Mw. Moncrieff et Ch. Liu, Convection initiation by density currents: Role of convergence, shear, anddynamical organization, M WEATH REV, 127(10), 1999, pp. 2455-2464
Steady-state analytic models establish two key points concerning the impact
of vertical shear on density currents and the implication for convection i
nitiation. First, shear decreases the horizontal convergence, and therefore
the mean ascent, associated with downshear propagating currents. Second, s
hear has a basic effect on the dynamical organization. If the downshear cur
rent travels at the speed of the ambient how at a critical (steering) level
, an overturning circulation provides deep lifting. Although mean ascent is
increased by shear in the case of upshear propagating currents, the liftin
g is comparatively shallow because jumplike ascent occurs rather than deep
overturning. The convection initiation mechanism involving the downshear cu
rrent is therefore very different from the upshear case.
These basic principles are borne out in two-dimensional numerical simulatio
ns. Density currents generated by a stationary cold source imposed on an in
itially horizontally homogeneous, sheared, and neutrally stratified ambient
how are explored. Results show that (i) if the surface flow and low-level
shear vectors are in the same direction, as in a low-level jet, the effects
of shear and surface flow on the density current head height counteract on
e another; and (ii) if they oppose one another, as in a surface jet, both c
onspire to lower the density current head on the downwind side but raise it
on the upwind side.
As regards convection initiation by sea breezes, point (i) above shows an a
pproximately equal but weak preference for convection exists on the leeward
and windward coasts. Point (ii) shows that initiation is strongly suppress
ed on the windward coast, but strongly enhanced on the leeward one. The hyp
othesis that sea breezes are more intense in offshore flow therefore holds
only if shear and surface how have opposite sign or if the flow is unsheare
d.
Concerning convection initiation by thunderstorm outflows, downshear propag
ating outflows provide the deepest lifting if they move at the speed of the
ambient flow at a critical level, despite the fact that low-level converge
nce is decreased by shear. While shear strengthens the mean ascent in upshe
ar propagating outflows there is no steering level to anchor the incipient
convection to the organized ascent.