Capability of WRF-LES Mode on Realistic Atmospheric Surface Layer

We investigated the surface layer wind velocity using Large-eddy simulation (LES) mode of a numerical weather prediction model. Simulation was initialized to suit the realistic time and space, where and when wind profiles of large Reynolds number boundary layer under neutral condition were measured by the field observation (Surface Layer Turbulence and Environmental Science Test, SLTEST). LES mode of Weather Research and Forecasting（WRF-LES mode） was used with the 24 m grid resolution for horizontal spacing in the finest (5^th) domain downscaled by one-way nesting. In order to improve an expression of the surface layer, the two-part subgrid-scale model (Sullivan et al., 1994) under neutral condition was implemented, and the cell perturbation (Muñoz-Esparza et al., 2014) was added on the margin of the LES domains; domain 4 connected from PBL domain, and domain 5 connected from coarser　LES domain. We detected time interval of neutral stability on observation point by checking the Monin-Obukhov length scale. The boundary layer height is estimated to be 160m and Reτ = 10⁶. Predominantly owing to the subgrid-scale modeling, the velocity profiles fitted the logarithmic assumption of corresponding roughness in grid points near the surface, with a slight veering in hodograph. Streaky structures were observed in a time series of streamwise velocity outputted on east-west line nearly perpendicular to the flow direction, presumably representing the coherent turbulence structure, however, the resolved scale velocity fluctuation near the surface seems to dissipate because of vertically coarse grid resolution (16 m for the first grid interface) within a confined boundary layer. Observations of large Reynolds number boundary layer have been revealing the velocity fluctuation scaled by the characteristics of the outer layer, which is located on a lower several grid points in the current WRF-LES. Assigning more grids on the logarithmic layer may resolve turbulence production near the surface, and possibly exhibit a better correspondence to the observation, which will be of the future work.