What is CAPE??

Larry J. Kosch

From time to time I hear storm chasers and nowcasters talk about CAPE. Just what is CAPE?? And how does this figure into the chaos of severe weather forcasting?? Thanks. LJK.
 
CAPE stands for Convective Available Potential Energy, and is a measure of the buoyancy of a layer. The larger the CAPE, the greater the potential speed of the updraft. (maximum updraft speed = sqrt ( 2 * CAPE ) ) CAPE can be visualized on a thermodynamic diagram by lifting a parcel dry adiabatically until it becomes saturated (Lifted Condensation Level). From there, lift pseudoadiabatically until the parcel temperature crosses the environmental temperature (Level of Free Convection). The area enclosed by the two curves between the LCL and the LFC is the CIN, or negative area. From the LFC up to where the parcel temperature again crosses the environmental temperature and becomes cooler than the environment (Equilibrium Level) is the positive area, more commonly called CAPE.

Ben
 
CAPE Values:
0001-1000 J/kg: Marginally (Weakly) Unstable
1000-2500 J/kg: Moderately Unstable
2500-3500 J/kg: Very Unstable
3500 J/kg and higher: Extremely Unstable

CAPE is measured in J/kg (Joules per kilogram).

CAPE does not take into account a cap, mixing, water loading, etc
You can still have values of CAPE and have no thunderstorms.

Souce: Various Literature

Mike
 
Types of CAPE:

MLCAPE: Mean Layer CAPE sometines refer to as Mixed Layer CAPE
CAPE is calculated using a parcel consisting of mean layer values of temperature and moisture from the lowest 100 mb above the ground level.

MUCAPE: Most Unstable CAPE:
CAPE is calculated using a parcel from a pressure level in the lowest 300 mb that will give you the most unstable CAPE there is.

SBCAPE: Surface-Based CAPE
CAPE is calculated using a surface based parcel.

Other Types of CAPE

DCAPE: Downdraft CAPE used to estimate the potential strength of rain-cooled downdrafts

NCAPE: Normalized CAPE is CAPE that is divided by the depth of the buoyancy layer.

Souce: Various Literature

I like using the MLCAPE over the SBCAPE and MUCAPE.

Mike
 
Dec 9, 2003
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Note the the "type" of CAPE to use depends a LOT on the situation at hand. This is especially true when the situation isn't a typical situation. For example, MLCAPE may not be as representative of the energy available to an updraft for low-topped or mini-supercells. Heck, Jon Davies has a good mini case-study about the tornado outbreak earlier this year in IL/IN and it explains how various CAPE measures may not have shown the situation very well at all. For a "typical" deep, moist convection environment, MLCAPE, however, is probably a more true representation of the energy available to an updraft.
 
CAPE

All of us know that a CAPE of 6000 J/Kg is considered a large number
that is often associated with severe weather once initiation is
achieved. However, what does this number mean in tangible terms?
First, consider that 1 Kg of air – the standard atmospheric density –
has a volume of 0.784 m^3 at 0C and 1000mb. Using the ideal gas law
and neglecting the effects of water vapor in our parcel, this volume
becomes 0.780 m^3 at 30C (86F), which is the about size of a cube
having 36 inches or 3 feet to a side.

By definition, this parcel has a maximum of 6000 J (Joules) of energy
available to it when it is lifted from the LFC (Level of Free
Convection, perhaps around 10,000 ft) to the EL (Equilibrium Level,
on the order of 30,000 ft), the region over which it is positively
buoyant. How much energy is this? 1 J = 1 Watt/second = 0.2389
calories. In terms of watts, this 0.780 m^3 parcel with 6000 J can
light a 100 Watt light bulb for 1 minute. Equivalently, it has 1433
calories of potential energy to contribute to convection.

CAPE is directly proportional to the maximum attainable speed of an
updraft:

Updraft speed (m/s) = square root of (2 X CAPE (J/Kg) ).

Therefore, our CAPE of 6000 J/Kg = 6000 m^2/s^2 can have a maximum
updraft speed of 109.5 m/s = 213 kt = 244 mph. In practice, the
updraft speed is much less then this – typically on the order of 50%,
as there are several other effects such as mixing, water and ice
loading, pressure effects, and vertical distribution of CAPE.

The vertical distribution of CAPE is important. The CAPE area (on a
sounding) can be long and "skinny" or it can be "fatter". For
equivalent CAPE values, the former would typically result in a lower
LI (Lifted Index = difference between environmental and parcel
temperatures at 500 mb) then the latter. The "fatter" CAPE
potentially has a greater degree of buoyancy over a smaller
altitude. The SPC "normalized CAPE" parameter in their meso analysis
web page provides a measure of the CAPE "fatness".

bill
 
Nov 12, 2004
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Remember that "not all CAPE is created equal". Skinny CAPE so-to-say is less favorable for severe weather events than when you have a very large positively buoyant region on the skew-t diagrams not only in terms of height but width. When CAPE is increasingly distributed vertically, the updraft becomes much weaker.

