It has been an exciting year in the National Weather Service with some new advances in radar operations technology. Some additional tools are now in use by the National Weather Service staff at the Des Moines Weather Office. A recent upgrade to the WSR-88D Doppler Radar included Dual Polarization and now AVSET and SAILS have been added to the list of available tools to the analysis toolkit of the storm interrogation meteorologist.
AVSET is a short-hand for Automated Volume Scan Evaluation and Termination. This feature can be turned on or off during the normal operation of radar and is particularly useful for lessening the time of one complete radar volume scan. First, let’s back up a minute and review some terms! One complete volume scan is the pattern of vertical scanning the radar makes from near the ground to the top of its elevating cycle. The example below is for Volume Coverage Pattern 12 (VCP 12):
In this example the radar begins to scan at 0.5 degree above the horizon and continues scanning through elevating angles up to 19.5 degrees to complete one volume scan. This process takes about 4 minutes and 30 seconds to complete. So, in our standard operations of the WSR-88D radar system, a storm interrogation meteorologist would expect to see new data arriving every 4 minutes, 30 seconds, regardless of how far away or close a storm is located to the radar. With AVSET, the data arrives faster! AVSET can be best explained by looking at the diagram in Figure 2, which shows how AVSET would work for VCP 212 with a storm far from the radar.
With AVSET running, the radar scans until it no longer detects much return from the target of interest – in this case, a thunderstorm located about 100 nautical miles from the radar site. The radar would scan from 0.5 degrees up to an elevation of 5.3 degrees and then would stop moving upward and not complete any other elevation slices from 6.4 degrees to 19.5 degrees because the radar no longer detects much of a measurable radar return signal. At this point, the radar is complete with the present volume scan and then moves onto the next volume scan by beginning near the ground level of 0.5 degrees above the horizon and starting all over again.
The advantage is pretty significant since we are able to cut the time of one complete volume scan from 4 minutes and 30 seconds to as little as 3 minutes and 30 seconds – a sizable savings in time! With the standard operations of having AVSET turned off, the storm interrogation meteorologist might see up to 13 scans per hour using a VCP 212 radar configuration. With AVSET on, it is possible to see up to 17 scans per hour. Now this might not seem like that great an improvement over standard operations, but any additional scans that we NWS meteorologists can view during a rapidly changing severe storm environment means that we have a much greater ability to complete our mission of “Protecting Life and Property.” This has been a welcomed change in the Des Moines NWS Weather Office that enhances not only storm interrogation, but can also result in greater lead times when issuing severe weather warnings which in turn gives you additional time to prepare for dangerous weather events such as damaging thunderstorm winds, large hail, and tornadoes.
AVSET is just one great recent addition to our radar toolkit…but wait…there’s more! There is another new method of storm interrogation introduced this year called SAILS. SAILS stands for Supplemental Adaptive Intra-Volume Low-Level Scans (SAILS). Now you know why it goes by the short-hand term SAILS! So – what does SAILS do for the radar’s operation and how does it benefit the storm interrogation meteorologist? First, let’s take a look at how SAILS works. In normal radar operations, the radar scans from near the horizon at 0.5 degrees up to a specified height using elevation cuts to complete one volume scan. This is the same process shown above in Figure 1. With SAILS active, the radar would do the following as illustrated by the diagrams in Figure 3.
As shown in Figure 3, when SAILS is operating, an additional low-level volume cut is added to the list of available products for use by the storm interrogation meteorologist. This benefits the staff at our office because many of the important features that lead to a severe thunderstorm or tornado warning often show up in the lowest volume cut at 0.5 degrees. This would be true of a rapidly rotating meso-cyclone that may be lowering in the process of producing a tornado or perhaps more examination of strong to intense base velocities (strong thunderstorm winds) that are lowering toward the ground in a wind producing storm, such as a derecho. Again, with the addition of another slice at 0.5 degrees during each complete volume scan, there are many more available 0.5 degree slices available to the storm interrogation meteorologist per hour. But hold on, you say! Wouldn’t it take longer to finish one complete volume scan if we add another 0.5 degree slice in the middle of each volume scan? The answer is “yes”, it does reduce the total number of complete volume scans per hour, but the trade-off is well worth this slight disadvantage in most severe weather applications. Even though with SAILS active, the number of complete volume scans is decreased by two in VCP 212 compared to the standard operation with SAILS off – we gain 9 additional 0.5 degree slices per hour! The benefits of both reducing the time between low-level slice updates and the nearly doubling of 0.5 degree slices per hour allows for more low-level observations of intense thunderstorms during severe weather events. This gives our staff an opportunity to better monitor the trends of the lower portion of the thunderstorm and decide how quickly the storm might be strengthening or weakening. This has major implications for warning operations and should subsequently result in more lead time for warnings and more time for you to take shelter for severe weather.
Wouldn’t it be nice to run AVSET and SAILS together? Yes! In fact we can and do run them together. In some cases the net advantage is even better than SAILS alone or AVSET alone. The whole can definitely be greater than the sum of the parts. Take a look at the following table for comparison of the original standard operating mode compared to AVSET and SAILS, and then to SAILS and AVSET operating together:
Figure 4 (page 10) shows that for VCP 212 either AVSET or SAILS working alone provide higher numbers of 0.5 degree slices per hour – AVSET (13-17) and SAILS (22) compared to the Standard Operation (13). Figure 4 also shows the slight disadvantage of complete volume scans for SAILS (11) compared to the Standard Operation (13) and AVSET (11-13) per hour. However, with SAILS and AVSET both operating – the number of 0.5 degree slices per hour increases even more – SAILS and AVSET together (22-28) compared to AVSET alone (13-17) compared to SAILS alone (22). Looking back at the last column of Volumetric Product Updates per Hour shows that the combination of SAILS and AVSET both running together brings the total number of complete volume scans per hour back to 11 to 14 – nearly equal or slightly exceeding the Standard Operation of 13 per hour! It might seem a bit odd to see a range of 0.5 degree slices and a range of complete volumetric product updates per hour when AVSET is being used. However, this is completely normal because the early termination of one complete volume scan depends both on the height of the storm being viewed and the distance the storm is from the radar. If the storm is captured in only three or four elevation cuts due to being not as tall or far away from the radar, then AVSET will terminate the current complete volume scan earlier and more completed volumetric product updates per hour will be available to the radar meteorologist. This same process carries over to the case when both AVSET and SAILS are working together. One more fact about SAILS is that it is only used when the radar is in severe storm interrogation mode – that is, when the radar is in Volume Coverage Patterns VCP 12 or VCP 212. These are the coverage patterns used when significant severe weather – that which a warning might be issued – is anticipated or already occurring.
The additional number of low level elevation slices at 0.5 degrees can be critical to more lead time and earlier warnings. By issuing warnings faster with more confidence due to all of the additional weather data observed in those low level 0.5 degree slices, this will ultimately provide better warning services to you and enhance our ability to protect life and property!
Blog post by Roger Vachalek – NWS Des Moines