Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 128

Figure 3.28. Conceptual model of a heat burst as a deformation of a shallow, cool, stable layer
at the surface by a descending current of warm, dry air from aloft. Solid arrows (left) represent
winds; dashed arrow represents heat burst wind. Dotted lines represent the upper boundary of
the stable layer. Sounding (right) at Russell, KS, near a heat burst event. It is seen in the
sounding that if saturated air near 500 hPa is cooled by the wet-bulb process until it becomes
unsaturated, then below 500 hPa if it is forced downward by mesoscale circulation, it would
continue to descend along the 313K isentrope driven by negative buoyancy and, in the absence
of mixing, become positively buoyant at low levels, but still strong enough to penetrate down to
the surface (from Bernstein and Johnson, 1994).
3.2.1.3 Cumulonimbus mammatus
Sometimes downdrafts are initiated very high above the ground, from the base of
the anvil of a convective storm or above the base of the anvil. The downdrafts
may make the underside of the anvil look like an upside-down cumuliform cloud,
but smoother and with longer protuberances ( Figure 3.29 ). These features are
called in general ''mamma'' or ''mammatus''. The latter is short hand for the
adjectival modifier applied to the parent cumulonimbus, which in Latin comes
after the noun. It is tempting, when looking at them, to believe that they are a
consequence of upside-down Rayleigh-Be´ nard convection. Mammatus may
appear to be smoother than upright convection because negatively buoyant
thermals continue to move downward beneath their equilibrium level into stable
air. It has also been hypothesized that mammatus appear smooth when precipita-
tion falls at the same rate as the downdraft (i.e., when the downdraft and
hydrometeor size are both constant) and therefore the precipitation particles
remain together. A third hypothesis is that when there is spectrum of ice crystal
sizes the smaller crystals sublimate first, mostly along the leading edge of the
mamma, leaving the larger crystals behind to appear opaque and smooth: The
tops of upright convective elements, on the other hand, do not appear smooth