Downslope warming and convective initiation

[Rocky Rascovich]

There is never enough to learn when it comes to the mechanics of severe weather initiation. Below is an excerpt from the latest SVR watch that was issued at 1600cdt. I always thought that a cap can be strengthened with down slope warming. Are their exceptions to the rule? Is this a typical scenario in a set up like this?

Being I've been chasing over 30 years, studied meteorology to the best of my ability, I fear for someone like myself, I should know the answer. But I'll bite into my humble pie and learn from someone more knowledgeable about this. Thanks.



DISCUSSION...STRONG DOWN SLOPE WARMING HAS EFFECTIVELY REMOVED CAP
ALONG DRY LINE. WITH MOISTURE CONTINUING TO STREAM NW E OF DRY LN

 

[David Wolfson]

Since nobody seems to have ventured an answer, I'll give it a shot. I don't think the forecaster meant adiabatic downslope flow -- at least I hope not. I read it as a badly phrased way of saying that strong warming was occurring at lower altitudes, i.e. off east of the higher Plains regions. FWIW.

 

[cdcollura]

Good day all,

I believe this was from 9-30-2009, and I was in fact on that dryline chasing. I do not think a downslope wind would remove a cap, but make it stronger as mentioned.

What caused the initiation was the leading edge of the moisture axis AHEAD of the dryline, and the storms were very high based / elevated.

The dryline itself had minimal convection in the form of altocumulus with bases at least 15K AGL ... There was no low-level convection. With the passage of the dryline, winds veered rapidly from SE to SW, gusting to 40 Kts with blowing dust, and temperatures shooting into the upper 90's (down-slope), with dewpoint near 60 in the SE flow, and the 30's behind the dryline in the SW flow.

I see no connection on the cap weakening in this environment ... It actually got stronger near the dryline! Maybe the forecaster meant "overcome" the cap, not weakening it. The moisture, though, never returned as the models forecasted so.

 

[Rocky Rascovich]

Thanks Chris for your input. This was why I posted this question as I thought this would increase the cap. Any other comments would be welcome.

 

[Jim Bishop]

DISCUSSION...STRONG DOWN SLOPE WARMING HAS EFFECTIVELY REMOVED CAP
ALONG DRY LINE. WITH MOISTURE CONTINUING TO STREAM NW E OF DRY LN

I'd have to see the full discussion to make a fair judgement call. But based on the above quote no, downsloping will not remove a CAP.

The CAP barely broke on this day near/ahead of the dryline in areas where convergence was greatest, surface temperaures were quite warm, and I believe there was even a weak pertubation in the upper levels helping to cool the mid-levels ever so slightly. But the cap didn't completely break - storms struggled.

So even saying any reason for "the CAP was removed" probably isn't accurate either.

 

[Ian Miller]

Good day all,

I believe this was from 9-30-2009, and I was in fact on that dryline chasing. I do not think a downslope wind would remove a cap, but make it stronger as mentioned.

What caused the initiation was the leading edge of the moisture axis AHEAD of the dryline, and the storms were very high based / elevated.

The dryline itself had minimal convection in the form of altocumulus with bases at least 15K AGL ... There was no low-level convection. With the passage of the dryline, winds veered rapidly from SE to SW, gusting to 40 Kts with blowing dust, and temperatures shooting into the upper 90's (down-slope), with dewpoint near 60 in the SE flow, and the 30's behind the dryline in the SW flow.

I see no connection on the cap weakening in this environment ... It actually got stronger near the dryline! Maybe the forecaster meant "overcome" the cap, not weakening it. The moisture, though, never returned as the models forecasted so.

That sounds like a good analysis of what happened, thanks for that Chris

 

[Matthew Grzych]

I've been thinking about this one a little bit and came up with a theory. Maybe the forecaster was suggesting that low-level warming and mixing (from downslope flow) effectively made a inverted-V type profile along and behind the dryline allowing for high based convection to develop. A surging low-level layer of warm dry air under a cap certainly could remove it.

