around the time of sunrise lends strong support to this conclusion. Instances of this phenomena have been recorded at every single radar location in the US.

*Ninth Image - Central US Composite (most likely .5 degree slice)
Date/Time: Nov. 10, 1998 / 2007 GMT (207 PM CST)
Caption: Streaks criss crossing at 90 angles. Do meteors do this? (no). And
note the stationary "streak." A "transient radar anomaly" that isn't transient?
Analysis: The only thing I see at all unusual with this image is the strange west-east oriented streak over southern Missouri, seemingly emanating from the radar over southwest Missouri. (Those thin north-south streaks over OK are obviously bogus and probably associated with the post-processing of the data by Unisys.) The southern Missouri return looks like it could be a sun spike, but that's not possible at 2 in the afternoon. The only thing I can even draw on to make a conclusion is the fact that a similar line appeared in the same area in the seventh image (loop) above. The nature of the return in that loop, a weird looking line with precipitation obviously moving through it, lends itself to processing problems with Unisys in trying to provide that composite loop. In this image, it could just be a bad radial in a scan, but it would be most helpful to be able to see the images from before and after that one.
Short and Sweet: No concrete conclusion possible due to lack of data. My best educated guess would be that it's either a post-processing problem with Unisys or a simple bad radial scan from the radar itself.

*Eleventh Image - Northeast US Composite Loop (most likely .5 degree slice)

Date/Time: Nov. 11, 1998 / 0915-1008 GMT (415 - 508 AM EST)

Caption: The unmoving Michigan/Iowa storms and "radar blanks." If this is just "ground clutter," why do the clouds circulate around the circles?

Analysis: This is a pretty clear case of either clutter suppression gone mad or poor post-processing by the radar image vendor. Again, without knowing the settings of those radars at that particular time, I can't be sure. However, it is known and has been frequently seen before. And the clouds are not "circulating around the circles." What you're seeing is lake effect snow in the wake of a cold upper level low centered somewhere over Quebec in this loop. The actual echoes are moving east-southeast in off the lakes, but are dissipating as they move further inland and constantly re-forming back over the lakes. Hence, you're seeing movement within the echo pattern, but the area of precipitation as a whole is pretty much stationary. The precip over northwest Illinois looks a little funky. I'd surmise that it was actually snowing across a good portion of northern Indiana and Illinois at this time, but it appears that at least one radar there is out of commission as we're only seeing partial returns. Impossible to say if these data gaps were caused by the radars themselves or by vendor post-processing.
Short and Sweet: Data gaps in this loop were mostly likely caused by a combination of misapplication of radar clutter suppression algorithms, problems with vendor post-processing and a radar outage over portions of Indiana and Illinois. While no concrete evidence can be shown, a comparison of the images from the individual radars against this loop and investigation into the status of the radars in Illinois and Indiana would be enlightening.

*Twelfth Eleventh Image - .5 Degree Loop (apparently composite) centered on Greenville, SC

Date/Time: Aug. 5, 1998 / 1038 - 1147 GMT (638 - 747 AM EDT)
Caption: A "direct hit" on Greenville, South Carolina during the Persied Meteor shower. But of what, by what?
Analysis: First off, this is probably a composite loop owing to the circular echo patterns around the adjacent radars at Columbia, SC, Charlotte, NC and the site in far northeast Georgia. (Also, note the sun spike caused by the rising sun!) The nature of the circular patterns and the fact that this is a .5 degree slice indicates that we're seeing something rather uniform lifting from very near the ground to several thousand feet into the air before dissipating. (Something exploding aloft would create very wide rings which would either contract around the radars with time or be swept away by prevailing winds.) Given the time of year and day, I believe we're either seeing the lifting and dissipation of low clouds and some fog as the sun rises (common in this area in August). This pattern isn't occurring directly around the Greenville-Spartanburg radar probably because the fog/low clouds may have developed only on the Piedmont and lower elevations, while the radar (I think) is at 1500 - 2000 feet elevation.
Short and Sweet: Loop shows lifting and dissipation of low level phenomena with the sunrise. It is most likely low clouds and fog. This pattern is the exact opposite of the one you'd see if something were exploding aloft and falling to the ground.
Update on Analysis of this Loop - 1/24/99: I was contacted by a noted biologist who informed me that my analysis of this loop was incorrect! What is seen here are not clouds and low fog lifting uniformly, but thousands of Purple Martins taking off at sunrise. (But the sunrise spike is still real!) This kind of thing has been documented on numerous occasions. After doing some checking, I realized that loop IS only the GSP image. The other locations, which I thought might have been radar sites, were instead roosting sites for the birds. They must fan out in those patterns after the mass take-off! (See:

Overall Conclusion: The images on show no glaring signs of anything at all unusual or unexplainable. There are a few that are not fully explainable by me, but I think I could nail them all down given more data to work with.

Well, that about wraps it up for this version! I plan to further examine another loop on this page which claims to focus on Turret Mt. in Arizona (HERE is the analysis). I'll ship that separately when I can get to it. I hope this effort will inspire those who are looking to do so with a bit more of a scientific method in the future. There is a whole heck of a lot out there that we don't know, but let us not disregard what we do know for the sake of a cause.

Questions and comments are more than welcome. Let us enlighten each other!

Charlie Maxwell
NWS, New Mexico


DISCLAIMER: The opinions and analyses offered by me are in no way those of the NWS or any other part of the US government. (standard disclaimer). 


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