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James D. Doyle and Dale R. Durran

equations expressed using tensor notation are where c p is the specific heat at constant pressure, R d is the dry gas constant, g is the acceleration due to gravity, ρ is the density, c is the speed of sound, ν is the hyperdiffusion coefficient, and f the Coriolis force. In this study, the Coriolis force is specified as 10 −4 s −1 . The overbar variables correspond to the reference state, which is horizontally homogeneous and in hydrostatic balance. The turbulent subgrid-scale fluxes for

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Laurence Armi and Georg J. Mayr

sill . J. Phys. Oceanogr. , 33 , 1446 – 1461 . Koop , C. G. , and F. K. Browand , 1979 : Instability and turbulence in a stratified fluid with shear . J. Fluid Mech. , 93 , 135 – 159 . Kuettner , J. , 1958 : The rotor flow in the lee of mountains . Organisation Scientifique et Technique du Vol à Voile (OSTIV) Publ. IV, 3–8 . Kuettner , J. , 1959 : The rotor flow in the lee of mountains . Geophysics Research Directorate, Air Force Cambridge Research Center, Air Force Research

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James D. Doyle, Qingfang Jiang, Ronald B. Smith, and Vanda Grubišić

1997 ). The Louis (1979) surface layer parameterization, which makes use of a surface energy budget based on the force-restore method, is used to represent the surface fluxes. Subgrid-scale moist convection is represented using the Kain and Fritsch (1993) parameterization. The grid-scale evolution of the moist processes is predicted explicitly from budget equations for cloud water, cloud ice, raindrops, snowflakes, and water vapor ( Rutledge and Hobbs 1983 ). The short- and longwave radiation

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Georg J. Mayr and Laurence Armi

. Zhong , S. , J. Li , C. B. Clements , S. F. J. De Wekker , and X. Bian , 2008a : Forcing mechanisms for Washoe Zephyr: A daytime downslope wind system in the lee of the Sierra Nevada. J. Appl. Meteor. Climatol. , 47 , 339 – 350 . Zhong , S. , J. Li , C. D. Whiteman , X. Bian , and W. Yao , 2008b : Climatology of high wind events in the Owens Valley, California. Mon. Wea. Rev. , 136 , 3536 – 3552 . Fig . 1. Map of the Sierra Nevada and Owens Valley. The locations

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Qingfang Jiang and James D. Doyle

heat release in the midtroposphere associated with precipitation plays a crucial role in forcing the low-level flow into the downslope windstorm regime and provides a pathway to the foehn. The interaction between a two-dimensional ridge and a nearly neutral moist airflow was recently studied by Miglietta and Rotunno (2005) and they found that the saturated airflow over the upwind slope can transition to unsaturated in the lee associated with strong descent. Terrain blocking of moist airflow and

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Qingfang Jiang, Ming Liu, and James D. Doyle

as w ′ u ′ = − K M ∂ U /∂ z and w ′ θ ′ = − K H ∂ θ /∂ z . The surface fluxes are calculated from a surface energy budget based on the force–restore method ( Louis 1979 ; Wang et al. 2002 ). The computational domain contains five horizontally nested grid meshes of 91 × 91, 133 × 133, 133 × 133, 157 × 157, and 157 × 157 grid points, and the corresponding horizontal grid spacings are 40.5, 13.5, 4.5, 1.5, and 0.5 km, respectively ( Fig. 9a ). The aerosol model is activated only in the

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Lukas Strauss, Stefano Serafin, and Vanda Grubišić

is absent; instead, channeled up-valley flow prevails. Note that the idealized diagrams of scenarios A and B are somewhat similar to two of those by Jiang and Doyle (2008 , their Fig. 12). The latter, however, refer to a weak westerly wind event during the Sierra Rotors Project (SRP) IOP 12, primarily determined by thermal forcing in the valley. A schematic of daytime turbulent interaction of in-valley westerlies with channeled up-valley flow (scenario C) is shown in Fig. 16c . The wave

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Peter Sheridan and Simon Vosper

forcing by the upstream wind and stability profile, and the wavelength of the resulting waves, in determining the degree of penetration of westerly flow and suggests that the thermal mechanism assists or controls westerly in-valley flow when dynamic forcing is insufficient to enable complete penetration across the valley floor ( Billings and Grubisic 2008b , a ). Intensive observation period (IOP) 6 contained the most intense downslope windstorm observed by the valley instruments during T

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Patrick A. Reinecke and Dale R. Durran

circulations in the initial conditions. Anthes et al. (1985) argued that the predictability of many mesoscale phenomena can substantially exceed that suggested by Lorenz if they are sufficiently organized by the large-scale flow (e.g., fronts) or controlled by well-known external forcing such as orography, thermal contrasts, or other land-use characteristics. Klemp and Lilly (1975) presented some of the earliest results suggesting that topography may enhance mesoscale predictability. They predicted

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

difficulty maintaining an accurate pressure altitude. In Fig. 7 , we compare the observed wind speed, averaged for the two legs, with the geostrophic speed for all the racetracks in the six flights. For slower winds, the flow is slightly supergeostrophic, while for fast winds aloft, the flow is significantly subgeostrophic. We suggest that the strong subgeostrophy aloft is caused by streamline curvature and southward centripetal force in the upper-level troughs above the Sierras. To evaluate this

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