Moisture and Sierra Waves: Observations and modeling

Several wave/rotor events documented during the recent Terrain-induced Rotor Experiment (T-REX) were associated with deep westerly or southwesterly flows. In these cases, a saturated or nearly saturated moist layer in the lower troposphere ascended over the southeast-northwest oriented Sierra ridge. GOES imagery often indicated well-defined wave clouds induced by trapped waves located to the west of the main Sierra ridge, likely excited by relatively low terrain along the west coast of California and persistent convective clouds over the windward slope of the main Sierra ridge.

In this study, the role of moisture and precipitation in modulating mountain waves and rotors is investigated using observational data from TREX, COAMPS reanalysis simulations (1.5 km horizontal resolution), high-resolution idealized COAMPS simulations, and linear solutions. Preliminary results suggest that while latent heat release over the windward slope enables the air to more readily ascend over high terrain and decreases the blocking, low-level moisture also tends to destabilize the lower troposphere and weakens the gravity wave response. 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 results indicate that the relative importance of moisture varies with the depth of the moist layer, terrain height, and moist stability.