Stability of the Atmosphere
Convective Available Potential
Energy (CAPE)
CAPE assumes Parcel Theory, in that 1)
a rising parcel exhibits no environmental entrainment, 2)
the parcel rises (moist) adiabatically, 3) all precipitation
falls out of the parcel (no water loading), and 4) the
parcel pressure is equal to the environmental pressure at each
level. Parcel Theory can have significant errors, especially for
large parcel displacements, at cloud edges, and for significant
water loading. However, the method often works quite well in the
undiluted core of a thunderstorm updraft.
CAPE represents the amount of buoyant energy available to accelerate
a parcel vertically, or the amount of work a parcel does on the
environment. CAPE is the positive area on a sounding between the
parcel's assumed ascent along a moist adiabat and the environmental
temperature curve from the level of free convection (LFC) to the
equilibrium level (EL). The greater the temperature difference
between the warmer parcel and the cooler environment, the greater
the CAPE and updraft acceleration to produce strong convection.
EL
CAPE = g { [(Tparcel - Tenvir) / Tenvir]
dz
LFC
in Joules/kg. The "{"
symbol here represents a vertical integration between the LFC
(level of free convection, above which the parcel is warmer than
the environment, i.e., the parcel is positively buoyant and will
rise) and the EL (equilibrium level, below which the parcel is
warmer than the environment).
|
CAPE below 0: |
Stable. |
|
CAPE = 0 to
1000: |
Marginally unstable. |
|
CAPE = 1000
to 2500: |
Moderately unstable. |
|
CAPE = 2500
to 3500: |
Very unstable. |
|
CAPE above 3500-4000: |
Extremely unstable. |
The above values are based on a parcel lifted
with the average temperature and moisture of the lowest 50 to
100 mb layer (i.e., the boundary layer). The value of CAPE is
dependent on the level from which a parcel is lifted. Parcels
lifted from the surface usually exhibit a higher (sometimes significantly
higher) CAPE value than for those lifted using mean boundary layer
characteristics.
While CAPE is sensitive to the properties utilized to initialize
a parcel, CAPE often is a much better indicator of instability
than indices which depend on level data (e.g. lifted index, total
totals index, etc). CAPE involves an integration over a depth
of the atmosphere and is not as sensitive to specific sounding
details.
Using CAPE, the maximum updraft speed in a thunderstorm (w-max)
at the equilibrium level can be calculated. In general, w-max
= square root of [2(CAPE)] . For example, a range of
CAPE of 1500-2500 J/kg gives a w-max range of about 50-70 m/s
(100-140 kts). However, due to water loading, mixing, entrainment,
and evaporative cooling, the actual w-max is approximately one-half
that calculated above.
Finally, the profile or shape of the positive area is important,
besides the actual CAPE value. Two soundings could have the same
CAPE value, but lead to different convective characteristics due
to differences in the shape of the area between the LFC and EL.
For example, given the same CAPE value in each, a longer, narrower
profile represents the potential for a slower updraft acceleration
but taller thunderstorms which is best for high precipitation
efficiency. However, a shorter, fatter profile would lead to a
more rapid vertical acceleration which would be important for
potential development of updraft rotation within the storm.
Source: NWS
Other UKAWC Stability Indices:
Ag Weather Center, Department of Biosystems &
Agricultural Engineering, University of Kentucky
