The anvil's role in lowering LCLs

[Kevin Walter]

I'm bored right now, so I want to throw out something I've been pondering for some time. When we all look at surface maps, we often note high temperature/dewpoint spreads and can sometimes disregard an environment as one that would exhibit "too high of an LCL for tornadogenesis". However, in a typical setup for supercells and tornadoes, you have wind shear, which means the anvil will be blown off out ahead of the storm, providing some "shade" for the air east of the storm that will later become the storm's inflow. Thus, in a storm-local sense, you may have surface conditions of 73 over 65, rather than 90 over 65 like all the "synoptic" observing networks in the region (ASOS) are reporting (the obs we're using to make our assumptions). It would be interesting to go back and look at some of the mobile mesonet temperature and humidity trends we took here at TTU in our storm intercepts this year to look into this.

Just throwing this out, feel free to comment at will.

 

[Kyle Masters]

I'd agree with that, but the only issue I see is those anvil clouds providing shade.

Yes, it'll stop the heating, but I think it would be just like a cloudy night...since the clouds are there the heat can't escape the atmosphere so temperature pretty much is just sustained...not increased.

It would be interesting to see some research on this matter.

 

[Steve Miller TX]

This is a very interesting topic for discussion and I eagerly look forward to other's input. Below, I refer to "parcel" as a sampling of the air being ingested into a storm.

Afer thinking about this for a few moments, it seems to me that the whole question really boils down to exactly what parcel is being lifted and in what part of the storm. I mean, there are certainly different "parcels" being ingested into the updraft, right? No where is this more evident than a developing wall cloud as the rain cooled air gets sucked in...and this "parcel" definitely has a lower LCL.

So, in that train of thought, I certainly believe that the cooler anvil shadow region aids in lowering LCLs in the same way but I think with much less of an impact than other "parcels" as I descibed above with the warm, moist "virgin" air being a large majority of "parcels" lifted....the rain cooled air being second and the anvil shadow air being a distant third...but still important! In fact, Eric Rasmussen did a nice study on the effects of anvil shadows creating a baroclinic boundary for the storm to relate too. I'm not sure where to find this offhand, but it would be great to read through that again.

Too much rain cooled air or anvil shadow cooled air being ingested would certainly lower LCLs, but also severely impact the necessary CAPE for violent updrafts. Of course, you still have air around the 5000 foot level being ingested at this point (largely as an effect of a low level jet) which can still offset rain-cooled surface air and maintain significant amounts of CAPE. But, doesn't it now become more elevated now? I think so. I can recall many storm events with this type of transition.

But, back to the anvil shadow affecting LCLs. I think in summary that it is a very small influence with other things like forward flank precip and rain cooled air being the dominating influence.

My $0.02 worth

 

[Mike Peregrine]

I feel like the area that becomes shaded by the anvil or affected by rain-cooled air may affect a supercell's lifecycle to a degree (depending on storm motion and steering winds), but we have to remember that when we are talking about long-lived supercells, or cyclic storms, the updraft is being fed from an angle that generally protects the storm's source of fuel - at least for a while, and perhaps for a great length of time. With a classic supercell, surface winds are feeding the storm from the south or southeast usually, while cooled air remains off to the north/northeast. Highly tornadic storms often ride boundaries with significantly cooler air on one side of the storm as opposed to the other. Depending on a chaser's location in the storm itself, it is not uncommon to experience serious temperature fluctuations. But if you remain under inflow, you'll feel the continuous warm, moist air adding buoyancy to the updraft. In an HP storm, the rain and rain-cooled air often wraps the meso, which can more to affect the lifecycle of the cell. In the right circumstances, supercells can live for hours, covering vast areas under the shade of an anvil, and yet the source of inflow is able to remain protected. Storm motion and steering winds may come into play when it comes to the storm eventually encountering an unfavorable, self-induced environment that seriously affects its ability to perpetuate a healthy, single rotating updraft. Anyway - this isn't really what you are asking about in your thread -

As far as the anvil's ability to successfully lower LCLs, this is an interesting point and hopefully we'll see more discussion - I know there are plenty of days with what I would consider to be an unacceptable LCL number that ends up producing tornadic supercells, and this may be one explanation.

 

[mikegeukes]

Observations of Low-Level Baroclinity Generated by Anvil Shadows

-Authors: Markowski, Paul M., Rasmussen, Erik N., Straka, Jerry M., Dowell, David C.
-Issn: 1520-0493 Journal: Monthly Weather Review Volume: 126 Issue: 11 Pages: 2942-2958
-Authors: Markowski, Paul M., Rasmussen, Erik N., Straka, Jerry M., Dowell, David C.

The article you have requested is available via subscription.

The link to the abstract is found below:
http://ams.allenpress.com/amsonline/?reque...BG%3E2.0.CO%3B2
http://ams.allenpress.com/amsonline/?reque...BG%3E2.0.CO%3B2

Mike

 

[Shane Adams]

I agree more with the philosohpy that an anvil shadow more sustains a favorable storm environment as opposed to hurting it, in the case of an isolated storm with no other development around, say within 50 miles all directions. Especially in situations where storms form near a dryline, and take time to root as they mature; you have a base that's continaully lowering as the storm moves into better air, as the storm continues to mature, with the anvil taking shape and becoming the "shadow" well-after the storm establishes itself as a tornadic threat. Having sen this several times, I find that isolated storm anvils have no real effect on lessening a storm's tornadic ability once that ability has been reached.

However in situations with storms in close proximity, anvils can be a very bad thing for other storms. Storm anvils that are upstream can seed their neighbors and effectively kill that storm's local environment, by robbing instability through rain-cooled inflow.

