"H7 temps" and Assessing Convective Potential

[Jeff Hawkins]

Well, here's a question from a "semi-newbie" who's learned the basics but has read threads where there's technical references that I don't "get" and can't find any previous discussion about. This is just one item, there may in the future be more!!!!

I have read of "H7 temps" referenced when discussing forecasts. What are they? Where can you find them? When should they be used and relevant? Why are they important?

 

[rdale]

H = height, aka millibars. H7 = 700mb.

 

[Patrick Marsh]

Hi Jim,

Anytime you see a reference to "H_" where the _ is a number, the author is typically referring to the height of a specific pressure level in the atmosphere. H7 is the "H"eight of the "7"00 mb level. Other common levels are H85 = Height of the 850 mb level, H5 = 500 mb level, H25 = 250 mb.

The H7 level is often examined for signs of rising or sinking motion (omega). Also, this is one of the levels that can be used to estimate the strength of the convective inhibition (via the temperature). (The "nose" of the cap typically resides somewhere between the 850mb and 700mb levels.)

Before explaining too much, I'd suggest reading http://www.jondavies.net/700mbTcap/700mbTcap.htm first and then coming back with any questions you might have. Cheers.

 

[Jeff Hawkins]

LOL, thanks for the replies...I'm having one of those "duh!" moments...but I wanted to be sure I wasn't missing something.

 

[Jeff Passner]

Jim, great question... Of course, just remember that 700 hPa temps are relative. Generally, anything over +13C makes it difficult for storms to form in the Plains...However, that is just a random number. The atmopshere really does not care about rules and numbers. The "magic" number is very different between Denver and Oklahoma City because of elevation. It also varies for seasons. A +10C 700 hPa temp in March might be a huge cap but in June it can be broken because of surface-based heating. I look for +10C in March as a "cap" and +14C as a cap in June. However, occasionally, if there is enough dynamic lifting you can break a cap. It's risky as a chaser to sit under a +14C cap... Most of the time you watch cumulus clouds and not much else.

 

[Skip Talbot]

Using just the H7 temps is a rather crude way to look at the strength of the cap isn't it? Its like judging instability by looking at just the H5 temps. Sure the inversion consumes a lot less area in the vertical profile than the area of instability does, but how stable the inversion is still relative to the rest of the profile. Its going to be influenced by things like the time of year, time of day, geographic location, and the synoptic pattern. I like to look at the Cap Index (the Lifted Index equivalent of the inversion's strength as opposed to instability), and of course the CINH (the CAPE equivalent of the inversion's strength). Earl Barker's plot of the Cap Index is one of my favorites:
http://wxcaster.com/smallfiles_central_svr.htm

 

[afischer]

Jim,

Here's an additional resource on H7 temps & capping: http://www.crh.noaa.gov/unr/?n=700mbTempsSvr

 

[Jeff Hawkins]

Man, leave it to the scholars of StormTrack to make a simple question a really technical discussion...thanks! That's what I love here! (of course hope in the future to do the same and give back as I learn more too) So if H7 temps (or H5 temps etc) are worth looking at/monitoring, what other parameters are your 'favorites' to study when forecasting?

 

[Brandon Sullivan]

Using just the H7 temps is a rather crude way to look at the strength of the cap isn't it? Its like judging instability by looking at just the H5 temps. Sure the inversion consumes a lot less area in the vertical profile than the area of instability does, but how stable the inversion is still relative to the rest of the profile. Its going to be influenced by things like the time of year, time of day, geographic location, and the synoptic pattern. I like to look at the Cap Index (the Lifted Index equivalent of the inversion's strength as opposed to instability), and of course the CINH (the CAPE equivalent of the inversion's strength). Earl Barker's plot of the Cap Index is one of my favorites:
http://wxcaster.com/smallfiles_central_svr.htm

Agreed.. The thermal profile in its entirety up to 500mb is something I look at when making an in depth forecast...

There has been a few setups this year where the "CAP" doesn't actually reside at 700mb.. and it lies lower around 800 - 850 mb... So looking at a color fill map of H7 temps isnt always a good deal..

No matter what, always look at a Skew T!! Contour maps just cant always do it.. A warm layer hidden in there will kill your lapse rates, or maybe depending on the location, completely stop convection from forming, OR, lead convection to be elevated if the boundary layer is stable. Ive found that a warm 850 layer often yields a quick transition to a forward propogating mcs.

Other Things... NEVER forget about the importance of synoptic features.. Dont get lost in all the parameters all the time.. Find the boundaries, the upper features, find where the support is going to be, then go from there..

Unless you have a vast understanding of the parameters and what goes into each of them, they can be useless.. They are usually driven by multiple variables...

Not a full list of everything I use, of course. But just some pretty general things to keep in mind when forecasting.

 

[Jeff Duda]

I think CIN is the best indication of capping, as it is basically a vertical integral of cap strength at each layer. Temps at one level may be good for a cursory evaluation, but to really get an in-depth sense of the resistance to free positive buoyancy, you can't argue with CIN.

