RAMS and WRF sensitivity to grid spacing in large-eddy simulations of the dry convective boundary layer. (21st December 2015)

• Record Type: Journal Article
• Authors: Ercolani, G.; Gorlé, C.; García-Sánchez, C.; Corbari, C.; Mancini, M.
• Is Part Of: Computers & fluids. Volume 123(2015)
• Page Start: 54
• Page End: 71
• Publication Date: 2015-12-21
• Subjects: WRF -- RAMS -- Large-eddy simulation (LES) -- Grid resolution -- Convective boundary layer; Fluid dynamics -- Data processing -- Periodicals; 532.050285
• DOI: 10.1016/j.compfluid.2015.09.009
• Languages: English
• ISSNs: 0045-7930

Abstract

Highlights: LESs of an idealized dry convective boundary (CBL) layer are presented. The results of two commonly used NWP-LAMs, RAMS and WRF, are compared. The impact of grid spacing is investigated, combining three horizontal and vertical cell sizes. It is shown that both codes can run LESs of the CBL with reasonable computational load. Guidelines on suitable grid spacings to perform similar LESs are formulated. Abstract: Large-eddy simulations (LESs) are frequently used to model the planetary boundary layer, and the choice of the grid cell size, numerical schemes and sub grid model can significantly influence the simulation results. In the present paper the impact of grid spacing on LES of an idealized atmospheric convective boundary layer (CBL), for which the statistics and flow structures are well understood, is assessed for two mesoscale models: the Regional Atmospheric Modeling System (RAMS) and the Weather Research and Forecasting model (WRF). Nine simulations are performed on a fixed computational domain (6 × 6 × 2 km), combining three different horizontal (120, 60, 30 m) and vertical (20, 10, 5 m) spacings. The impact of the cell size on the CBL is investigated by comparing turbulence statistics and velocity spectra. The results demonstrate that both WRF and RAMS can perform LES of the CBL under consideration without requiring extremely high computational loads, but they also indicate the importance of adopting a computational grid that is adequate for the numerical schemes and subgrid models used. In both RAMS and WRF a horizontal cell size of 30 m is required to obtain a suitable turbulence reproduction throughout the CBL height. Considering the vertical grid spacing, WRF produced similar results for all the three tested values, while in RAMS it should be ensured that the aspect ratio of the cells does not exceed a value of 3. The two models were found to behave differently in function of the grid resolution, and they have different shortcomings in their prediction of CBL turbulence. WRF exhibits enhanced damping at the smallest scales, while RAMS is prone to the appearance of spurious fluctuations in the flow when the grid aspect ratio is too high.