Supplemental Adaptive Intra-volume Low-level Scan - Huh?


The NWS radar network, which includes the NWS Milwaukee/Sullivan WSR-88D, has received some very important technology upgrades over the last few months.


The two most important ones are:


  • AVSET (Automated Volume Scan Evaluation and Termination)
  • SAILS  (Supplemental Adaptive Intra-volume Low-level Scan)

The common thread in each of these is giving radar users faster updates of critical radar information, which can lead to more accurate and timely warnings by NWS radar operators.

What is AVSET?

  • Simply put, the RDA will calculate the echo area above 18dBZ at elevations over 5 degrees and will gracefully terminate the volume scan one tilt after when the area is less than a specified threshold.

Here's what that means. When our radar is operating in severe weather mode, by default it will scan the atmosphere at 14 different elevation angles, starting at .5° elevation and peaking at 19.5°. It does this in 4 to 4 1/2 minutes (depending on the specific mode it is in). The completion of all these elevations is called a Volume Scan. There are times when the radar beam will overshoot all of the storms when it is scanning at the higher elevation angles.  At this point there is no operational benefit to continuing the volume scan at these higher elevation angles.  With the installation of AVSET, the volume scan will terminate early with the net effect of shortening the elapsed time between data collection at the more critical lower levels. 

All users of our radar data will benefit by getting more critical data at a faster interval (so long as upper elevation angles are clear of significant radar returns).  For a radar operator responsible for issuing warnings, this can help to get warnings out sooner and with more accuracy. 

For more information about AVSET, go to this document from our Radar Operations Center.

What is SAILS?

Almost without question, one of the most important elevations to scan is the bottom one at .5° (1/2 degree above horizontal).   This allows us to scan the lowest part of storms, and if a storm is going to produce damaging straight-line winds or tornadoes, this is where we want to get as frequent a look as possible.  That scan is closest to the ground where damage might occur. 

SAILS is a strategy that inserts an extra .5° low-level scan during every volume scan, when we are operating in a severe weather mode with the radar.  For those that care, this is Volume Coverage Pattern 12 and 212. 

So how does that work?  Remember from above, when the radar is operating in severe weather mode, it will scan through 14 elevations (unless AVSET terminates the VCP early) in 4 to 4 1/2 minutes.  When the radar reaches roughly the middle of the volume scan, the radar will drop back down to the .5° elevation angle and do a quick scan, then return to where it left off and complete the volume scan.  This will double the number of critical .5° elevation scans for all users of radar data. 

Here is a graphical representation of how SAILS will work:

For more detailed information about SAILS, go to this document from our Radar Operations Center.





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