Weather or Not

Severe Outflow by R. Edwards

Maria’s Destruction of San Juan Radar

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Recall that when Hurricane Maria moved into Puerto Rico, the San Juan WSR-88D (Doppler radar for my non-weather friends) failed in the outer eyewall. Here’s what resulted:

The dome and antenna blew away to who-knows-where, and the steel framing itself appears slightly torqued. Trees all around are shredded to varying degrees, as should be expected with eyewall winds of an upper-echelon hurricane. The mowed-down trees in the foreground point uphill toward the radar, telling us the wind direction at the time they fell, and part of the source for embedded debris. The image below represents the last 0.5-degree reflectivity data acquired and transmitted before the radar was lost.

Now, thanks to a series of photos from NWS San Juan, two of which I include here, we can confirm why, and apply knowledge gained from damage surveys of other structures (and the Miami radar in Hurricane Andrew) to this weather-sensing structure. That is quite unfortunately ironic.

From available velocity data prior to failure (example below), we know that the radar — which sits high on a hill exposed to stronger flow than lower elevations receive — was hammered by sustained winds over 100 mph and gusts at that elevation near 150 mph. Those winds certainly contained leaves, sticks, and perhaps branches torn from nearby trees. The resulting debris-peppered airstream battered the dome piece by piece and gust by gust, until panels started loosening and coming apart under the combined stress of wind forces and high-velocity “sandblasting” effects.

Once the first panels started prying apart, the flow got inside and the entire dome shredded, exposing the antenna fully to forces for which it wasn’t designed. Being an airfoil, the dish tore away from the base easily and blew away. The entire process of destruction probably took only a few seconds, at most, from initial breach of the panels. To my knowledge, a WSR-88D hasn’t been struck directly by a significant (EF2+) tornado; however, given the windspeeds likely involved in this event, and the wind-resistance specifications of the radar structures (below, source link) one should expect similar results.

A speed of 60 m/s is the same as 134 mph. Of course the failure alone doesn’t prove the windspeed. We don’t know if the radar was built to those specs, nor to what extent the near-certain presence of flying debris played a role. Regardless, can the radar structures be engineered retroactively and cost-effectively against stronger flow?

Of course, the staff at San Juan is dealing with far more pressing matters personally and logistically than missing radar pictures and mechanical engineering of radomes. Still, it won’t help them in their public-warning mission to be without this crucial tool for at least some number of months, depending on how long it takes parts can be scavenged from elsewhere for reconstruction purposes, then shipped in and reconstructed, amidst all the island’s other damage.

Video of and Tribute to Don Burgess

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Roy Britt, longtime storm enthusiast and occasional chaser since 1981, recently posted a 54-minute 1998 interview with atmospheric scientist Don Burgess, formerly of NSSL and OSF (now WDTB).

Here’s the link (embedding is disabled):
http://youtu.be/ztus6BMQTcM

This is absolutely fascinating. Don offers a richly personalized look into the history of radar meteorology, scientific storm chasing and severe-storm science in general. His account is full of entertaining stories, including compelling chasing anecdotes about Al Moller, Erik Rasmussen and Gene Moore, and interesting tales about other famous severe-storms meteorologists of the era like Les Lemon, Joe Golden, John McGinley, and Chuck Doswell. I have heard most of these stories over the years, from Don, Erik, Al, Chuck and others involved; but it’s great to have them online for posterity.

Admittedly, I would have done the very same thing as Don on the Union City day (24 May 1973)–briefly leave my post at the radar to run up to the roof and see the tornado in the distance. And like Don, I would have found the resulting performance-evaluation penalty a small price to pay for the experience! Fortunately, since Don was my own supervisor as a Federal student-employee of NSSL, I’m pretty sure that I wouldn’t have gotten in trouble. Alas, we didn’t have visible tornadoes in central Oklahoma in the late 1980s to test that hypothesis.

Like Don, as a student, I simply and audaciously showed up at the lab and asked for a job. It turns out they had one open at the time, unlike Don’s lengthy waiting experience. Rodger Brown interviewed and hired me…and got me to working directly for Don in the Forecast Applications Research group. As he did 15 years prior, I also analyzed Doppler radar data at times, and chased with the storm-intercept teams at every possible opportunity (including TOTO in 1986, DOPLIGHT-87, and IOT&E2 of 1989).

