6.5
Numerical simulations of stratospheric gravity waves and turbulence during T-REX IOP 6
David R. Vollmer, North Carolina State University, Raleigh, NC; and Y. L. Lin and M. L. Kaplan
IOP 6 (24-26 March 2006) during the Terrain-Induced Rotor Experiment (T-REX) was characterized by strong interactions between mountain-waves and rotors with gravity waves breaking in the upper troposphere and lower stratosphere, resulting in clear-air turbulence (CAT) from 11 to 14 km MSL. The synoptic situation included a 300-hPa cyclonic jet streak crossing the Sierra Nevada just north of the T-REX field experiment site in the Owens Valley, CA. The present study uses the Weather Research and Forecasting (WRF) model to simulate the wave activity and CAT from near-surface to the lower stratosphere using horizontal grid intervals ranging from 18 km to 71 m. Of particular interest is the effect of the jet streak location and propagation on the mountain wave structure and regime. Basically, there were three flow regimes observed by the T-REX and simulated by WRF model. In the first stage, early in IOP-6 (approximately 0000 UTC 25 March 2006) the 300 hPa trough axis was situated well offshore with a 50+ ms-1 jet streak at the base. At this time the mountain wave consisted of a couplet of downslope wind and hydraulic jump over the Owens Valley in the lower troposphere and a minor signal in the stratosphere. In the second stage, by 1200 UTC 26 March 2006, however, a large-amplitude vertically-propagating gravity wave developed in the troposphere, with strong wave breaking occurring in the lower stratosphere (100 hPa) by 2000 UTC. Significant lower stratospheric turbulence was observed by the HAIPER aircraft during this timeframe and reproduced by the model simulations. At this time the jet streak/trough axis at 300 hPa was northwest of the Owens Valley, near the San Francisco Bay area. By 0000 UTC 26 March, the jet streak axis, which was embedded in a trough, had propagated directly north of the T-REX area of interest, and the flow regime transitioned to a complex “leaky” lee-wave situation with low-level trapped lee waves, multiple hydraulic jumps on the order of 10 ms-1 in the middle troposphere, and strong gravity-wave breaking above the tropopause. The turbulence associated with these breaking waves is hypothesized to be associated with parcels undergoing: 1) significant buoyancy perturbation associated with the upward propagating mountain waves in the jet streak-modified lower stratospheric airstream, and 2) the juxtaposed vertical parcel stretching and shrinking. In the third stage, by 0600 UTC 26 March, the jet streak axis had passed to the east of the Owens Valley and the atmosphere quickly adjusted to a relatively quiescent state. We hypothesize that the interaction of the jet streak circulations and the mountain wave created a favorable environment for gravity-wave breaking and severe turbulence in the stratosphere, and that the relative position of the jet streak to the mountain was responsible for the changes in mountain-wave regimes. .
Session 6, T-REX
Tuesday, 7 August 2007, 1:30 PM-3:30 PM, Waterville Room
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