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Qingfang Jiang and James D. Doyle

slopes and neighboring plateaus, most of which assume “undisturbed” conditions; that is, the large-scale flow is relatively weak (e.g., Gleeson 1951 ; Whiteman 1990 ; Rampanelli et al. 2004 ), with a few exceptions. For example, the relationship between valley winds and the synoptic-scale flow has been examined by Whiteman and Doran (1993) using a numerical model, and mechanisms including thermal forcing, downward mixing of momentum, valley channeling, and pressure-driven channeling have been

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Shiyuan Zhong, Ju Li, C. David Whiteman, Xindi Bian, and Wenqing Yao

thermal forcing. As expected from the theory of thermally driven circulations, daytime winds generally blow up valley and upslope while nighttime winds generally blow down valley and downslope. Figure 3 shows daytime and nighttime wind roses at Keeler and Lone Pine using all available data from their individual periods of record. The daytime winds at Keeler are mostly from the southerly quadrants indicating up-valley and upslope flows. At night, wind directions are generally down valley (from the

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Juerg Schmidli, Brian Billings, Fotini K. Chow, Stephan F. J. de Wekker, James Doyle, Vanda Grubišić, Teddy Holt, Qiangfang Jiang, Katherine A. Lundquist, Peter Sheridan, Simon Vosper, C. David Whiteman, Andrzej A. Wyszogrodzki, and Günther Zängl

between the minimum and maximum vertical grid spacing was given by where Δ z min = 20 m, Δ z m = 110 m, a = (1 + n )/2, α = 0.5, and n = 20. The lateral boundary conditions are periodic. A Rayleigh sponge layer, starting at 5 km, was specified as the top boundary condition. All simulations were run with the Coriolis force turned off. The models were integrated for 12 h from sunrise at 0600 local time (LT) to sunset at 1800 LT. The temporal evolution of surface sensible heat flux is determined

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James D. Doyle, Saša Gaberšek, Qingfang Jiang, Ligia Bernardet, John M. Brown, Andreas Dörnbrack, Elmar Filaus, Vanda Grubišić, Daniel J. Kirshbaum, Oswald Knoth, Steven Koch, Juerg Schmidli, Ivana Stiperski, Simon Vosper, and Shiyuan Zhong

underlying terrain. For example, mesoscale predictions of landfalling fronts were found to be very sensitive to small changes in incident flow, as deduced through simulations made with small modifications to the topography orientation by Nuss and Miller (2001) . Two-dimensional idealized adjoint ( Doyle et al. 2007 ) and ensemble ( Doyle and Reynolds 2008 ) model results indicate large sensitivity to the initial state as the mountain height increases, forcing wave breaking, where perturbation growth

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Patrick A. Reinecke and Dale Durran

, with the fifth- and sixth-order schemes yielding values, 1% and 3% stronger than the corresponding continuous solution. (Despite the implicit numerical diffusion associated with the fifth-order scheme, the wave amplitude is slightly stronger than in the continuous solution.) We now consider the impact of discretization on hydrostatic mountain waves. Figure 4a shows the normalized vertical velocity in the continuous Boussinesq solution for δ = 10. The influence of the Coriolis force is included

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Vanda Grubišić and Brian J. Billings

January and May ( Fig. 4 ), can also be related to the synoptic-scale forcing. As Fig. 7 shows, differences in the number of wave events observed in January and May 2000 and 2001 is reflected well in the persistence of strong westerly winds at the 700-hPa level in the 35°–40°N latitude band in these two months in 2000 and their general absence in 2001. This is not surprising given that moderate-to-strong ridge-perpendicular wind (≥8–10 m s −1 ) at ridge-level height is one of the necessary

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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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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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Yanping Li, Ronald B. Smith, and Vanda Grubišić

at 1500 LST). This 3-h time lag between the pressure minimum and temperature maximum is an indication of the valley heat transport mechanism. 5. Numerical simulations a. The setup of idealized 2D simulations To explain the unique phases of the surface pressure observations from the valley stations, the WRF, version 2.1.2, is employed to simulate an idealized diurnal valley circulation and the characteristics of the valley thermal forcing. The model setup is similar to Rampanelli et al. (2004

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