Subsidence and Water Vapor Imagery

[Jake Wallentine]

Hi all,
First off, wasn't able to post this question in the education forum for some reason, so decided to put it here. Feel free to move this thread mods if you feel the need to. I wasn't able to find my answer to this really anywhere else on ST after performing a search, so I thought i'd go ahead and ask it.

Back on Thursday 4/24, when there were a lot of chasers out hoping for something to break the cap in central KS, SC KS, and NC OK, I was monitoring the conditions closely back from Norman. I understand that the cap was what held convection down overall in the system, but I read several NWSFO's discussions through HWOs and AFDs and they continued to mention the cap would be gone or was weakening significantly by 0z. This really confused me as to why nothing was initiating even with a weak or weakening cap on the dryline. So, I asked one of my grad student friends here at OU and he didn't have time to explain it thoroughly, but he said that subsidence was the main contributor to the lack of convection. I asked him where I might look see where subsidence was occuring and he told me the best thing to look at would be water vapor imagery. Unfortunately, he didn't have time to further explain this. So, I was wondering what I would look at in the water vapor imagery that would suggest subsidence in the atmosphere? I remember that Mike Umscheid and Shane Adams mentioning something about the subsidence that day also. Is the cap and subsidence interrelated, or are they two completely different things?...and was the veered 850mb winds what caused a lot of issues as far as no initiation that day? Thanks ahead of time for any answers!

 

[rdale]

Cap and subsidence can be related - there's a nice training session on using WV imagery at http://cimss.ssec.wisc.edu/goes/visit/water_vapor.html

Go to the bottom for the downloadable version.

 

[Phil Kurimski]

Subsidence means there is sinking motion...typically associated with a high pressure system. In an area of subsidence you will typically see clear or mostly clear skies.

The cap is where temperatures in the mid levels are warm enough that they stop convection at a certain level. You can have a cap in place with cumulus clouds...however the cap will stop their growth at a certain level. This will be the level where the air is warmer than the parcel rising from the ground.

So as you can see the cap and subsidence can be two different things.

 

[Jeff Snyder]

I understand that the cap was what held convection down overall in the system, but I read several NWSFO's discussions through HWOs and AFDs and they continued to mention the cap would be gone or was weakening significantly by 0z.

Actually, I'm not sure the cap was the thing that kept convection in check along the dryline. The 00z OUN sounding shows essentially no cap (even as seen by the MLCINH), and the 00z DDC sounding modified for surface conditions just east of the dryline (just E of DDC) also implies no cap. There is the possibility that warmer 850mb temps nosed in just above and immediately east of the dryline, but model analyses from that (supported by the DDC soudning) doesn't suggest that to have been the case (particularly N of the OK/KS border). Alas, we see here that there is more to convective initiation than capping (or the lack of). Honestly, I don't really know what prevented initiation along the dryline, except it's a good bet that large-scale subsidence behind the lead vort max played a role. It is possible that the deeper moisture was never able to advect westward enough to get right ahead of the dryline, as sfc obs showed a mixing zone between the dryline and deeper moisture to the east (similar to what we saw in central and southern Oklahoma earlier last week). You can infer areas of subsidence and ascent by looking at water vapor in a lagrangian reference frame. In other words, is the very dry air aloft (deep red) expanding in size? Note that areas of moist and dry air aloft moving around (i.e. only advection) does not necessarily imply subsidence or ascent -- you should look at how those areas are changing with time as they move around.

A recent case from last year similar to this was 6/7/07, when storms tried to develop in central Oklahoma. The 00z OUN sounding that day showed negligible CINH, yet the towers the developed (one or two even glaciated, IIRC) did not persist for more than an hour, and it was largely a bust. Maybe there is something to be said of the conceptual model of organized severe weather and tornado events that places the anticylonic shear side (i.e. warm side) of the upper-level jet in an unfavorable area... Large-scale subsidence is much, much weaker in scale (i.e. on the order of cm/s) than deep moist convection (on the order of 10s of m/s), so one wouldn't that that, on scale analysis alone, synoptic-scale vertical motion can directly affect convection (aside from modifying the thermodynamic profile that affects said convection). It does seem plausible that large-scale subsidence aloft would weaken low-level convergence (by mass continuity alone) and it may significantly limit the dryline circulation that we often rely on to initiate convection.