I won't explain in detail but will instead direct you to the METED site where they have a great presentation on this sort of thing:

http://www.meted.ucar.edu/mesoprim/cape/
 
Apr 22, 2004
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Since I didn't see mention of it, in regards to forecasting the potential for tornadoes, the total CAPE is less valuable than knowing the low-level CAPE. Sure, you need some buoyancy to have deep convection, yet in the presence of strong dynamic forcing it doesn't really take that much. With the goal of getting low-level rotation in a storm, you are generally going to need the help of stretching vorticity (spin) from buoyancy as close to the surface as possible - so having more CAPE available in the lowest 3 km will definitely help. Values of 150 J/Kg are more than adequate when combined with other favorable parameters for severe tornadic storms, but rarely will you have tornadoes (if at all) when this index value is not positive.

I'd also add that there is some controversy over parcel theory, on which CAPE is based. For the technical minded, here is a discussion on the subject:

http://www.cimms.ou.edu/~doswell/relbuoy/relbuoy.html

Glen
 

Joe Nield

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Remember, too, that even relatively weak instability, in the right configuration, can lead to low topped tornadic supercells, such as what occurred on April 20, 2004, in Illinois and Indiana. Relatively low CAPE, with the level of maximum buoyancy low in the atmosphere and the vast majority of the instability concentrated below 500 mb provided a prime environment for tornadic storms.

See Jon Davies' excellent case study of this event for more:

http://members.cox.net/jondavies3/042004il.../042004ilin.htm
 
Apr 22, 2004
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Originally posted by Joe Nield
low topped tornadic supercells

See Jon Davies' excellent case study of this event for more:
I would disagree that this is a good case to consider as a benchmark for low-topped supercells. This event really doesn't fit the mold, and I don't think this particular analysis is some of Jon's best work - though his case studies are generally top-notch. I think this link of his is more typical of the types of mini-supercell tornado events most often observed in the plains.

http://members.cox.net/jdavies1/041803cne/...e/041803cne.htm

Glen
 

Joe Nield

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Originally posted by Glen Romine+--><div class='quotetop'>QUOTE(Glen Romine)</div>
<!--QuoteBegin-Joe Nield
low topped tornadic supercells

See Jon Davies' excellent case study of this event for more:
I would disagree that this is a good case to consider as a benchmark for low-topped supercells. This event really doesn't fit the mold, and I don't think this particular analysis is some of Jon's best work - though his case studies are generally top-notch. I think this link of his is more typical of the types of mini-supercell tornado events most often observed in the plains.

http://members.cox.net/jdavies1/041803cne/...e/041803cne.htm

Glen[/b]
I was pointing it out more as an illustration of the importance of the configuration of CAPE in the sounding rather than a focus just on low topped supercells. I, too, have found his work to be quite good. I was wondering what about this analysis left you wanting?
 

Larry J. Kosch

Thanks for the Responses

Hey, Thanks for the great responses, guys. I knew CAPE meant something to the storm chasers since they talked about it so much during the chase season. For some of you, it gets really DEEP.

Any newbie greener than me probably would have thought you were talking about a cape, like the kind that Superman wears. Perhaps he was using the CAPE lifting potential for his flying abilities? Or does he need the wind drag resistance of the cape to keep his flying more stable?? LOL.

LJK.
 

Mike Smith

Here is a "meteorologically incorrect" thought from someone who has been forecasting severe storms for 35 years. CAPE is the most over- analyzed parameter in meteorology.

What, exactly, do you forecast differently when the CAPE is 3,325 versus 3,000? The answer is, nothing. Nor should you. In the late 1960's to around 1970, NSSL had a mesonetwork of hourly rawinsondes and found there were very signficant spatial and temporal variations in lifted index (more in a moment) in central and western Oklahoma during severe weather situations. With today's rawinsondes 250 miles apart and twice/per day, trying to recalculate CAPE to the tens and units places (and I have seen forecasters use programs to do that on many occasions!) is silly.

What, exactly, is the advantage of CAPE versus the lifted and Showalter indices? The latter two can be easily calculated by hand. Supposedly CAPE is "more precise." OK, but what do you forecast differently for a CAPE of 3,500 versus a lifted of -7? Again, given the documented spatial and temporal variations in these parameters, what does the "more precise" CAPE yield in terms of forecast value?

