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Ronald B. Smith, Bryan K. Woods, Jorgen Jensen, William A. Cooper, James D. Doyle, Qingfang Jiang, and Vanda Grubišić

Abstract

Using the National Science Foundation (NSF)–NCAR Gulfstream V and the NSF–Wyoming King Air research aircraft during the Terrain-Induced Rotor Experiment (T-REX) in March–April 2006, six cases of Sierra Nevada mountain waves were surveyed with 126 cross-mountain legs. The goal was to identify the influence of the tropopause on waves entering the stratosphere. During each flight leg, part of the variation in observed parameters was due to parameter layering, heaving up and down in the waves. Diagnosis of the combined wave-layering signal was aided with innovative use of new GPS altitude measurements. The ozone and water vapor layering correlated with layered Bernoulli function and cross-flow speed.

GPS-corrected static pressure was used to compute the vertical energy flux, confirming, for the first time, the Eliassen–Palm relation between momentum and energy flux (EF = −U · MF). Kinetic (KE) and potential (PE) wave energy densities were also computed. The equipartition ratio (EQR = PE/KE) changed abruptly across the tropopause, indicating partial wave reflection. In one case (16 April 2006) systematically reversed momentum and energy fluxes were found in the stratosphere above 12 km. On a “wave property diagram,” three families of waves were identified: up- and downgoing long waves (30 km) and shorter (14 km) trapped waves. For the latter two types, an explanation is proposed related to secondary generation near the tropopause and reflection or secondary generation in the lower stratosphere.

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Vanda Grubišić and Brian J. Billings

Abstract

A large-amplitude lee-wave rotor event observationally documented during Sierra Rotors Project Intensive Observing Period (IOP) 8 on 24–26 March 2004 in the lee of the southern Sierra Nevada is examined. Mountain waves and rotors occurred over Owens Valley in a pre-cold-frontal environment. In this study, the evolution and structure of the observed and numerically simulated mountain waves and rotors during the event on 25 March, in which the horizontal circulation associated with the rotor was observed as an opposing, easterly flow by the mesonetwork of surface stations in Owens Valley, are analyzed.

The high-resolution numerical simulations of this case, performed with the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) run with multiple nested-grid domains, the finest grid having 333-m horizontal spacing, reproduced many of the observed features of this event. These include small-amplitude waves above the Sierra ridge decoupled from thermally forced flow within the valley, and a large-amplitude mountain wave, turbulent rotor, and strong westerlies on the Sierra Nevada lee slopes during the period of the observed surface easterly flow. The sequence of the observed and simulated events shows a pronounced diurnal variation with the maximum wave and rotor activity occurring in the early evening hours during both days of IOP 8.

The lee-wave response, and thus indirectly the appearance of lee-wave rotor during the core IOP 8 period, is found to be strongly controlled by temporal changes in the upstream ambient wind and stability profiles. The downstream mountain range exerts strong control over the lee-wave horizontal wavelength during the strongest part of this event, thus exhibiting the control over the cross-valley position of the rotor and the degree of strong downslope wind penetration into the valley.

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James D. Doyle and Dale R. Durran

Abstract

The internal structure and dynamics of rotors that form in the lee of topographic ridges are explored using a series of high-resolution eddy-resolving numerical simulations. Surface friction generates a sheet of horizontal vorticity along the lee slope that is lifted aloft by the mountain lee wave at the boundary layer separation point. Parallel-shear instability breaks this vortex sheet into small intense vortices or subrotors.

The strength and evolution of the subrotors and the internal structure of the main large-scale rotor are substantially different in 2D and 3D simulations. In 2D, the subrotors are less intense and are ultimately entrained into the larger-scale rotor circulation, where they dissipate and contribute their vorticity toward the maintenance of the main rotor. In 3D, even for flow over a uniform infinitely long barrier, the subrotors are more intense, and primarily are simply swept downstream past the main rotor along the interface between that rotor and the surrounding lee wave. The average vorticity within the interior of the main rotor is much weaker and the flow is more chaotic.

When an isolated peak is added to a 3D ridge, systematic along-ridge velocity perturbations create regions of preferential vortex stretching at the leading edge of the rotor. Subrotors passing through such regions are intensified by stretching and may develop values of the ridge-parallel vorticity component well in excess of those in the parent, shear-generated vortex sheet. Because of their intensity, such subrotor circulations likely pose the greatest hazard to aviation.

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