Jet Streak Quadrants - Significance?

[brody_clifton]

The right front quadrant of a 500mb jet streak..I have recently pondered this question more and more for some reason (especially after yesterday): Why is it that the RFQ of a streak is not always a bad location for severe/supercell storms to develop? From the data I looked at yesterday, it seems the entire outbreak area was in the right front quadrant of a 100+ kt 500mb jet streak that was positioned from western Oklahoma up through northeast Kansas, southeast Nebraska and extreme northwest Missouri, with the nose of the jet streak up into central/eastern Iowa. The outbreak of tornadic sups Sunday occurred in MO and western IL, and both places were in the right front quadrant of this jet streak.
I guess my question is, why do significant events like yesterday commonly, or even often, occur in right front quadrants of jet streaks as opposed to the LFQ or RRQs, both which are supposed to be the prime areas for upper level divergence and corresponding upward vertical velocities? Does it really matter what quadrant of a given jet streak a warm sector lies beneath, or was yesterday (and several other past events I've analyzed) a special case in which the cold frontal forcing, instability, and erosion of the cap from strong upper cold air advection overrode the negative UVV values (subsidence) generally associated with the RFQ of a jet streak? I mean, to me and pretty much everyone else it was obvious yesterday from looking at 500 MB wind vectors that strong diffluence existed in both the left front, right front, and right rear quadrants (eastern OK) of the jet streak at the time. So maybe it just depends on the structure and associated wind vectors of each indivual upper trough moreso than saying "Oh no, temp's at 86, dewpoint's at 74, 20 kt SSE gusty surface winds feeding into the low, and with a sharp dryline buldging through intersecting this morning's outflow boundary, BUT, unfortunately we're under the wrong (RFQ) quadrant of the jet streak, so bummer it's a bust today due to subsidence

 

[Jeff Snyder]

Brody,

It's probably a little better to use the 200-300mb winds to find divergence and vertical motion, since it's easier to infer vertical motion with divergence in the upper-levels (afterall, the air that "fills" that divergence isn't going to come from the stratosphere in the vast majority of cases). Oftentimes, the 300mb jet won't be too dislike the 500mb winds, but just a note. If there was 500mb divergence, you couldn't really imply much in the way of vertical motion, since you could have subsidence above 500mb or upward motion below 500mb. This is much the same reason why we can say that convergence in the low-levels can imply upward motion -- we invoke the imperbeability condition, meaning that we assume that air cannot go into the ground (which is a pretty good assumption LOL), so it must go "up").

There has been some research into the climo of severe and tornado outbreaks relative to jet streaks. Here's one bit from http://www.srh.noaa.gov/mrx/research/tornc...tornadoclim.htm

Of the 90 total events with available 300 or 250 hPa charts, 83 (92%) were located near a jet streak. Of the 80 events where it was determined that the southern Appalachian region was located near a particular quadrant, 63 (79%) were found on the right side of the jet streaks with 30 near the entrance region and 33 near the exit region. Only 17 events (21%) were found on the left side of the jet streaks, with 16 near the exit region and only one near the entrance region. Around 54% of the significant tornado events in the southern Appalachian region (or 61% of those near jet streaks) occurred near the exit region of an upper-level jet streak. All eight outbreak events in this study were located near a 300 or 250 hPa jet streak, with seven events found on the right side of the jet streaks and only one on the left side. Four outbreak events occurred near the right-exit region, three events near the right-entrance region, one event near the left-exit region, while none occurred near the left-entrance region.[/b]

Here's another good publication on this:
Rose, S. F., P. V. Hobbs, J. D. Locatelli, and M. T. Stoelinga, 2004: A 10-yr climatology relating the locations of reported tornadoes to the quadrants of upper-level jet streaks. Wea. Forecasting, 19, 301-309.

In this paper, we have shown a significant correlation between one type of severe weather, namely tornadoes, and the quadrants of upper- level jet streaks. Tornadoes occurred most frequently below the exit region of a jet streak. Figures 4 –6 show that this maximum in tornado occurrence appears as a fairly uniform, elliptical area approximately 1000 km wide, centered below the left side of the jet axis. Most of the tornadoes occurred below the left-exit quadrant, but a significant number also occurred below the right- exit quadrant. Most of the tornadoes that occurred below the exit region of a jet streak were located significantly closer to the jet center than tornadoes that occurred below the entrance region. In the entrance region, tornado occurrence was strongly favored below the right-entrance quadrant, although this maximum was not as well defined as that below the right- and left-exit quadrants. Regardless of the quadrant of the jet involved, tornadoes generally occurred in proximity to frontal, trough, or dryline boundaries and/or surface low pressure systems.[/b]

In both cases, there were more tornado events in the right-exit region (typically the convergent quad with subsidence per transverse circulation) than in the right-entrance region (typically the divergence quad). Of course, if the jet streak is curved, there may still be upper-level divergence of the 250mb jet streak since upward motion associated with DPVA (differential positive vorticity advection) may compensate for (and exceed) the subsidence from the transverse circulation from the right-exit region. Even in straight jet-streaks, the transverse circulation results in an indirect thermal circulation, where the warm side (right side) of the jet is seeing subsidence, while the cold side (left side) is seeing upward motion. This may enhance the southerly low-level flow, helping advect deep Gulf moisture northward and increasing low-level shear. On the flip side, there is a direct thermal circulation on the entrance half of the jet streak, where transverse circulation indicates less favorable contribution to the low-level flow.

I really suggest anyone interested in this check out this MetEd module about jet streaks.

 

[Matthew K. Hartman]

I guess my question is, why do significant events like yesterday commonly, or even often, occur in right front quadrants of jet streaks as opposed to the LFQ or RRQs, both which are supposed to be the prime areas for upper level divergence and corresponding upward vertical velocities? Does it really matter what quadrant of a given jet streak a warm sector lies beneath...[/b]

Brody... whether you're looking at the entrance regions (rear quadrants) or the exit regions (front quandrants) makes a big difference. It's mainly because of the dynamic vertical wind profiles they create. I, personally don't take the 500mb entrance/exit regions into much consideration when forming my convective forecasts. My preference is more toward the 300mb jet dynamics... The right exit (RFQ) and left entrance regions (LRQ) are areas of divergence, while conversely, the left-exit (LFQ) and right-entrance regions (RRQ) are areas of convergence. Divergence at 300mb will enhance vertical motion and should, in most cases, indicate areas of convergence at the surface. And convergence at the surface is certainly what you want, especially when you are looking for a triggering mechanism for convective initiation.

The reason why the right-exit region is typically preferrable is because you have the stronger jet winds upstream in this scenerio. The best mid/upper-level energy (vorticity) is upstream and often, this leads to greater veering in the vertical wind profiles, which likewise increases your helicities.

I also have a brief tutorial/explanation of jet kinematics and dynamics on my list of forecasting tools at my site, which includes a few visuals that may help some.