Abstract
Elongated and quasi-stationary cloud bands capable of producing heavy precipitation have recently been observed in the lee of midlatitude mountain ridges. Herein, idealized explicit-convection simulations are used to investigate such bands. A methodical sampling of environmental parameter space reveals that the bands are favored by a multilayer upstream static-stability profile, with a conditionally unstable midlevel layer overlying an absolutely stable surface-based layer. Such profiles promote the formation of leeside hydraulic jumps, with deep upright ascent that initiates elevated moist convection. Over smooth ridges, isolated bands develop past each ridge end due to a local superposition of cross-barrier and along-barrier pressure gradients. This superposition enhances leeside vertical displacements compared to parcels traversing the ridge midsection. In the Northern Hemisphere, the Coriolis force favors the left band over the right band (relative to the incoming flow) due to opposite-signed relative-vorticity perturbations past the two ridge ends. Whereas the negative vorticity anomaly past the left end enhances forcing for ascent, the positive vorticity anomaly past the right end suppresses it. For the environmental flows considered herein, the simulated bands are the most persistent over medium-height (1.5-km high) ridges, which force stronger leeside ascent than taller or shorter ridges. Over more rugged terrain, additional bands form past deep gaps or valleys, again due to a local superposition of horizontal pressure gradients. In contrast to some recent studies of orographic cloud bands, these simulated bands owe their existence to the release of moist static instability, indicating that neither slantwise nor inertial instability is required for their formation.
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