SAILing

It has been nearly 24 years since the first NEXRAD radar was deployed and today the network continues to be regarded as “state of the art” and the best S-band operational network of radars in the world. Simply having identical hardware (such as radar frequency, beam-width, manufacturer, etc.) and software builds across the network results in a high degree of reliability and usability of radar data. Commonality among all radars is typically not seen in many other radar networks. But the NEXRAD network also remains on the cutting edge of science through continual refreshes of radar technology. Scientific upgrades including advances in radar algorithms, the addition of dual-polarization technology, and new scanning strategies continually provide new operational uses of NEXRAD radar data.

Some of the more exciting upgrades to the network have come through increases in spatial and temporal resolution of the data. New Volume Coverage Patterns (VCP) have been introduced to shorten volume scan completion times such as with VCP 12 in 2004.  Radars in VCP 12 could provide a full volume scan for the first time in under five minutes, reaching an update time of about 4.2 minutes. Super-resolution data increased resolution to 250 m radially and half-degree azimuthally in 2008. But even further increases in temporal resolution could be made not just through the introduction of new VCPs, but by providing new dynamic scanning techniques.  In 2012, the Automated Volume Scan Evaluation and Termination (AVSET) method was implemented to terminate a current volume scan as a radar tilts upward once radar echoes are no longer observed. Thus, low level updates where precipitation is occurring are made available more often without sacrificing vertical sampling. AVSET can provide volume scan updates as fast as every 190 seconds (just over 3 minutes).

AVSET set the stage for further improvements through dynamic scanning in 2014 with the introduction of the Supplemental Adaptive Intra-Volume Low-Level Scan or SAILS. SAILS provides a supplemental elevation scan at the lowest tilt (0.5 degrees) for VCP 12 and 212 scanning strategies. The additional scan is inserted into the middle of the volume scan thereby halving the latency of data closest to ground. When not using SAILS, the 0.5 degree update time in VCP 12 is 253 seconds, but with SAILS the average update time would improve to approximately 147 seconds on average (in table below). When SAILS is combined with AVSET, the frequency of 0.5 degree scans could increase to a maximum of 101 seconds.

VCP12-SAILS

The video below compares radar returns for KICT (Wichita, KS) from a convective event on April 17, 2015.  The image on the left shows KICT scans at 0.5 degrees for a two hour period (0300-0500 UTC) while the radar was operating in SAILS mode using VCP 12.  The image on the right is what the KICT returns would have looked like without SAILS.

With SAILS, the average update frequency of the lowest elevation angle improves to 162 seconds, with some updates available as fast as 140 seconds.  Faster update times is reflected in a smoother looking transition between images. The figure below gives the update time of the lowest elevation angle in seconds for KICT over the two-hour time period.  Update times ranged from 140 to 181 seconds.

KICT-SAILS

Believing you can never have enough of a good thing, the concept of SAILS has been pushed even further with MESOSAILS or the Multiple Elevation Scan Option for SAILS.  MESOSAILS is currently being tested operationally for several radars in 2015 with plans to deploy this capability throughout the whole network in 2016. In MESOSAILS mode, a radar returns to scan at 0.5 degrees 2 or 3 times within one volume scan. A study was performed to determine if the antenna/pedestal assembly of a NEXRAD radar could withstand the acceleration/deceleration strain in MESOSAILS mode.  It was found that performance characteristics were all within original design specifications for the NEXRAD radars. With MESOSAILS, a maximum of 4 total scans can be made at the lowest elevation angle within one volume scan. Original testing of this new method has shown that update times at 0.5 degrees could be reduced to as low as 89 seconds on average and potentially as low as 72 seconds a certain points within a volume scan.

SAILS Test VCP Definitions

One of the first radars to employ the MESOSAILS technique is KDDC (Dodge City, KS). KDDC was running in MESOSAILS mode for the same convective event shown above for April 17, 2015. A comparison of the KDDC lowest angle returns (shown on the left) with KICT (without SAILS, on the right) is made in the video below. The time period is over the same two hours as shown in the prior video. Clearly, the improvement in update time can be seen in the animation.

The following figure shows the update time for KDDC at 0.5 degrees for a 20-hour time period encompassing the two hours in the video. KDDC was operated in SAILSx3 and SAILSx2 modes on April 16th and 17th and the transition is easily seen in the graph.  In SAILSx3, 0.5 degree scans are updated every 100 seconds on average and 116 seconds in SAILSx2.  It is interesting to note that the update time approaches 1 minute at several points with some updates provided after only 70 seconds.

KDDC-MESOSAILS

Obviously approaching 1-minute updates for the lowest level scan could prove crucial in severe weather scenarios especially for the detection and tracking of tornadoes. But the MESOSAILS technique should benefit a host of radar products ranging from rotation tracks, storm cell identification and tracking, hail detection and tracking as well as providing more accurate QPE products. It will be intriguing to determine the extent of improvement in such algorithms once MESOSAILS is deployed across the entire network.

But MESOSAILS is just one of a very long line of improvements made to the NEXRAD network. The continual refresh of new technologies and operational control of radars keep the network at the forefront of scientific radar development. This also ensures that users from NWS forecasters to private sector companies will have plenty of new tools and opportunities to provide cutting edge products for the benefit of its clients.

-Chris Porter

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