Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 121

Figure 3.22. Sense of horizontal vorticity (curved arrows) near the ground underneath a
precipitation-laden downdraft in a convective storm over Oklahoma City, Oklahoma on July
26, 1978. The vorticity depicted is part of a vortex ring like the one shown at the bottom of
Figure 3.21 (photograph by the author).
Ramesh Srivastava later (at the University of Chicago) suggested that in wet
microbursts, when ice is included in modeling studies, the breaking up of larger
raindrops into smaller raindrops, which evaporate more quickly than larger rain-
drops, may also be an important mechanism for microburst formation, in addition
to melting of graupel. He also found that as the environmental lapse rate
decreases, more precipitation and more ice particles in particular are necessary to
form intense downdrafts. It is noted that melting of ice particles can occur com-
pletely in a fall through just a few kilometers in a layer above freezing, while
raindrops of the same size do not evaporate completely during a fall through the
same layer: Although the absorbed latent heat of freezing during melting is much
less than the absorbed latent heat of condensation during evaporation, more
cooling is effected by the melting, so the melting of graupel in downdrafts is a
significant contributor to intense downdrafts.
When the lapse rate is very high (near dry adiabatic), both dry and wet
microbursts can occur; when the lapse rate is relatively low, only wet microbursts are
possible, and they must have significant amounts of ice-phase precipitation. While
one can forecast dry microbursts when convective storms are possible (i.e., there
is CAPE and the LCL/LFC are attainable with expected daytime heating or
mesoscale ascent) and the LCL/LFC is relatively high, differentiating between the