Ordinary-cell convective storms - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 150

used with extreme caution when extrapolating to the more complicated conditions
in real convective storms.

3.2.3 Gravity waves forced by a density current

Since, relative to the leading edge of the gust front, air is forced up and over it,
gravity waves may be triggered there when the necessary conditions for gravity
wave propagation are satisfied. The reader is referred elsewhere for details on
gravity wave propagation. When the Scorer parameter l , defined for an

incompressible atmosphere as

2

¼ N 2 u 2 ud 2 u dz 2 l = 1 = = ð 3 : 56 Þ

decreases rapidly with height, where u is the wind speed in a unidirectional wind
profile, it is possible that gravity waves may propagate vertically and then be
reflected back downward, leading to resonance and a series of ''trapped gravity
waves'', which do not tilt with height like pure internal gravity waves. In the
atmosphere, the Scorer parameter decreases rapidly with height when N decreases
with height (there is low lapse rate at low altitude and high lapse rate at high
altitude), u increases with height, and when d 2 u dz 2 is large and negative at low

altitude and zero at higher altitude. The latter condition occurs when there is a
low-level jet. The ascending branches of the gravity waves may lift air to satura-
tion and produce clouds or even trigger elevated convection above the gust front.

=

3.3 MULTICELL CONVECTIVE STORMS

Keith Browning, in an influential hailstorm monograph published in 1977,
proposed that there is a continuum of storm types based on the lifetime of convec-
tive cells. His proposal was pre-dated by work done by him and his collaborators
in England, by A. Chisholm and colleagues, who studied Alberta hailstorms, by
John Marwitz, who studied hailstorms in Alberta, and by A. S. Dennis and col-
leagues, who studied hailstorms in South Dakota. Sometimes discrete new
''secondary'' cells form periodically along the leading edge of a gust front on the
right, rear flank of the storm and reach a quasi-equilibrium state, as cells mature
and decay on the storm's left, front flank ( Figure 3.44 ). At other times, under
other conditions (when low-level vertical shear is weak, according to RKW
theory), convective cells simply decay as the gust front spreads out and no second-
ary cells are triggered. In the following chapter, the case in which convective cells
persist for a long time period is discussed in detail. In the case of ''single'' Byers-
Braham cells, precipitation is the ''brake'' acting on a cell's life. It was postulated
that the brake is released somewhat when there is enough vertical shear in the
environment so that precipitation does not fall back into the updraft. It was
thought, however, based on the Thunderstorm Project, that too much shear inhi-
bits storm formation because it rips an updraft apart so that precipitation does
not have a chance to form.