Upstream Desaturation of Moist-Neutral Flow Over a Ridge

In a full-physics numerical model of nearly moist-neutral atmospheric flow over a mountain ridge, the established flow has upstream regions of downward-displaced, unsaturated flow. This unusual feature is seen to be associated with an upstream propagating wave of subsidence that precludes the upward-displaced and saturated flow that might be expected upstream of the topography. The figure (from Miglietta/Rotunno 2005) illustrates such an evolving flow where a) indicates downward vertical velocity at mid-levels upstream, and b) shows an advancing region of low cloud water mixing ratio. This phenomenon is shown to be a consequence of the peculiar property of saturated, moist-neutral flow: an upward air-parcel displacement produces zero buoyancy, while a downward displacement desaturates the air parcel and produces a positive buoyancy anomaly. A simple implementation of this saturated-atmosphere property in various reduced models verifies the mechanisms associated with the desaturation of the upstream flow. In addition, an ultimate reduction to a nonlinear hyperbolic wave system suggests that the propagating subsidence can be interpreted as a upstream travelling shock amid dispersing inertia-gravity waves.