Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 154

Figure 3.47. Conceptual model of stages in the discrete propagation of a multicellular squall
line in the forward direction (to the right) as high-frequency gravity waves (hf GWs) are forced
by the squall line and are trapped (below the ''ducting level'') beneath the forward anvil (owing
to reduced static stability and jet-like flow in the anvil, which results in the Scorer parameter
decreasing with height); the gravity waves then trigger new cell growth (''daughter clouds'')
ahead of the storm in regions where there is enhanced lift (from Fovell et al., 2006).

An alternative to Fovell's explanation of multicell periodicity is that gravity
waves are triggered as air is lifted over the stable cold pool and new-cell growth is
initiated in the ascending regions of the wave, as proposed by Bob Houze and his
colleagues. Fovell and his colleagues have found observational and numerical
evidence that gravity waves can be ducted ahead of the storm when the boundary
layer is capped by a stable layer, expecially at night, under the anvil outflow, and
that low-frequency gravity waves may trigger new cells far ahead of the storm. In
this case, the storm may appear to jump ahead of itself ( Figure 3.47 ).
When the period of new-cell development is very short, the convective cells
are no longer independent of each other. Such an evolution is termed ''weak evo-
lution''. Multicell storms propagate in part by cell translation and in part by
''discrete propagation'' ( Figure 3.48 ) along an advancing gust front. When the
period of new-cell development is very short, the cells propagate continuously
( Figure 3.49 ). When new cells form, either by discrete or continuous propagation,
the entire storm as a whole propagates to the right of the mean wind ( Figure 3.50 )
when new-cell growth is along the right flank of the storm.
According to RKW theory, the optimum condition for new-cell growth is that

D u, the low-level shear multiplied by the depth of the shear layer, equals c, the

speed of the cold pool. With time, more and more precipitation falls into un-
saturated air and cools evaporatively and frozen precipitation falls below the