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.

 

[Ben Cotton]

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

 

[mikegeukes]

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

 

[mikegeukes]

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

 

[Jeff Snyder]

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.

 

[Bill Schintler]

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

 

[Alex Lamers]

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/

 

[Glen Romine]

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]

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

 

[Glen Romine]

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]

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?

 

[Rockwell Schrock]

Where can CAPE data be found on the 'net?

 

[David Draun]

SPC mentions CAPE in their forecasts. I like to go look at soundings from this site though.


http://www.wxcaster.com/model_skewt.htm

also this one....

http://www.stormchaser.niu.edu/machine/textsound.html

http://www.stormchaser.niu.edu/machine/tex...tfcstsound.html

 

[Rockwell Schrock]

I'd like to reply to my own post to say that I also found some nice CAPE maps (and CAPE forecasts from various models) here:

http://www.rap.ucar.edu/weather/model/

 

[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.