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San Juan Puerto Rico Radar Loops — San Juan Radar
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…NWS San Juan (San Juan, Puerto Rico) weather Alert for San Juan… …Advisory Type: Small Craft Advisory… …Issued: December 27, 2022, 1:57 pm…
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Radar Reflectivity Explained
The colors are the different echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan. «Reflectivity» is the amount of transmitted power returned to the radar receiver. Reflectivity (designated by the letter Z) covers a wide range of signals (from very weak to very strong). So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used.
The dBZ values increase as the strength of the signal returned to the radar increases. Each reflectivity image you see includes one of two color scales. One scale (far left) represents dBZ values when the radar is in clear air mode (dBZ values from -28 to +28). The other scale (near left) represents dBZ values when the radar is in precipitation mode (dBZ values from 5 to 75). Notice the color on each scale remains the same in both operational modes, only the values change. The value of the dBZ depends upon the mode the radar is in at the time the image was created.
The scale of dBZ values is also related to the intensity of rainfall. Typically, light rain is occurring when the dBZ value reaches 20. The higher the dBZ, the stronger the rainrate. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rainrates which are associated to the dBZ values.
These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Hail is a good reflector of energy and will return very high dBZ values. Since hail can cause the rainfall estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.
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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.