Outline Introduction CAPE Description Parcel Choice Fat vs
Skinny Other Forms Conclusion
Slide 3
Introduction So why do we need CAPE? To Fly?To look Cool? To
Win Slam Dunk Contest?
Slide 4
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.
Slide 5
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***
Slide 6
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)
Slide 7
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.
Slide 8
Fat vs Skinny Cape Effect of CAPE on a parcel is influenced by
the distribution of the buoyancy and moisture in the sounding
***Equal CAPE***
Slide 9
Effects of Entrainment and Water Loading Identical vertical
profiles of CAPE
Slide 10
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.
Slide 11
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.
Slide 12
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!