 

[Greg Blumberg]

I believe that the downslope surface winds, when they move down the mountain are subject to adiabatic heating. The technical term is called a katabatic wind. My guess is that the strong downslope heating pushes up under the warm moist air mass ahead of the dryline. This density inversion between the warm moist air and the warm dry air (the warm dry air is typically above the moist air.)

The pockets of enhanced moisture instability in that area due to this could help overcome the cap, but not remove it.

In relation to the forecaster's words, the only way I could think of the process removing the cap, would be by adiabatic heating assisting radiational heating within the warm sector. That in itself could create an environment warm enough to remove the cap as a factor in convective initiation, but not erode it. The cap itself would not be modified, but the environment below it.

I wish I knew the scenario regarding this situation, (soundings, surface data, visible satellite.) It might make things easier to understand with the forecaster's terminology.

 

[Matthew Grzych]

I believe that the downslope surface winds, when they move down the mountain are subject to adiabatic heating. The technical term is called a katabatic wind.

Actually a katabatic wind is density driven wind (like drainage flow) that originates from cold air descending downslope as fast speeds like what is seen in Antactica. This was more likely a Chinook type wind, which is not a katabatic wind.

If you take a look at a sounding with a cap in place you can "remove" the cap by following the temperature line down to the surface dry adiabatically. If the temperature at the surface were warmed to this temperature, then the cap is effectively removed. I am thinking that's what was forecasted on this day. They were thinking that the cap would be removed behind the DL by warm dry air mixing out the boundary layer. But storms would fire behind the DL because the convective temperature was met, not convergence in my theoretical scenario.

 

[Greg Blumberg]

Thanks Matt. I get my specialized winds mixed up sometimes between the Chinooks and katabatic winds. I always thought that a Chinook was a type of katabatic wind, but now I can see how they would be different.

So in your scenario, the cap would be eroded behind the dryline and thus would produce elevated convection as opposed to the surface based convection ahead of the dryline? In this situation, though, would the boundary prevent surface based convection by this method of cap erosion?

 

[Jeff Snyder]

So in your scenario, the cap would be eroded behind the dryline and thus would produce elevated convection as opposed to the surface based convection ahead of the dryline? In this situation, though, would the boundary prevent surface based convection by this method of cap erosion?

No quite. As the near-surface air is warmed as a result of downslope flow, remaining convective inhibition may be removed along and behind the dryline (as Matt pointed out). Convection can develop in this very well-mixed, but relatively dry, environment. Such convection can still be surface-based (i.e. parcels near the surface can still make up part of the updraft), though it likely will be very high-based. The term "elevated convection" is used most often to refer to deep, moist convection in which the parcels that comprise the updraft(s) do not originate near the surface. For example, to the north of a warm front, strong warm-air advection just off the surface may result in significant CAPE for air parcels near, for example, 850 mb. Meanwhile, the very low-levels (i.e. below the 850 mb height) is often quite stable, so the stable, near-surface parcels do not rise through the updraft.

It is quite common for surface-based storms to develop initially behind the dryline, in the hot, well-mixed environment. With time, these storms can move over and into the warm/moist sector.

 

[Matthew Grzych]

So in your scenario, the cap would be eroded behind the dryline and thus would produce elevated convection as opposed to the surface based convection ahead of the dryline? In this situation, though, would the boundary prevent surface based convection by this method of cap erosion?

If you mix out the planetary boundary layer behind and along the DL such that the cap is removed (by reaching the convective temperature), convection would be high-based, but still surface based. The LCL would be high, but storms would not be considered elevated. If upper level flow was sufficient and the DL was not surging eastward, then storms could move east into the deeper moisture and become more productive.

Essentially, the DL forces convection by 2 methods. 1. surface convergence 2. Deep mixing. I assume people don't often think about deep mixing and convective temperature being met as an initiation factor with the DL.