Casual Observation and question re: VIL

[Mike Johnston]

While in cyberchase mode, I have often observed that a spike in magnitude of VIL signatures is often very closely associated with the issuance of tornado warnings. It seems not so much the absolute value of the VIL signature, but relative in both space and time to the surrounding readings. I first noticed while nowcasting for some guys out on the dryline in W. TX last summer, took note, and have made a point since to observe the VIL scan often since. Even with the episodes over this past winter, it seems that a temporary spike in VIL signatures relative to precedent and surrounding signatures has often coincided with the issuance of tornado warnings - within minutes, it seems.

I understand that VIL represents a volumetric measure of liquid, and quickly inflates when hail is reflected back, and also that hail is associated with strong updrafts, and strong updrafts may be generally associated with tornadic storms. Beyond this simple crude correlation, though, is there any - more specific - theoretical hypothesis out there regarding VIL and tornadogenesis?

 

[rdale]

Not that I can imagine... Nothing more than a coincidence, there are probably MANY more storms with VIL spikes that have nothing to do with tornadoes. And VIL is very dependent on scan strategies / storm location / etc, better off using hi-resolution VIL like NWS has access to (and GRLevel2AE will offer.)

 

[Kiel Ortega]

I understand that VIL represents a volumetric measure of liquid, and quickly inflates when hail is reflected back

Just as a note, VIL essentially excludes reflectivity above ~55 dbZ because it is supposed to be vertically integrated liquid; above ~55 dbZ you usually get ice contamination (i.e., hail). So yes it inflates when hail is reflected (high dbZ) but VIL does nothing in terms of telling you anything about the hail present in the storm.

 

[rdale]

I don't think hi-res VIL has the cutoff, I know it doesn't ignore low (< 18.5 dbZ) echoes like conventional VIL does.

- Rob

 

[Greg Stumpf]

I don't think hi-res VIL has the cutoff, I know it doesn't ignore low (< 18.5 dbZ) echoes like conventional VIL does.

Rob, you are correct. "hi-res" VIL (which is VIL on the 1 degree X 1 km polar grid) does not have the upper reflectivity bound in the integration. However, this was an oversight, and most likely is going to be corrected in a future ORPG build. Another however...hi-res VIL values are capped at 80 kg m-2, and the 8-bit scale heavily favors very low values of VIL. The reasons behind the choice of scale are embedded in FAA requirements - their studies have shown that VIL is a very good indicator of impending convective initiation, and having a lot of precision at very low values helps. It is quite possible a second HRVIL product (e.g, not capped at 80) will be made available in the future, that will have a scale favoring use for severe weather diagnosis.

Hopefully, GR will compute its own VIL and not use the ORPG VIL product. In fact, compute both a capped VIL (removing ice contribution) and uncapped VIL, or perhaps the delta between the two. I will recommend this to Mike G. (I'm a beta-tester).

However, the Severe Hail Index (SHI; Witt et al. 1998) was designed for detecting the contribution to the reflectivity profile by hail cores aloft by weighting higher reflectivities and those above the melting layer higher, and is a better indicator of storm severity and hail than any VIL (or VIL Density). Beyond that, will be hail diagnostic parameters incorporating conventional radar variables as well as other sensor data (thermo and shear profiles, etc), and eventually polarimetric variables, mostly still under R&D.

More info is available in this paper (and followups that Kiel is involved with - he can post the links):

http://ams.confex.com/ams/pdfpapers/81451.pdf


greg

 

[rdale]

Since NWS hi-res VIL isn't available outside of NWS - GRAE's is the one I was referring to.

"In fact, compute both a capped VIL (removing ice contribution) and uncapped VIL, or perhaps the delta between the two."

I'm sure his is uncapped now, I like the idea of all three of those graphics.

 

[Greg Stumpf]

Since NWS hi-res VIL isn't available outside of NWS - GRAE's is the one I was referring to.

Gosh, I forgot about that. Hopefully that will change soon, but I have no news.

 

[rdale]

What would the delta map show us?

 

[Greg Stumpf]

What would the delta map show us?

Unsure how well that would correlate with storm severity, but at least it would be available for others to study.

 

[Nic Wilson]

Many folks at Lincoln Lab at MIT (leading research center for meteorological applications used by the FAA) are convinced that VIL is a strong proxy for lightning activity. But, this relationship is flawed due to scenarios of wet hail growth that skew VIL values very high, yet don't allow for effective charge exchange.

Traditionally, VIL has been most commonly used as a forecast tool for hail. Forecasters would wait until the first hail report of a severe weather outbreak and note the VIL associated with it. This value of VIL became the "VIL of the day" and the theshold that forecasters would use to provide Severe Thunderstorm Warnings due to hail potential. The advancement of WSR-88D hail algorithms (e.g. Kiel and other at NSSL's work) that incorporate space and time specific thermodynamic profiles is phasing out the old guard's use of VIL in this manner.