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EJSSM 7.2: Operational Uses of Spectrum Width


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#1 Roger Edwards

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Posted 02 March 2012 - 04:37 PM

EJSSM Forum comments are welcome on this manuscript.

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Abstract

Spectrum width is a WSR-88D product that has been available to operational forecasters since the radar was deployed. In 2008, super-high-resolution reflectivity, velocity and spectrum width data became available. Six cases exemplifying operational use of spectrum width are presented; five are from after the upgrade. The cases were selected to depict the wide array of uses of spectrum width (SW). In one case, use of SW improved forecaster capability to evaluate the strength of horizontal shear within a bow echo's mesovortex. One case shows that SW can be extremely helpful in determining location of boundaries, which aids in overall situational awareness. In another case, SW aided forecaster confidence to issue a tornado warning with lead time. If a storm is close to the radar (55 km in this example), SW can be used to clarify the location of the rear flank downdraft, assess where its wind damage may be a threat, and discern subsequent cutoff of the tornado from the warm, moist inflow. Finally, when used in a derecho case, SW helped a forecaster to identify more quickly where wind damage threats were likely.

#2 Matt Bunkers

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Posted 20 March 2012 - 02:25 AM

One thing that I have found especially useful regarding spectrum width (SW) is analyzing three-body scatter spikes (TBSSs) and their contamination of velocity data.  This is especially important when analyzing areas of potential azimuthal shear.  There have been times when, because of velocity contamination, I have observed false mesocyclones or mesoanticyclones.  I've even witnessed others issue tornado warnings based on these aberations.  This topic wasn't discussed in Spoden et al.'s paper, perhaps because they were focussed on how SW can be used to identify real features.  Thus, I want to point out the work by Chris Smallcomb here,

http://www.nwas.org/ej/2008/2008.php

Chris has several pertinent examples where TBSSs show up clearly in the SW data, even when the TBSS was not readily apparent in the reflectivity data (i.e., his Fig. 6).  He also has examples of bad velocity data leading to incorrect identifications of rotation.  Correlation coefficient (CC) also is a great tool for identifying TBSSs, but that isn't fully available yet, nor does it mean we should abandon SW when CC does become available.

Finally, SW can be useful at times in identifying large, strong midlevel updrafts.  This was suggested by Jon Zeitler in his comments, but the authors chose not to discuss it.  At any rate, the viewing angle seems to be important for this, and thus the SW is not nearly as reliable for identifying updrafts as it is for pointing out TBSSs.  Nevertheless, it is something that is worth examining in an operational setting as time permits.  An NWA presentation by Les Lemon and me can be found here:

http://www.crh.noaa....eting_final.ppt

This this paper provides a good assessment of some operational uses of spectrum width, but I'd also encourage others to use it for TBSSs and UDs as well.




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