Storm heights, LCLs, and T/Td spreads

[Rob H]

I have a few questions on storm heights and specifically what makes a setup favorable for tornadogenesis. Is looking at the surface T/Td spreads basically a rough indicator of where the LCL is at? Are there any other methods besides LCL that should be used to measure/estimate a storm's height? What LCL or T/Td spreads are you looking for in tornadogenesis? I'm guessing this last question is similar to "What temps at 700mb indicate a strong cap" and that there are no definitive answers, but more of a set of rough guidelines.

 

[Darrin Rasberry]

I have a few questions on storm heights and specifically what makes a setup favorable for tornadogenesis. Is looking at the surface T/Td spreads basically a rough indicator of where the LCL is at? Are there any other methods besides LCL that should be used to measure/estimate a storm's height? What LCL or T/Td spreads are you looking for in tornadogenesis? I'm guessing this last question is similar to "What temps at 700mb indicate a strong cap" and that there are no definitive answers, but more of a set of rough guidelines.

I, too, am a beginner, so take this with a grain of salt.

LCLs occur when the moisture saturates due to cooling in the atmosphere as the air parcel (the old "car-sized chunk of air" thing) is lifted from near (50mb of pressure less) than the ground level. As it cools, it will, on a stormy day, eventually saturate and begin condensing to clouds all the way up to the top of the anvil, so long as the layer of free convection (LFC) is relatively close to the LCL.

A high T/Td spread does not mean you won't have high LCLs or at most elevated storms and whatnot. These packets cool in the atmosphere, so if there's a hot cap up above, well, they won't cool at all. Additionally, since LCLs depend on how cold the atmosphere is, you can have some pretty low LCLs and even surface-based convection with ridiculous dewpoints, like in Nebraska on the 4th of this month. Cold core setups do that for ya

Also, keep in mind that high-based does not mean elevated. A high based storm (LCLs 1000m or greater AGL IIRC), to borrow some familiar language, can and occasionally do produce tornadoes. But it's more likely with lower LCLs than with higher ones.

More info: http://www.theweatherprediction.com/habyhints/299/

 

[Rob H]

Thanks for the post, Darrin. The URL you linked to is actually what inspired this thread:

In a supercell thunderstorm situation, a low LCL (closer to surface) increases the likelihood of tornadogenesis since the region of CAPE will be closer to the surface.

I got to wondering what constitutes a "low LCL" and how does that relate to the setup (as you mentioned cold core setups are different). To be more precise, the kind of information I was looking for was along the lines of what you said in your last paragraph, but with a little more surrounding context:

high based storm (LCLs 1000m or greater AGL IIRC)

 

[Rob H]

I finally came across this image on Jon Davies' site, which is pretty much what I was looking for.

From the image, here is a summary of general parameters involving LCLs and favorable tornado development:

>1500 = Poor
>1250 = Marginal
>1000 = Good
<1000 = Strong

 

[Jeff Duda]

A high T/Td spread does not mean you won't have high LCLs or at most elevated storms and whatnot. These packets cool in the atmosphere, so if there's a hot cap up above, well, they won't cool at all.

A matter of clarification for you, Darrin:
If you have high T/Td spreads, then LCL heights will be high. This is due to the dry adiabatic lapse rate, which is about 9.8 K/km near the surface, and is pretty much the same no matter what the temperature is. Also, mixing ratio values will remain constant throughout the rise of an unsaturated parcel. Consult a skew-T on how that works. Also, if a parcel of air is rising, it will cool no matter what kind of environment it is rising into. If an air parcel rises into a cap, it will become stable relative to the cap and will want to sink back down. However, if enough forcing exists to keep the parcel rising, it will continue to cool as it rises through the cap.

 

[Skip Talbot]

A side point that I have noticed is that you often get more photogenic opportunities with higher LCL's rather than lower LCL's, provided that the storms produce. Low LCL's often produce ground hugging, scud infested, grungy grey storms. This happens alot on many high risk setups. Some higher LCL events, on the other hand, can produce beautiful supercell structure and very tall, impressive looking stovepipe/rope tornadoes, like you see with slight and moderate setups on the high plains.

 

[Darrin Rasberry]

A matter of clarification for you, Darrin:
If you have high T/Td spreads, then LCL heights will be high. This is due to the dry adiabatic lapse rate, which is about 9.8 K/km near the surface, and is pretty much the same no matter what the temperature is. Also, mixing ratio values will remain constant throughout the rise of an unsaturated parcel. Consult a skew-T on how that works. Also, if a parcel of air is rising, it will cool no matter what kind of environment it is rising into. If an air parcel rises into a cap, it will become stable relative to the cap and will want to sink back down. However, if enough forcing exists to keep the parcel rising, it will continue to cool as it rises through the cap.

Thanks for the info Jeff! I did not know that about the high T/Td spreads (although, I did make a typo - I meant to say they won't cool WELL at all).

How much surface RH corresponds to high-based storms?