Moisture and Sierra Waves: Observations and Modeling

Several wave/rotor events documented during the recent Terrain-induced Rotor Experiment (T-REX) were associated with moist westerly or southwesterly flows in the lower troposphere ascending over the southeast-northwest oriented Sierra ridge. During these events, satellite images indicated the presence of clouds over the windward slope of the Sierra ridge, with foehn-induced clearing and lee waves over the Owens Valley. The observed waves and rotor structure were inevitably influenced by the latent heat release associated with moist processes.

In this study, the role of moisture and precipitation in modulating mountain waves and rotors is investigated using observational data from the Terrain-Induced Rotor Experiment (T-REX), the Coupled Ocean/Atmospheric Prediction System (COAMPS), and linear theory. Preliminary results highlight two competing impacts of low-level moisture on Sierra waves: a) latent heat release over the windward slope that enables the air to more readily ascend over high terrain and decreases the upstream blocking, and therefore, enhances vertical motion near the surface; and b) moisture tends to destabilize the lower troposphere and weakens the gravity wave response aloft. For the cases considered in this study, the second process dominates and the low-level moisture is found to weaken mountain waves. In the presence of trapped waves, the moisture increase in the lower troposphere tends to modify the Scorer parameter profile by decreasing the low-level moist Scorer parameter, and can disrupt the wave-ducting condition. The relative importance of moisture varies with the depth of the moist layer, terrain height, and moist stability. The relationship among these variables within the broader moist mountain wave parameter space will be discussed.