Sensitivity of Orographic Precipitation to Concave Terrain in Idealized Simulations

Sensitivity of Orographic Precipitation to Concave Terrain in Idealized Simulations

This study examines how variations to the non-dimensional mountain height (Ĥ) and the horizontal aspect ratio (β) of a straight ridge and a concave ridge influence orographic precipitation. An idealized three-dimensional model is used to simulate a moist flow impinging upon these two alpine-scale ridges with Ĥ = 0.66 to 2.0 and β = 1.0 to 8.0. It is found that when the approaching flow is unblocked, precipitation strengthens for both ridges when Ĥ increases, however when the approaching flow becomes blocked precipitation significantly weakens. Precipitation generated by the concave ridge is found to be more sensitive to changes in Ĥ, especially when the ridge is long and narrow (large β). The concave ridge generates substantially more precipitation than the straight ridge via an established precipitation-enhancing funneling mechanism near the ridge vertex. It was believed that when the approaching flow is blocked, the strength of the precipitation enhancement by the concave ridge relative to the straight ridge is negligible; however this study reveals that when Ĥ is sufficiently large to induce flow-reversal on the windward slope, an established secondary circulation develops that is strengthened by the concave ridge and precipitation is subsequently enhanced by the concave ridge. It is also shown that strength of the precipitation enhancement is sensitive to β, however only when the flow is very unblocked. Finally, a regime diagram describing the dynamics of the flow for concave terrain is constructed.