As the National Weather Service continues to improve upon doppler radar scanning strategies during severe weather events, there is a new one for you to know about and understand. SAILS was introduced a few years ago and was a great help in rapidly changing conditions. But now, MRLE is coming, and is being tested at 15 radar sites around the country.
What is MRLE? MRLE stands for Mid-Volume Rescan of Low-Level Elevations. If you are aware of SAILS, you know how that works. The radar scan starts at the lowest tilt, or level, and progressively scans tilts at higher and higher angles, giving you a 3-D view of a shower or thunderstorm. Halfway through the volume scan, the radar comes back down and rescans the lowest level, so you can see what’s going on as close to the base of the showers and thunderstorms as possible. This data is what people commonly use, and what is seen on AllisonHouse Maps and AllisonHouse-supported radar software, as well as on TV. However, MRLE allows for up to four low elevation scans, instead of just one extra, in any volume scan. This lets us see what’s happening at the lowest part of the storm more accurately with more frequent updates, to see how, for example, a rotating thunderstorm called a supercell is developing or evolving, and how ground threats are potentially changing.
So how does the new MRLE scan strategy work? It goes like this: the radar scans the lowest 8 tilts (up to approximately 5.1 degrees at tilt 8). Then, the radar goes back down to roughly .5 degrees (or, more accurately, tilt 1), and scans the lowest 4 elevation angles one more time, in 30 second intervals, depending on how the radar operator configures it. After that, it continues to scan with the remaining 6 upper tilts as usual, ending at 18.5 degrees.
If you are like “this is just like SAILS, but I get additional lower level scans halfway through the volume scan every 30 seconds”, you are correct!
Now, a person may ask, “why don’t you have a mode that just sticks to doing lowest level scans?”. If you do that, you don’t get a 3-D view of the storm, you don’t get products like digital VIL, for detection of hail and hail size, and you don’t see how a mesocyclone (the rotating updraft portion of the storm) is behaving at mid-levels (or to see if there is one present or forming). As it is, the total number of scan tilts is reduced to handle the additional low-level scans.
More details on MRLE is available on the Radar Operations Center’s website:
The test is expected to run approximately one year on these 15 sites:
KAMA – Amarillo, TX
KDGX – Jackson, MS
KJKL – Jackson, KY
KLSX – St. Louis, MO
KEMX – Tucson, AZ
KLRX – Elko, NV
KICT – Wichita, KS
KIND – Indianapolis, IN
KDVN – Quad Cities, IA
KILN – Wilmington, OH
KPUX – Pueblo, CO
KEWX – Austin/San Antonio, TX
KENX – Albany, NY
KRAX – Raleigh, NC
KUDX – Rapid City, SD
If proven successful, this scan option will be adopted in 2019 nationwide. Some software may require updates to handle this new scanning strategy properly; an update is coming for GR2Analyst to get the extra scan updates to you more quickly.