Orographic Clouds in Terrain-Blocked Flows: An Idealized Modeling Study

Idealized numerical simulations of moist strongly stratified flow over topography are used to study the processes that control orographic clouds in terrain-blocked flows as a joint function of the nondimensional flow parameter Nh/U, the horizontal topographic aspect ratio beta, and the Rossby radius of deformation Nh/f. The simulations show the competition between enhanced upstream condensation in the secondary vertically propagating gravity wave and the reduction of condensation owing to enhanced low-level flow deflection. As Nh/U increases above about 1.5, the tendency for flow to be deflected around the barrier reduces cloud formation in the primary gravity wave over the ridge, while increasing beta expands low-level clouds over a broader upstream area. Ice clouds may form aloft in the secondary vertically propagating gravity wave and extend upstream for several hundred kilometers. In terrain-blocked flows, more than half of the condensate mass develops upstream of the barrier in the secondary gravity wave, while in unblocked flows most of the condensate is downstream of the barrier in the primary lee wave. In 2D, none of the flow can be diverted around the barrier, and it therefore produces a much more vigorous hydrologic cycle than over long (beta = 8) 3D ridges, increasing upstream lifting and cloud water content by at least a factor of 2, and generating primary wave clouds that are not produced in the 3D case. Rotation reduces the upstream extent of condensation in blocked flows to a region on the order of the radius of deformation and in 3D induces a marked asymmetry in the lifting and condensation upstream of the terrain.