Maria’s Destruction of San Juan Radar

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.



Comments

Leave a Reply

You must be logged in to post a comment.