EDIT: Just to address a minor pet peeve of mine... "cap" is not an acronym (*wink wink, nudge nudge, Kenny /below*)! CAPE is an acronym (Convective Available Potential Energy), but there is no need to capitalize all the letters in "cap" as CAP. The cap is a bit of slang, I suppose, to refer to a capping inversion. So, folks should write "The cap is weakening" instead of "The CAP is weakening". This isn't directed at anyone who has posted above me, but I read it quite a bit in FCST and NOW posts.

 

[Jake Wallentine]

Appreciate it guys...definately cleared some things up for me on the issue.

 

[Kenny Drake]

You cannot diagnose whether convection will initiate simply by looking at thermodynamic profiles. There can be absolutely no CAP and yet no convection. There needs to be some form of dynamic forcing in order to initiate. If there is no CAP and there is forcing, there will be widespread convection in that area. This is why a lot of chasers look for a CAP, but a CAP in which adequate forcing can overcome on a conditional basis yielding the possibility for discrete storm mode. Water vapor imagery is a great way to see forcing, both subsidence and ascent. Subsidence will show up as an area with relatively no water vapor. WV imagery from Thursday showed a giant brown area over most of OK, indicative of subsidence. Ascent is harder to pick out to the untrained eye. Generally I look for areas of rotation (which is not easy sometimes). Areas of rotation on w/v show shortwaves. Just ahead of the shortwave there will be enhanced vertical ascent. This does not mean you have overall vertical ascent. For example, a shortwave embedded in NWerly flow associated with a much larger wave will generally still yield net subsidence.

From an opposite perspective, there can exist plenty of vertical ascent but the CAP is simply too strong to yield convection. I'm guessing there must be a relationship between the kinetic energy associated with vertical ascent to CINH. (vertical velocity)^2 should yield energy per unit mass. I guess this could be compared to CINH (energy per unit mass). I haven't thought about it too much but it is worth mentioning.

 

[rdale]

Kenny - good insight, but read the last paragraph of Jeff's POST again

 

[Glen Romine]

There needs to be some form of dynamic forcing in order to initiate. ..... Water vapor imagery is a great way to see forcing, both subsidence and ascent.

And this is one of my pet peeves.... While I would agree with some of the ideas mentioned - I'm going to pick a bone with the idea that the forcing to initiate convection comes from the broad ascent associated with a shortwave. Instead, this mid-tropospheric ascent is typically on the order of 0.1 m/s, very modest ascent with mostly horizontal wind motion, where gentle isentropic ascent leads to adiabatic cooling that may result in reduction in the cap strength (although not always cooling that low down). Thus, the large scale ascent, such as one might infer from WV imagery, primes the environment, perhaps lowering the amount of lifting needed for convection to initiate. Forcing to initiate surface-based convection typically needs to come from the surface (there are exceptions to everything in the atmosphere) - be it from frontal /boundary lifting, orographic, thermals, etc... and these lifting features generally cannot be identified using WV imagery. Also, just because you see a WV image suggesting mid-level subsidence, it does not mean you cannot have convection. But, that gets into a longer discussion, so I'll cut things off here.

 

[Chris Bowman]

This is a good discussion, especially since I was sitting at my office thinking things had decent shot of going. I was watching some subsidence on the WV imagery that I thought would start to erode the thick cloud cover we were having. This indeed happened and with good moisture flowing to the north CAPE started to rapidly increase. I was actually confident enough to head out to about 30 miles east of the really good dryline convergence noted on radar around Barber county. Obviously nothing happened so the next day I went and looked at the data. One thing I think had a factor was going back and looking at the 1.5 potential vorticity anomaly pressure plot and seeing that there was basically northing south of I-70. The best PV and associated forcing was across northern Kansas. So despite getting good convergence on the dryline with plentiful CAPE and other parameters that all looked favorable there just didn't seem to be much/ anything to help push the domino over across southern KS. This is what really struck me. I'm sure there may be other factors but that's what I noticed.

That 00Z OUN sounding was amazing though. Too bad it couldn't have been realized.