Analysis of Horizontal Shear and Mixing at a Range of Length Scales Using Filtered Large-Eddy Simulation of a Flow over Complex Terrain
Abstract
Atmospheric boundary layer turbulence in most numerical weather prediction models is currently parameterized using one-dimensional planetary boundary layer (1D PBL) schemes. Such parameterizations are based on the assumption of horizontal homogeneity. If grid cell sizes are relatively large (greater than a few kilometers) such assumption can be justified, however, when grid cell size is less than a couple of kilometers such assumption breaks down. Accurate characterization of flows in complex terrain requires resolving terrain effects on the flow. This requires high-resolution numerical grids with grid cell sizes or 1 km or less in the range of scales labeled "gray zone" or "Terra Incognita" (Wyngaard 2004). As the grid-cell size of mesoscale simulations decreases, however, the assumption of horizontal homogeneity is violated and the effect of horizontal gradients should be accounted for. These three-dimensional effects at mesoscale grid-cell size are particularly pronounced in flows over complex terrain.
To study turbulence effects at different length scales in complex terrain we carried out large-eddy simulations (LES) based on the Wind Forecast Improvement 2 (WFIP2) field study in Columbia River Gorge area. Field study observations and validated LES results are then used to refine and assess the new 3D-PBL parameterization implemented in the WRF model. We focus on a selected case when physical phenomena of significance for forecasting in complex terrain including mountain waves, topographic wake, and gap flow were observed. We use filtered LES fields to compute turbulence quantities at different scales and provide an estimate of a scale below which a 3D PBL parameterization should be used.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMGC0590002K
- Keywords:
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- 3329 Mesoscale meteorology;
- ATMOSPHERIC PROCESSES;
- 3394 Instruments and techniques;
- ATMOSPHERIC PROCESSES;
- 0545 Modeling;
- COMPUTATIONAL GEOPHYSICS;
- 1630 Impacts of global change;
- GLOBAL CHANGE