Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 109

pools are not as intense as at mid-latitudes, owing to comparatively high relative
humidity in midlevels of the troposphere and a warm ocean surface that heats the
low levels.
CAPE can be thought of as the integrated potential buoyancy in a convective
cloud; the buoyancy is only ''potential'' because it may or may not be realized in
entirety. Equation (3.8) yields CAPE for the environment: for calculating vertical
velocity in a convective cloud, one needs also to include both the dynamic and
buoyancy-related vertical perturbation pressure terms, precipitation loading, and
turbulent mixing. In a resting atmosphere, the latter two act to reduce buoyancy,
so CAPE in the environment, as calculated from a proximity sounding, may be
thought of as an upper limit. Thus, the vertical velocity calculated from (3.7) may
be thought of as an upper bound in most cases and used to estimate only in a very
qualitative way the intensity of convection that might occur. Estimates of vertical
velocity using (3.7) are in accordance with parcel theory, in which the assumptions
noted above are used.
In highly sheared environments, it is possible that the vertical perturbation
pressure gradient acts upward and is substantial, so that CAPE may not be a
good indicator at all of the intensity of convection. This is the case, for example,
in the environment of landfalling tropical cyclones, when low-level vertical shear is
strong, but CAPE is relatively low. A discussion of this situation is delayed until
Chapter 4, when supercell dynamics are discussed.
Estimates of the vertical velocity in convective storms using (3.7) are typically
O(10m s 1 ); in the strongest storms, vertical velocity may exceed 50m s 1 .In
numerical models such high vertical velocities are found in convective updrafts.
Verifying such high vertical velocities in nature is more dicult. A limited number
of measurements have been made using storm-penetrating aircraft ( Figure 3.14 ).
Other estimates have been made from radiosondes released directly into storm
updrafts ( Figure 3.15 ); it is assumed that updraft velocity is given by the balloon
ascent rate (determined either from the rate of decrease in pressure or the rate of
increase of GPS height) less the neutral (considering the buoyancy of the balloon
alone less its drag) ascent rate of the balloon. Icing can slow the balloon down
and it is not known whether or not the balloon passed through the center of the
updraft.
Measurements have been made of updrafts using Doppler radar data. In most
instances, however, the beams from Doppler radars are directed in nearly per-
pendicular directions to the vertical, so that vertical velocity must be inferred
kinematically from the divergence of the horizontal wind (cf. Appendix); errors
in divergence accumulate when vertical velocity is estimated from upward
integrations. Downward integrations are dicult unless one knows for sure the
upper boundary condition and at what level it is valid: Updrafts may penetrate
the stratosphere, so that one would have to know the equilibrium level to be able
to set a zero upper boundary condition (which would be above the EL). Another
serious problem with multiple Doppler radar-based, kinematically computed ver-
tical velocities is that it is unlikely that multiple radars sample the identical volume
at the same time, owing to the scanning patterns of the radars and the spreading