Mesoscale convective systems - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 282

Figure 5.15. Idealized depiction of (top) weak and (bottom) strong surface horizontal pressure
gradients in association with rear-inflow jets that continue forward toward the leading con-
vective line or are blocked, respectively (from Johnson, 2001).
There is also a theory in which low-level convergence associated with gravity
waves drives convection when the atmosphere is conditionally unstable, while the
diabatic heating-cooling couplet induced by latent heat release-evaporation (or
sublimation or melting) excites gravity waves: many modes of gravity waves are
possible, but there could be constructive coupling between gravity waves and the
convective system. Such a mechanism is called ''Wave CISK'', where CISK stands
for ''conditional instability of the second kind''; it was first postulated to explain
tropical convection by Dick Lindzen back in the 1970s, and demonstrated in an
idealized model by his student Dave Raymond. It has been suggested that Wave
CISK may explain the surface mesoscale pressure features in MCSs. On a varia-
tion of the Wave CISK theme, it has been suggested that gravity waves excited in
a linearized dynamical framework by the cooling that takes place underneath a
stratiform precipitation area (without the need for diabatic heating above) may
explain the mesohigh-wake low couplet and its propagation.
While we have described the more common ''trailing stratiform'' linear MCS
( Figure 5.17a ), there are also MCSs in which the stratiform area develops ahead of