Nested Large-Eddy Simulations of the Displacement of a Cold-Air Pool by Lee Vortices
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Nested Large-Eddy Simulations of the Displacement of a Cold-Air Pool by Lee Vortices Alex Connolly1
· Fotini Katopodes Chow2 · Sebastian W. Hoch3
Received: 15 October 2019 / Accepted: 5 August 2020 © Springer Nature B.V. 2020
Abstract Mesoscale simulations are typically performed at coarse resolutions that do not adequately represent underlying topography; nesting large-eddy simulations within a mesoscale model can better resolve terrain and hence capture topographically-induced stable flow phenomena. In the case of the Mountain Terrain Atmospheric Modelling and Observations (MATERHORN) program, large temperature fluctuations were observed on the slope of Granite Peak, Utah, which partially encloses a cold-air pool in the east basin. These flow features are able to be resolved using large-eddy simulation within the Weather Research and Forecasting (WRF) model with x = 100 m, allowing accurate representation of lee vortices with horizontal length scale of O(1 km). At this resolution, terrain slopes become quite steep, and some model warm biases remain in the east basin due to limits on terrain-following coordinates that prevent the model from fully resolving drainage flows with this steep terrain. A new timestep limit for the WRF model related to these steep slopes is proposed. In addition, the initialization of soil moisture is adjusted by drying the shallowest layer to assist the formation of a cold pool in the large-eddy simulation. These real case simulations compare well to observations and also to previously published simulations using idealized configurations to study similar phenomena. For instance, the values of non-dimensional mountain height, which characterize flow regimes in idealized studies, are similar in the real case. Keywords Cold-air pool · Large-eddy simulation · Lee vortices · Mountain meteorology · Numerical weather prediction
1 Introduction Stably stratified flows, typical of the nocturnal atmospheric boundary layer, are affected by the presence of complex terrain in ways not seen under neutral or unstable conditions
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Alex Connolly [email protected]
1
Department of Civil and Environmental Engineering, University of California, Berkeley, 205 O’Brien Hall, Berkeley, CA 94720, US
2
Department of Civil and Environmental Engineering, University of California, Berkeley, CA, US
3
Department of Atmospheric Science, University of Utah, Salt Lake City, UT, US
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(Baines 1998). In large part, this is due to the decoupling of synoptic flows and low-level flows within a near-surface inversion, which inhibits vertical mixing of momentum from aloft. Flow blocking and channeling also play major roles when air parcels are too dense to rise up and over obstacles (Chow et al. 2013). Likely due to this multiplicity of phenomena requiring parametrization, stable conditions also pose special challenges to numerical weather prediction (NWP) models. Planetary boundary-layer (PBL) schemes, the turbulence closures employed in NWP models, are more successful during daytime when con
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