Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 112

falls through unsaturated air, then evaporative cooling (or melting cooling, if
frozen precipitation melts, or sublimation cooling, if frozen precipitation subli-
mates, or a combination of all of the aforementioned) further reduces buoyancy
(increases negative buoyancy) and the downdraft is further strengthened. If the
condensed water substance is in the form of ice crystals, however, the terminal fall
velocity is smaller and, as upward vertical velocity decreases with increasing
height, the ice particles remain aloft and diverge horizontally to form an anvil.
When there is vertical shear, the anvil may be advected downstream far ahead
of the storm ( Figure 3.16 ) and forewarn it. When there is only weak or no vertical
(a)
(b)
Figure 3.16. Long anvils. (a) Cumulonimbus with a long anvil downstream, as viewed from an
aircraft over southwestern Nebraska on July 19, 2009. (b) Developing cumulonimbus in
Oklahoma on April 30, 2003 (photographs by the author).