 

[Jeff Snyder]

I agree with the thoughts of Steve and Mike... The anvil shadow can certainly locally cool temperature, thereby decreasing T-Td deficits and lowering LCLs (assuming Tds don't change). However, as Steve noted, by changing the temp of the updraft "parcel", you'd (in this case) dramatically decrease CAPE and updraft intensity. So, I think a lot of this comes down to how much of the anvil-shadow air is ingested into the updraft. Obviously, we see evidence of rain-cooled air being reingested into the updraft in the form of a wall-cloud. If we examine the typical supercell, with FFD/precip to the northeast (downshear) and the updraft to the southwest, the "typical" parcel source region is to the south or southeast of the updraft -- away from the anvil shadow. This is good in many cases, since it'll allow the supercell to ingest "virgin" (to steal Steve's terminology), unstable air.

Now, I havent' read the Markowski et al. paper that Mike linked above, but the anvil shadow does help create a local baroclinic boundary on which the supercell can propagate. The baroclinically-generated vorticity can be ingested (tilt and stretch) into the supercell updraft, which in turn can increase updraft rotation and updraft strength. There used to be a theory that tornadoes developed largely because of the baroclinically-generated vorticity along the southern edge of the FFD gust front (where the FFD gust front met with the inflow). However, this vorticity doesn't appear to be the main "cause"/source needed for tornadogenesis now. So, as Shane noted above, I do think that the anvil shadow can certainly aid in the persistance of a supercell, though I'm not sure of it's effect on lowering LCLs and increasing tornado potential.

 

[Kevin Walter]

Great discussion so far -

This all came to mind when I saw a post in another forum about someone writing off the tornadic storm in the NE panhandle the other day b/c the LCLs would be too high. Indeed, ASOS obs from the area were reporting T/Td deficits of at least 24 degrees, much higher in some cases. At the same time, the storm was slowly propagating to the SE, not in a direction where it ingested air that had resided for a long time under cirrus, but air that had spent maybe just enough time under the SE part of the anvil (sfc winds were from the ESE) to cool sfc temps and lower LCLs, but not enough to significantly affect potential instability. Also, the storm was isolated, so it wasn't as if there was alot of grunge cirrus cutting down surface heating everywhere.

If you'd like to take a look at the sfc obs, sat, and radar, I'll forgo posting them here (mainly b/c I don't know how to) and just point you to www.rap.ucar.edu/weather and particularly the Jun 27, 23Z single images of sat, sfc, and radar in the Nebraska panhandle (they are archived up to 6 or so days right from the dropdown menu). These are certainly interesting images and a very unique case of an isolated tornadic supercell in late June.

 

[Jim Tang]

I feel your pain Kevin. Man, if I knew how to post satellite and radar images, I would!

Everybody here go take a job at SPC! Still a newbie here, so what I am saying below could be false.

Anyway, about anvils and LCLs-

I believe that anvils can indeed lower LCLs greatly. Just a theory here, but wouldn't the virga falling in front the anvil increase humidity and therefore decrease LCLs? And the anvil itself could decrease temps by a few degrees.

However, most importantly, the virga and clouds wouldn't decrease instability. Again, just a theory- the ice crystals in the anvil would probably decrease temperatures that high up to cool a few degrees, which balances out the decrease of temperatures at the sfc.

For a sup that moves south as opposed to east or northeast, I'm saying that the anvil shadow could cool the degrees down by a few degrees.

 

[Jeff Snyder]

Originally posted by Jim Tang

However, most importantly, the virga and clouds wouldn't decrease instability. Again, just a theory- the ice crystals in the anvil would probably decrease temperatures that high up to cool a few degrees, which balances out the decrease of temperatures at the sfc.

The only problem with that theory is that the virga would fall with/near the front-flank downdraft, which is downshear from the updraft. I mean, for a typical southwesterly-flow-aloft situation, the precip falls northeast of the updraft, while inflow is from the southeast or south. With the typical 35-120kts in the mid and upper-levels, this "cooled' air aloft is quickly transported downstream and away from the updraft.

 

[Mike Johnston]

Perhaps the anvil could play a role in lowering surface temps around the storm, but seems like it would be a challenge to isolate this variable in a mesonet-type experiment. Alot of other factors - angle of the sun, other general cirrus cover, etc. would seem to have as much, if not more impact.

It's an interesting theory, but if you consider it's possible significance for the storm environment as a whole, I'm not sure as a chaser I'm looking for a thick anvil to lower surface temps out ahead of the storm. After all, aren't updrafts stronger w/ back-sheared anvil storms? I know there's a balance here, but seems like we would generally be looking for factors which might increase the dewpoint, as opposed to lowering the temp, to bring down LCL's.

 

[Ben Cotton]

Originally posted by Jeff Snyder+--><div class='quotetop'>QUOTE(Jeff Snyder)</div>

<!--QuoteBegin-Jim Tang

However, most importantly, the virga and clouds wouldn't decrease instability. Again, just a theory- the ice crystals in the anvil would probably decrease temperatures that high up to cool a few degrees, which balances out the decrease of temperatures at the sfc.

The only problem with that theory is that the virga would fall with/near the front-flank downdraft, which is downshear from the updraft. I mean, for a typical southwesterly-flow-aloft situation, the precip falls northeast of the updraft, while inflow is from the southeast or south. With the typical 35-120kts in the mid and upper-levels, this "cooled' air aloft is quickly transported downstream and away from the updraft.[/b]

Not only that, but the falling ice crystals would likely evaporate long before they hit the ground, so the increased RH would occur in the upper or mid levels, not a the lower level where it would be able to affect the LCL.


Ben