Admittedly, the most fun part of the job–besides the paid storm chasing–was sorting, categorizing and managing the NSSL slide and movie collections! Yet analyzing those huge reams of gridded radar-number printouts (known at the time as BSCANS), and tapping Don’s deep expertise, taught me a great deal about Doppler radar that has benefited every single step of my career. His guidance provided the early, foundational knowledge of Doppler concepts needed to stay abreast of the scientific advancements in that realm since. What a blessing it was, as an 18- and 19-year-old, to gain immersive understanding of concepts such as volume-scanning, spectrum width, velocity dealiasing, second-trip echoes, range folding, mesocyclone algorithm, TVS, dual-Doppler analysis, and all sorts of real-world storm signatures in both base moments and derived Doppler products. Credit Don for that.

I couldn’t have asked for a better supervisor. It wasn’t always easy to be patient with me, especially as a younger adult, but he was. He was the rare sort of manager who would inspire his charges to an extent that they would run through walls (or more appropriately, nighttime mesocyclones) for him. In 1989, while still an undergraduate student, Don somehow trusted me to supervise an NSSL chase crew and drive its van for the NEXRAD radar-verification program (IOT&E2). That was a gutsy call on his part, but I tried never to betray that trust. On a few occasions, we heard Don’s distinctive voice crackle over the radio while returning late at night from Texas storms intercepted during the day. I didn’t think twice about answering, “Control, this is Nessel-2…sure, Don, we’ll do it!”–to his radio queries on whether we were willing to plunge through a given potentially tornadic supercell. Obviously, we did survive, despite a few white-knuckled moments and sudden 180-degree wind shifts on those dark country roads of southern Oklahoma. Learning experiences…

Don also was and is a great guy (which should be self-evident simply via the video), and devoted family man. Folks like Don–and there aren’t many–stand as refreshing exceptions to the vice-ridden minefields of human nature. He is down-to-earth and humble in person, in contrast to his giant stature in radar meteorology. He is officially retired now, but remains heavily involved in the science, and also remains very deservedly and highly respected throughout the severe-storms community.

Radar Interference over Southern Nevada

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On Friday night, starting around 0330 Z (7:30 p.m. PST, 8:30 p.m. MST) and continuing for at least 3 hours thereafter, lots of non-precipitation interference was detected by several weather radars scattered across the Great Basin region. These signatures easily overlaid each other on composite images that use data from the most recent scans of multiple radars at nearly the same time. Here’s an annotated, multi-radar composite reflectivity image, with radar locations labeled.


The locations marked on the map are for the radars themselves, not necessarily the towns they serve. For example, the Elko radar actually is west of Elko itself. True north is parallel to the Nevada-Utah border on this map projection.

Here are un-annotated images from 0320Z and from 0400Z.

Now, this sort of thing typically sets the tinfoil-hat brigade aflutter with all manner of feverish paranoia. Some background will help here, I hope.

All receivers of radio waves are subject to radio-frequency (RF) interference. This includes the national network of WSR-88D (Weather Surveillance Radar, 1988 Doppler) units, which transmit and listen at the 10-cm wavelength. Research has shown this to be a really useful wavelength for weather radars, to balance good detection of hydrometeors (rain, snow and hail) with lack of attenuation (signal loss) through the same rain, snow and hail.

Radar beams also expand with distance. They bracket a line between the antenna and the source of the interference. It’s nothing unusual at all! This actually happens on a daily basis, all across the U. S., upon a most routine pair of events: sunset and sunrise.

The sun transmits at all wavelengths, including 10 cm. Therefore, when the sun is positioned low in the sky, just before setting and just after rising, a sweep of the radar antenna is low enough to “see” the sun. The radar detection of the sun’s 10-cm transmissions shows up as expanding beam signals pointing directly toward sunrise or sunset. Here’s a great example from NWS Louisville.

Other common sources of RF interference at 10 cm also can cause such beam signatures. Really, any substantial source of 10-cm interference can, if crossed by the sweeping radar beam. The magnetron in your favorite microwave oven, and millions of others, transmits at 12 cm–not far from that of the radars (albeit at very high frequency suitable for cooking in close quarters).

Triangulating the source of simultaneous interference on multiple receivers is easy. The beam lines converge at the source of the 10-cm interference over southern Nevada, specifically the area above the USAF Nevada Test and Training Range. Clearly something there was causing enough RF interference to be detected by several radars, for quite some time on Friday evening.

What was it? Don’t ask me; I have no way to know, nor even a good guess. Countless electronic devices exist–or can be made–that transmit fairly strongly at 10 cm. Barring some official explanation, I guess the tinfoil-hat crazies will have to take over from there, imagining all kinds of conspiratorial bogeymen under the bed…or more geographically correctly, near Area 51.