If the day comes (and I hope it will) where satellite or profiler technology will allow much more frequent soundings at much closer intervals than we have today, that are more accurate than current technology allows, we may well learn that using CAPE is a signficant step forward. In the meantime, you can save a lot of time and suffer from less information overload by just doing a quick LI and SI calculation, especially when in the field.
 
Dec 9, 2003
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What's the advantage of CAPE over LI? Lifted index is a measure of temperature difference at one fixed level (500mb usually). CAPE, on the other hand, is in integration of positive buoyancy (Tv parcel > Tv environment) through the entire layer... LI can be quite deceptive sometimes, such as for those cases with a warm layer near 500mb. Or when you are dealing with low-topped supercells in which the EQ level may only be 500mb (in which case LI could be 0), despite a fat CAPE profile between the LCL and the EQ level. Yes, you can't easily 'eye-ball' CAPE, so for that regard, LI is EASIER. However, I think the strength of CAPE in terms of proving a more accurature picture of potential energy available to an updraft makes it considerably better than a fixed-level index such as LI or Showalter's-Index.
 

Mike Smith

More accurate for what?

I re-ask my original question: What do you forecast differently with a CAPE of 3,250 versus 3,000? Especially (in the real world) if you were to launch a rawinsonde in the same airmass at the same time 20 miles away, the CAPE might vary by 1000 j?

I have seen times when computer-generated CAPE (I did not look at the sounding myself) was zero when the SI was negative and snow thunderstorms developed (granted, it may have been a problem with calculating CAPE in winter). Often, when we have to rely on a computer to do calculations, this type of error can occur.

My point is we spend far too much time worrying about CAPE, MUCAPE, etc., etc.
 
Apr 22, 2004
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I would agree with Mike that CAPE does seem to be overused, particularly model forecast CAPE. I'm not sure I'd sell out on LI and SI though - but really what seems most important is whether there is a thermodynamic environment supportive of deep convection. LI gives surface instability, and Showalter gives elevated instability in typical cases. However, because of the use of fixed layers in the calculations - you will likely miss a few events (e.g., when the moisture isn't at 850 mb), but for basic forecasting I agree it is probably the quick and dirty approach. CAPE and MUCAPE can serve as substitute for similar information, the former having similar problems with defining the surface parcel characteristics as in LI, and MUCAPE capturing elevated storm potential similar to SI.

I prefer CAPE and MUCAPE for the reasons Jeff mentioned - but really these parameters are low on my list relative to other forecast problems. Model forecasts of instability are often poor, and finding regions with otherwise favorable conditions and at least the 'potential' for parcels to become buoyant (even if the forecast model doesn't show it) is likely to work better, at least it does for me. Otherwise, small changes in moisture distribution, cloud cover, temperature, precipitation coverage, etc.... can lead to wholesale changes in instability profiles, and ruin a model forecast of them.

Glen
 
Dec 9, 2003
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More accurate for what?

I re-ask my original question: What do you forecast differently with a CAPE of 3,250 versus 3,000? Especially (in the real world) if you were to launch a rawinsonde in the same airmass at the same time 20 miles away, the CAPE might vary by 1000 j?
Mike, what do you forecast differently with an LI of -6.5 vs. -6? Both your question and my own are dealing with instability 'increases' of ~8%... But again, are you going to forecast differently for -6.5 than you would for -6?

I will agree that, because of the larger scale of CAPE (in the thousands) versus LI (in the single digits and occassionally double-digits), people tend to overanalyze model output (e.g. decide a target area based on ETA-forecast 2700 SBCAPE vs. 2500 SBCAPE), but from an objective standpoint, I certainly think that CAPE provides a 'better' view of potential instability. Yes, there's no way to eyeball it with much precision, so in that matter, LI is likely better if you are sitting out in a field looking at an 18z sounding. Then again, if you're out in a field looking at an 18z sounding, you obviously have access to a computer systme of some sort, and likely have access to CAPE calculations, as most sites put CAPE values on the sounding image.

Is it overused? Maybe... But instability is one of the key "ingredients" for deep convection... I tend to keep CAPE forecasts in mind when making a target area, since model forecast instability parameters are really all we have... I mean, you could just randomly pick a date and location sometime and hope for some action, but I'd rather take what we have (observations, model output, etc) and go from there... Yes, model forecasts of CAPE are sketchy, but again, that's the best we have. Until we have a surface observation network with very high spatial and temporal resolution (say, every 5-10 miles with obs every 5 minutes) nationwide, we have to make with what we have. Heck, one could argue that many convective forecast parameters (CAPE, helicity, bulk shear, etc) are meaningless given the high variability of said parameters, and given the paucity of the upper-air sounding network in the U.S. (and across the world for that matter), though that would make target-area forecasting success align more with luck than skill...