Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 280

Figure 5.13b. Squall line with wave (arrow) and mesoscale vortex (signature is circled) on May
24, 2011, as depicted by the Huntsville, AL WSR-88D radar; (left) radar reflectivity in dBZ;
(right) Doppler velocity in m s 1 .
evaporatively cooled cold pool underneath the storm. Melting or sublimation may
increase the amount of cooling. The pressure at the leading edge of the cold pool/
gust front also has a non-hydrostatic, dynamic component: air approaching the
leading edge must slow down and rise, which is consistent with an adverse hori-
zontal pressure gradient. Beneath the downdraft, there is also a non-
hydrostatic, dynamic component: air descending to the ground must spread out
laterally, which is consistent with a local maximum in pressure and diverging
surface pressure gradient force. Hydrometeor loading can also increase surface
pressure. When a symmetric MCS evolves into an asymmetric MCS, the mesohigh
shifts location along with the stratiform precipitation area.
A wake low is produced as dry air subsides, warming the air and adiabatically
producing a hydrostatic deficit of pressure. It is thought that a descending rear-
inflow jet warms the air adiabatically more than the evaporation of raindrops (or
sublimation of ice crystals or melting of ice crystals) cools it. When there is very
strong warming, as there is in a heat burst, descending air parcels may overshoot
their equilibrium level as they lose their negative buoyancy, but have enough
kinetic energy to continue to descend and warm even more (cf. Section 3.2.1.2).
Wake lows may contain transient decaying convective cells associated with
stronger, narrower low-pressure areas. A necessary condition for wake low forma-
tion is the existence of a stratiform precipitation area; the intensification of wake
lows occurs during the latter stages of an MCS.
A pre-squall mesolow is associated with subsiding air aloft ahead of squall
lines. This subsiding air warms adiabatically, and hydrostatically reduces the
surface pressure.