Another lightning question

[Robert Edmonds]

So I was googling lightning and I came upon this page,

http://science.nasa.gov/newhome/headlines/...sd24may99_1.htm

on it, it said,

On April 17, 1995, the OTD observed a steady rise in lighting activity in a supercell storm over Oklahoma. Just before the spacecraft passed out of view of the storm, the lightning rates dropped sharply. A few minutes later, a tornado was observed descending out of the storm.

The linkage is that strong upward convection leads to high lightning rates and increased vorticity. Then the convection collapses and generates downdrafts, leading to a sharp decrease in the lightning and setting the conditions for the onset of a tornado.

Since this was 10 years ago I was wondering if this a effect has been verified or not.

Comments welcome....

 

[Nic Wilson]

Archived Thread Regarding Supercell Updrafts and Lightning

This archived thread should offer some updated information regarding the relations between supercell updrafts and lightning production. The Lightning Imaging Sensor discussed in your article is an interesting platform of technology that has been further improved over the years and will be an important part of the payload on the upcoming GOES-R launch in late 2012. Its new official title is the "Geostationary Lightning Mapper" and will provide 80% detection of all lightning strikes across North and South America in psedo-realtime. More information regarding the GOES-R launch can be found doing a simple google search on GOES-R.

The simple explanation to your question is that "lightning jumps" are often found near localized increases in updraft strength. The supercell, as a self-sustaining storm, goes through cyclical updraft impulses. Often times these are associated with tornadogenesis, but many times they are simply indication of slight increases in storm intensity. Unfortunately, "lightning jumps" can't be used as a consistent predictor for tornadogenesis because of the high level of false alarms brought about by its use.

 

[Jeff Snyder]

I'm sure someone can offer a better explanation (e.g. Nic Wilson is working with lightning obs/data for his graduate work), but that sequence that you quoted seems about right. A strong updraft will help with charge separation, creating significant electric fields and lightning. The updraft may temporarily weaken, decreasing lightning activity. It's been observed many times that, in many cases, the updraft weakens before producing a tornado.

During tornadogenesis, the LDAR-II and radar data indicated the updraft was weakening. The radar maximum reflectivity height and radar top (30 dBZ) started to descend 5-10 minutes (1-2 volume scans) before tornado touchdown. Total lightning and CG flash rates decreased by up to a factor of 5 to a minimum during tornado touchdown. LDAR-II source heights all showed descent by a few kilometers during this same time period. These observations agree with tornadogenesis theory that updrafts may weaken and the mesocyclone may become divided (composed of both updraft and downdraft) when a storm becomes tornadic.

http://ams.confex.com/ams/Annual2005/techp...paper_84908.htm --> Total lightning and radar characteristics of supercells: Insights on electrification and severe weather forecasting. Scott M. Steiger, Texas A&M University, College Station, TX; and R. E. Orville, M. J. Murphy, and N. W. S. Demetriades.

 

[Nic Wilson]

My research has involved looking at cases from spring-summer 2005 in the Dallas-Ft. Worth area so unfortunately there was only one quality supercell case - April 25th. The sequence of activity stated above is generally correct.

Typical Timeline of Updraft and Lightning Interaction
1.) Updraft impulse occurs
2.) Elevated charge regions are separated
3.) Increased amount of in-cloud flashes due to elevated charge layers
4.) Updraft weakens
5.) Charge layers descend
6.) Increased amounts of cloud-to-ground flashes observed and increased likelihood for hail and/or tornadogenesis

"Lightning jumps" (increased amount of in-cloud lightning associated with the updraft impulse) almost always precede an increase in storm strength witnessed at the surface. However, the magnitude of the "lightning jump" has shown little correlation with expected observations at the surface (e.g. tornadogenesis, hail, wind damage).

Many WFOs in the NWS Southern region are now using total (cloud plus cloud-to-ground) lightning data in their real-time operations to identify "lightning jumps". Total lightning information has superior temporal resolution compared to the WSR-88D (2 min vs. 5 min) in severe weather situations. However, researchers have not developed a quantifiable method to use the lightning information as a means for earlier warning leadtime (e.g. Lightning observations need to be confirmed by WSR-88D observations in order to issue a warning 99% of the time).

I presented a paper at the Radar Conference in October that discusses a couple instances where the use of lightning information complements the WSR-88D in forecasting operations. It can be found at the link below

http://weather.ou.edu/~nwilson/radarconf2005.pdf

The Steiger et al. 2005 paper above is an excellent review of the potential total lightning exhibits for earlier detection of tornadoes - however, the signatures apparent in that paper are typically non-existant in weak tornadoes, e.g. the April 25th F0s in the Dallas-Ft. Worth area.