Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 106

trating tops can be detected from the blackbody temperature difference between
the water vapor and IR bands, but the regions where the blackbody temperature
difference is greatest may only locate the ''coldest'' regions—not necessarily the
penetrating tops. The ''warm spots'' downstream of penetrating tops are thought
to be created by gravity waves (wave breaking) or other effects. The difference in
brightness temperature between the cold areas and warm areas is several K or
greater. There is some model evidence that the ring signature occurs when vertical
shear is relatively weak and the U/V signature occurs when vertical shear is
relatively strong.
The vertical velocity at the center of a column of rising air at a given altitude
may be estimated thermodynamically, using the inviscid, steady-state version of
the vertical equation of motion (2.70) without any p 0 d or p 0 b (according to ''parcel
theory'' in which an air parcel's effect on its environment is neglected), as

1 = 2

7 Þ
where CAPE, the ''convective available potential energy'', is the vertically
integrated energy acquired by the rising air as a result of the upward buoyancy
force acting on it:

w ð z Þ¼½ 2 ð CAPE ð z ÞÞ ð 3 :

ð z ð z

LFC ½ T c ð z 0 Þ T 0 ð z 0 Þ=

Bdz 0 ¼ g T 0 ð z 0 Þ dz 0

CAPE ð z Þ¼ ð 3 : 8 Þ

LFC

where T 0 ð z 0 Þ is the vertical profile of temperature in the environment of the cloud,
and T c ð z 0 Þ is the vertical profile of temperature inside the cloud. The accuracy of
CAPE is increased if moisture and cloud liquid water content is accounted for by
using virtual temperature or cloud virtual temperature (cf. (2.21)-(2.23)) for tem-
perature.
Estimation of CAPE using (3.8) depends on how the LFC is computed. When
surface-based air parcels are used, CAPE is called ''surface-based CAPE'' or
SBCAPE. When the mixed-layer or some other boundary-layer mean is used,
CAPE is called the ''mixed-layer'' or ''mean-layer'' CAPE or MLCAPE. When
the highest combination of temperature and dew point are used, CAPE is called
the ''most unstable'' CAPE or MUCAPE.
In many instances the LFC is also at cloud base, but not necessarily so. When
the LFC is higher than the LCL, the outside of the cloud tends to assume a
smooth, laminar appearance ( Figure 3.12a ), suggestive of laminar lift of a stable
air mass, like air being lifted in an orographic wave cloud ( Figure 3.12b ) (cf. the
discussion on why mammatus look smooth in 3.2.1.3). Strictly speaking, (3.7) is
valid when there is no mixing, the atmosphere is resting (there is no vertical or
horizontal wind shear in the environment), and the air parcel is infinitely narrow.
However, the wider the air parcel, the less the importance of mixing. So, when the
air parcel is infinitely narrow, we can neglect the effects of vertical gradients of p b ,
but mixing must be accounted for, and vice versa. One would expect that there is
an intermediate range of aspect ratio for which the deleterious effects of both
mixing and upward-directed perturbation pressure gradient force are each mini-
mized with respect to each other (cf. (2.75) and Section 2.5.3).