 

[Jason Boggs]

Myself, I like looking at EHI, CAPE, Sfc-6km bulk shear and moisture convergence among a few others.

 

[Brandon Sullivan]

I think CIN is the best indication of capping, as it is basically a vertical integral of cap strength at each layer. Temps at one level may be good for a cursory evaluation, but to really get an in-depth sense of the resistance to free positive buoyancy, you can't argue with CIN.

Again, CIN is a great tool.. BUT.. It has its pitfalls.. Take a look at this image..

Both images have the same CINH.. BUT.. The one on the left has a stronger CAP..

As I think Skip mentioned, I REALLY like the Cap Strength index... Basically it is (Twhateverheight-Tparcel).. Basically it allows you to determine the strength of the CAP.. Pretty straight forward.

 

[Patrick Marsh]

Wow, this thread sort of took off... A couple of things...

For those who are arguing against using the 700mb temperatures for cap strength, if you are saying this based on my previous post, rest assured, I'm not advocating solely using the 700mb temperatures. In fact, I explicitly state this is one level that can be used to estimate the cap and the cap is often found below the this level.

The truth of the matter is, you have to evaluate everything in context. You need to think about this in terms of parcel theory.

Stable Parcel: An unstable parcel can be thought of in terms of a marble in the bottom of a bowl. No matter what force you exert on the marble, it will always return back to the same position.

Neutral Parcel: A neutral parcel can be thought of in terms of a marble on a flat plane. When a force is exerted on the marble, the marble will move away from the original location.

Unstable Parcel: An unstable parcel can be thought of in terms of a marble on top of an upside-down bowl. The slightest force exerted on the marble will cause the marble to accelerate away from its original location.

CAPE stands for Convective Available Potential Energy. It's potential energy that is available for release should a parcel be able to access it. Just because a parcel is unstable (or there is no convective inhibition) does not mean that you'll have a thunderstorm develop. You still have to have some initial focus of lift to dislodge the parcel. A CAPE of 5000 J/kg is meaningless unless you have something to initiate lift to release the Potential Energy.

So, no matter what the CAP, CINH, etc you have to assess the vertical motion in conjunction to get a more complete picture.

Lastly, there is an argument about CAP vs. CINH. CINH is proportional to the summation of (Tenv - Tparcel) < 0. In other words, the CINH value is proportional to the accumulation of the cap strength at every level that has a positive cap strength. CINH is an integral quantity, whereas cap strength is a single-level quantity. If you simply rely on low-level heating as your source of lift, the two soundings above are identical. The same amount of sustained heating would be required for initiation. The difference comes in terms of forced ascent. If you have strong enough low-level convergence to forcibly lift a parcel through the cap on the left, you can get convection before convection with the sounding on the right. On the flip side, weak forced ascent throughout much of the atmosphere might be enough to overcome the CINH on the right, whereas it wouldn't be able to force the parcel through the strong CAP on the left. (So, depending on the character of the forced ascent, i can get either sounding to convect.)

Edit:
I oversimplified things last night in my haste to post. In the last paragraph, it isn't low-level heating that I should have referred to. I was trying to come up with something concrete to use instead of the mysterious quantity of "energy". In reality, the sounding on the right would require less daytime heating that the sounding on the left. However, the same amount of "energy" in the parcel would be required to overcome the inhibition. (I probably confused things even more...)

 

[Patrick Marsh]

MOD NOTE: I have renamed this thread to account for the discussion relating H7 temperature to assessing the potential for convective initiation.

 

[Jeff Snyder]

I guess it depends upon how you define "cap". Traditionally, it's been defined analogous to the Lifted Index, in that the "maximum" cap is typically the greatest Lifted Index in the lower portion of the troposphere (or the greatest temperature difference between the warmer environment and the cooler parcel). However, similar to how many operational meteorologists typically prefer CAPE to LI, many tend to prefer CINH to "maximum delta-T" (or a cap-LI) since it is an integrated quantity (and, therefore, includes thermo information for a depth of the atmosphere as opposed to a single level). I haven't used the so-called "cap index" (shown on wxcaster.com , Earl's site), so I will yield to your experience using it if you find it useful.

As you noted, however, knowing the PROFILE of CINH can oftentimes be very useful. In your example image, while both profiles would require the same energy input to get a surface parcel to its LFC (assuming CINH is the same), seeing the skinny tall profile would make me feel a bit better about initiation (all other things being equal, and for low to moderate low-level convergence) since you'd only need a slight increase in surface temp or dewpoint to shift the surface parcel trace rightward to eliminate the CINH. In addition, you probably aren't going to lift JUST the "surface" parcel, so CINH for a parcel just off the surface may be significant weaker and may allow for the initiation of slightly-elevated convection (which may become surface-based when surface convergence increases along the developing storm's gust fronts). On the other hand, for the profile on the left, the surface temperature and/or dewpoint may need to increase significantly to eliminate CINH. So, if I'm worried about initiation, I certainly would like to know the low-level thermo and CINH profile / vertical structure to assess how much heating/moistening it would take to reduce or eliminate the CINH.