Three-dimensional characteristics of mountain waves during T-REX

The Sierra Nevada mountain range is a north-northwest to south-southeast oriented mountain range of approximately 650 km length, 100 km width, and features the tallest peak, Mt. Whitney (4,417 m), and the steepest orographic gradient along the eastern slope in the contiguous United States. The Sierra Nevada is well known for generating large amplitude mountain waves (i.e., the Sierra Wave). Although the Sierra Nevada is often considered nearly a two-dimensional barrier to southwesterly flow, the range has a number of peaks above 4 km along the crest and several deep passes.

Measurements from the NSF/NCAR High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) obtained during the recent Terrain-Induced Rotor Experiment (T-REX) indicate marked differences in the character of the wave response between northern and southern cross Sierra tracks, which were separated by a distance of approximately 50 km. Observations from several of the HIAPER research flights indicate that the vertical velocities in the primary wave exhibited variations up to a factor of two between the southern and northern portion of the racetracks in the lower stratosphere. The perturbation wind speed and potential temperature also exhibited large differences in the direction along the Sierra crest. Multiple racetracks at the 37000 ft. and 43000 ft. altitudes indicate that these differences were repeatable, which is suggestive that the deviations were likely due to mountain waves that varied systematically in amplitude rather than associated with transients. A few of the research flights indicated that the largest variations were on the northern flight leg, which traversed above Independence, CA, while the majority of flights showed larger variations on the leg to the south. The topography beneath the northern flight segments is approximately 800 m higher than the southern flight segments. However, a number of topographic peaks near the southern segment are among the highest in the Sierra.

In this study, we make use of real data and idealized nonhydrostatic numerical model simulations as well as linear theory to test the hypothesis that the observed variability in the wave amplitude and characteristics in the along-barrier direction is a consequence of the three-dimensionality of the Sierra. Results suggest that wave launching is sensitive to the overall three-dimensional characteristics of the Sierra barrier and impact the wave amplitude and characteristics in the lower stratosphere. The downstream Inyo Range influences the asymmetric stratospheric wave response as well. Real-time high resolution COAMPS forecasts successfully capture the along barrier variations in the wave amplitude (using vertical velocity as a proxy) as well as skillfully distinguish between large- and small-amplitude stratospheric events during T-REX. The success of the real-time forecasts and implications for predictability will be addressed.