A Brief Review of CAPE

Outline Introduction CAPE Description Parcel Choice Fat vs Skinny Other Forms Conclusion

IntroductionSo why do we need CAPE?

To Fly? To look Cool? To Win Slam Dunk Contest?

CAPE Description (C)onvective (A)vailable (P)otential (E)nergy Effective measure of the buoyancy of an air

parcel which is basic to the generation of convective updrafts and downdrafts

Textbook buoyancy is a relative quantity in that it is defined in terms of the density difference between an air parcel and its environment

Remember the development of textbook buoyancy may be misleading in that it is built around a 1 dimensional pure parcel theory.

So why do we use it? One of the more complete estimates of buoyant energy by determining

the temperature difference between the ascent path and the environment at all levels from the LFC through the equilibrium level.

***Only one part of an ingredients based forecasting method***

Parcel Choice SBCAPE – uses the surface air and

dewpoint temperatures to determine the parcel ascent path

MUCAPE – lifts the most unstable parcel in the lowest 300mb of the sounding

MLCAPE – lifts a parcel constituting a well-mixed layer of constant potential temperature and mixing ratio (most often sfc based but can be elevated)

When to use each? SBCAPE – when sfc based convection is expected. Useful

for low-topped supercell cases. Can be highly volatile on small time and space scale. Caution in shallow moisture situations.

MUCAPE – always produces the largest estimate of buoyancy. Most effective to assess the potential for elevated convection where SBCAPE and MLCAPE will be fairly small.

MLCAPE – less variable in space and time than SBCAPE due to averaging. Usually slightly larger than SBCAPE when nocturnal shallow sfc based inversion exists. Will be equal to SBCAPE in when boundary layer is well mixed throughout the lifting layer. Better assessment than SBCAPE in shallow moisture situations or shallow sfc based inversions.

Fat vs Skinny CapeEffect of CAPE on a parcel is influenced by the distribution of the

buoyancy and moisture in the sounding

***Equal CAPE***

Effects of Entrainment and Water Loading

Identical vertical profiles of CAPE

Normalized CAPE Normalized CAPE - CAPE that is divided

by the depth of the buoyancy layer (units of m s**-2). Values near or less than .1 suggest a "tall, skinny" CAPE profile with relatively weak parcel accelerations, while values closer to .3 to .4 suggest a "fat" CAPE profile with large parcel accelerations possible. Normalized CAPE and lifed indicies are similar measures of instability.

Downdraft CAPE The DCAPE (Downdraft CAPE) can be

used to estimate the potential strength of rain-cooled downdrafts within thunderstorm convection, and is similar to CAPE. Larger DCAPE values are associated with stronger downdrafts.

Conclusions SBCAPE can be several orders of magnitude

larger than MLCAPE due to averaging. Many of the research studies and

subsequent results tend to be based on MLCAPE as opposed to SBCAPE. Forecaster should understand the variables that are involved in various calculated indices.

All available tools should be utilized when making a convective forecast!