Interaction of a mountain lee wave with a basin cold pool

High-resolution numerical simulation of nocturnal terrain-driven flows was undertaken for the ridge-and-valley topography of the central Appalachians. These WRF simulations at 444 m horizontal grid spacing and with 10 layers below 50 m AGL captured the expected mesoscale phenomena of mountain meteorology, but also several interesting interactions between them. The expected phenomena included both drainage flows and mountain lee waves. The drainage flows from the long narrow ridges augmented and deepened the nocturnal inversion in the broader inter-ridge basins. This process resulted in both thermodynamic and kinematic changes to the mean state of the atmosphere between the ridges. On the thermodynamic side, the depth of the nocturnal inversion was greater than would be expected over level terrain. On the kinematic side, these inter-basin cold pools decoupled from the cross-ridge flow aloft, resulting in calm winds or reverse flow at levels well below ridge-top. Thus, cross-ridge wind speed increased with height while stability decreased. This led, as theory suggests, to the formation of horizontally propagating gravity waves in the lee of both the Allegheny Front (a downwind facing escarpment) and each of the major ridges. Thus, in early evening the model displayed those phenomena which previous studies have reported for larger mountain ranges elsewhere around the world.

More interesting is the interaction of the mountain waves with the inter-basin cold pool and its consequences for both the waves and the cold pool. As wave amplitude grew during the first half of the night the lower portions of the last (farthest downwind) wave in the train responded to the low-level reversal of the mean flow by tilting down shear. The isentropes eventually become vertical on the down-shear side of the wave crest. This wave breaking occurred at the level where the cross-ridge wind component was zero, suggesting that interaction between the lower portions of the gravity wave and the low-level flow reversal contributed to breaking.

Subsequent to breaking, the wave underwent a number of structural and behavioral changes. It began moving rapidly down shear, leaving its position at the down shear end of the wave train and eventually passing over the next ridge down shear. As it moved the local shear continued to intensify upwind of the advancing crest. This change caused the maximum in parameterized turbulence kinetic energy to move from under the wave crest (i.e. rotor turbulence) to the highly sheared up-shear side of the wave crest (i.e. Kelvin-Helmholtz turbulence). This turbulence and the high winds up-shear of the traveling wave crest penetrated progressively closer to the surface as the wave amplified, eventually eliminating the basin cold pool under the wave trough. The resulting low-altitude burst of warm air and strong winds then pushed the remnants of the basin cold pool up against the down-shear ridge where it was eroded by turbulent mixing. This sequence of events took most of the night to complete and resulted in a significant alteration of the diurnal cycle of valley-floor winds and temperatures from that which would occur on a night with no mesoscale disturbances.