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Title: High-resolution simulations of downslope flows over complex terrain using WRF-IBM

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Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
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Conference: Presented at: 17th Conference on Mountain Meteorology, Burlington, VT, United States, Jun 27 - Jul 01, 2016
Country of Publication:
United States

Citation Formats

Arthur, R S, Lundquist, K A, Mirocha, J D, Hoch, S W, and Chow, F K. High-resolution simulations of downslope flows over complex terrain using WRF-IBM. United States: N. p., 2016. Web.
Arthur, R S, Lundquist, K A, Mirocha, J D, Hoch, S W, & Chow, F K. High-resolution simulations of downslope flows over complex terrain using WRF-IBM. United States.
Arthur, R S, Lundquist, K A, Mirocha, J D, Hoch, S W, and Chow, F K. 2016. "High-resolution simulations of downslope flows over complex terrain using WRF-IBM". United States. doi:.
title = {High-resolution simulations of downslope flows over complex terrain using WRF-IBM},
author = {Arthur, R S and Lundquist, K A and Mirocha, J D and Hoch, S W and Chow, F K},
abstractNote = {},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2016,
month = 9

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  • Boundary layer flows are greatly complicated by the presence of complex terrain which redirects mean flow and alters the structure of turbulence. Surface fluxes of heat and moisture provide additional forcing which induce secondary flows, or can dominate flow dynamics in cases with weak mean flows. Mesoscale models are increasingly being used for numerical simulations of boundary layer flows over complex terrain. These models typically use a terrain-following coordinate transformation, but these introduce numerical errors over steep terrain. An alternative is to use an immersed boundary method which alleviates errors associated with the coordinate transformation by allowing the terrain tomore » be represented as a surface which arbitrarily passes through a Cartesian grid. This paper describes coupling atmospheric physics models to an immersed boundary method implemented in the Weather Research and Forecasting (WRF) model in previous work [Lundquist et al., 2007]. When the immersed boundary method is used, boundary conditions must be imposed on the immersed surface for velocity and scalar surface fluxes. Previous algorithms, such as those used by Tseng and Ferziger [2003] and Balaras [2004], impose no-slip boundary conditions on the velocity field at the immersed surface by adding a body force to the Navier-Stokes equations. Flux boundary conditions for the advection-diffusion equation have not been adequately addressed. A new algorithm is developed here which allows scalar surface fluxes to be imposed on the flow solution at an immersed boundary. With this extension of the immersed boundary method, land-surface models can be coupled to the immersed boundary to provide realistic surface forcing. Validation is provided in the context of idealized valley simulations with both specified and parameterized surface fluxes using the WRF code. Applicability to real terrain is illustrated with a fully coupled two-dimensional simulation of the Owens Valley in California.« less
  • A finite difference computer code (MATHEW) for ensuring mass-consistency in wind fields, developed by Sherman (1978), has proven successful in providing input for ADPIC (Lange, 1978), a particle-in-cell advection-diffusion code used in pollution transport studies. An analog of MATHEW is described to be used in finite element advection-diffusion codes, finite element planetary boundary models, and for wind energy assessment studies in complex terrain.
  • Evapotranspiration is one of the critical variables in both water and energy balance models of the hydrological system. The hydrologic system is driven by the soil-plant-atmosphere continuum, and as such is a spatially distributed process. Traditional techniques rely on point sensors to collect information that is then averaged over a region. The assumptions involved in spatially average point data is of limited value (1) because of limited sensors in the arrays, (2) the inability to extend and interpret the Measured scalars and estimated fluxes at a point over large areas in complex terrain, and (3) the limited understanding of themore » relationship between point measurements of spatial processes. Remote sensing technology offers the ability to collect detailed spatially distributed data. However, the Los Alamos National Laboratory`s volume-imaging, scanning water-vapor Raman lidar has been shown to be able to estimate the latent energy flux at a point. The extension of this capability to larger scales over complex terrain represents a step forward. This abstract Outlines the techniques used to estimate the spatially resolved latent energy flux. The following sections describe the site, model, data acquired, and lidar estimated latent energy ``map``.« less
  • To estimate three-dimensional wind distribution over complex terrain, the mass conservation method and four independent non-linear models based upon the fluid dynamic equations are applied to an area with complex terrain. Horizontal size of the domain is about 700 m by 500 m square with the maximum difference of the ground elevation is about 100 m. Calculated wind velocities estimated by each model are compared with wind velocity observed at 10 points in the domain including two profile measuring points. The results show that the non-linear numerical models do not always give better agreement with observations than MASCON method usingmore » single point wind data. Estimated wind distribution by MASON method using wind data at several points are often better than simulated wind distribution by the dynamic models.« less
  • An approximate formulation of the problem of nocturnal drainage flow has been incorporated into a computer code. This formulation, representing a generalization of the shallow fluid approximation, is applicable to complex terrain and accounts for many of the salient physical effects exhibited by drainage flow. These include the dynamics of the katabatic flow, radiation cooling, surface drag, entrainment of the overlying layer, the Coriolis force and interaction with the synoptic flow. Example calculations have been performed in one and two horizontal dimensions. These calculations exhibit a number of interesting qualitative flow features which have been observed, such as thinning ofmore » the layer over ridges and pooling in valleys. We also find that hydraulic jumps are present in the flow. The calculation shows qualitatively correct behavior at modest computational cost. Several processes which are represented parametrically can probably be tuned by comparison with data. Also, by virtue of the low cost of calculations, the model can be applied widely with a resulting gain in experience in how to best use the output. Having cited these advantages, however, it must be recognized that the model is markedly limited by its lack of vertical resolution. The more detailed description afforded by the truly three-dimensional model is necessary for a more complete understanding of this vertical structure.« less