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Juerg Schmidli, Gregory S. Poulos, Megan H. Daniels, and Fotini K. Chow

1. Introduction Understanding the winds within a valley and their interactions with the larger-scale forcings is of interest for a number of reasons. For example, the dispersion of pollutants in a valley depends strongly on local valley circulations (e.g., Whiteman 1989 ; Fast et al. 2006 ); nocturnal minimum surface temperatures depend strongly on the near-surface wind speed (e.g., Estournel and Guedalia 1985 ; Steeneveld et al. 2006 ) and hence on the strength of the valley wind; land

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Thomas Raab and Georg Mayr

the MAPR site. 3. Case studies A pressure gradient forces air to flow over or around a mountain range. We discern two major factors, namely the synoptic-scale pressure field, hence called the dynamic forcing because it causes the synoptic wind field, and the contribution from differences in temperature between the upstream and downstream side of the barrier, hence called the hydrostatic forcing. To distinguish the magnitude of the two forcings for the four cases, we compare the vertical structure

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Stefano Serafin, Lukas Strauss, and Vanda Grubišić

. 2008b ). Nevertheless, it is a reasonable expectation that mountain-wave activity generated by westerly flow over the Sierra Nevada can lead to various degrees of penetration of momentum to the valley floor on the lee side ( Jiang and Doyle 2008 ; Strauss et al. 2016 ). Dynamical and thermal forcing mechanisms are known to cause downslope winds on the lower mountain slopes (or the valley bottom or leeside plains) at many locations around the world. The connection between large-amplitude mountain

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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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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, 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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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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Stephen A. Cohn, Vanda Grubiššićć, and William O. J. Brown

over Owens Valley. Although there is an evolution of upwind conditions, we cannot confidently relate changes of the wave position in the valley to this variation in forcing. Fig . 6. (a) Wind speed, (b) wind direction, and (c) potential temperature from T-REX soundings taken approximately 140 km west-southwest (upwind) of Owens Valley. Migration of a wave crest directly over a wind profiler, as happened over the center profiler between 1600 and 1845 UTC, is extremely fortuitous. Rotor flow is

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