One of the best technological advances in the past year didn't involve any new technology at all — the advance is the result of an ingenious programming tweak developed by the National Weather Service. It will ultimately save thousands of lives over the coming years.
The programming tweak consists of two modifications called SAILS and AVSET. They are designed to dramatically cut down on the amount of time it takes a weather radar to perform a sweep of the skies, giving us more than double the coverage of the atmosphere just above the ground than we currently have.
Weather radar works by scanning the atmosphere at up to fourteen different levels, or "tilts," measured in degrees above the horizon relative to the radar itself. The lowest level it scans is 0.5°, which gives us the best look at what's going on closest to the ground. The highest level it scans is 19.6°, which lets us look inside storms that are practically sitting on top of the radar site. All fourteen radar tilts together give us a thorough look inside thunderstorms to see what they're doing.
For example, here's what a line of severe thunderstorms looked like as they were pushing through Grand Rapids, Michigan earlier this afternoon. This is the 0.5° tilt that we're all familiar with:
And this is the 19.5° tilt at the same time, showing the mid-levels of the thunderstorm immediately around the radar site:
When you look at a radar image online or on television, you're almost always looking at the 0.5° tilt. Since it's the level closest to the ground, it's obviously the most useful to us. It gives us the most representative look at what precipitation is actually reaching the ground, and it also lets us see rotation in a storm which could indicate the presence of a tornado.
How it works
Under normal circumstances, the weather radar will scan the 0.5° tilt first, then the 0.9° tilt, and so on up until it reaches 19.5°. Once it completes that scan, it will trace back down to 0.5° and start the process over again. This takes time — about 4 minutes and 10 seconds' worth, plus the time it takes to process the images and transmit them online.
This is an eternity in severe weather situations. As we've seen many times before, small tornadoes could form and dissipate in between these radar sweeps and we'd never know they were there until we hear reports of damage. Now, we have a way to hopefully stop that from happening.
Weather radars measure precipitation in decibels (dBZ). When you look at the legend on the side of a radar image, the units of the numbers are in decibels. Higher decibels equate to stronger returns — heavy precipitation, debris, bugs, birds, airplanes, anything that reflects more of the radar beam back registers a higher number.
AVSET stands for Automated Volume Scan Evaluation and Termination. This programming tweak tells the radar to stop scanning the upper levels of the atmosphere once it stops detecting precipitation heavier than a set amount. We'll use the example of 20 dBZ, which is the equivalent of a light rain shower. The radar will continue scanning each tilt until it stops registering returns higher than 20 dBZ. Once the radar detects that the precipitation is lighter than the threshold, it will jump back down to the lower levels and start scanning again.
This cuts down the amount of time it takes to make a complete sweep of the atmosphere by a full minute.
SAILS stands for Supplemental Adaptive Intra-volume Low-level Scans. When SAILS is activated, the code tells the radar to scan the lowest 0.5° tilt twice in one sweep, giving us a low-level scan about every 2 minutes and 20 seconds. The radar proceeds to scan the atmosphere as normal, but around the fourth tilt (usually on either side of 3.1°), the radar jumps back down and scans the lowest level again.
Since SAILS allows the radar to scan the 0.5° tilt twice, we get a low-level sweep about every 2 minutes and 20 seconds. This is extremely helpful in severe weather situations.
AVSET & SAILS
When both SAILS and AVSET are activated, it gives us a low-level sweep about every 1 minute and 45 seconds. This is a far cry from the almost 5 minutes we have to wait during normal operations.
How many more low-level scans do SAILS and AVSET provide us?
Under normal operations, we get a low-level scan 14 times per hour. When both AVSET and SAILS are activated, we can get more than double that number, clocking in between 24 and 32 low-level scans per hour. That is an incredible advance in the availability of low-level products, and it is already showing major success.
Just yesterday, a second round of major tornadoes struck northeastern Nebraska, and the weather radar in Sioux Falls, South Dakota had SAILS activated. With SAILS alone, the radar provided us with updates on the tornado's progress every couple of minutes. The results are immediately visible in the smooth, fluid animated radar image of the storm (it may take a moment to load).
The benefit of SAILS is also apparent in this animated radar image from Tulsa, Oklahoma at the beginning of this month, as a squall line approached the region. The radar updated the 0.5° tilt every two to three minutes, giving us a great view of the storms as they approached northeastern Oklahoma.
These two updates were a pretty clever way of utilizing existing radar technology to better serve its purpose of helping National Weather Service meteorologists issue warnings more quickly and effectively, saving thousands of lives in the process.
Offices across the country have rolled out the SAILS and AVSET updates en masse over the past month or two, and most (if not all) offices will have the update in place pretty soon.
If you're wondering what is "the next big thing" in weather radar technology (because we all know you are), it's called phased array and it will provide us with full radar scans in about one minute. The research is promising — it's detecting more severe weather than the radars we currently use, and it can produce a complete 16-tilt sweep of the atmosphere in the time it takes our current radars to complete two. It'll be quite a few years before it rolls out operationally, but SAILS and AVSET are pretty great to have in the meantime.
[Top image by NOAA, radar images via Gibson Ridge, crappy illustrations by yours truly]