Abstract
Abstract. A synthetic inflow turbulence generator was implemented in the idealised Weather Research and Forecasting large eddy simulation (WRF-LES v3.6.1) model under neutral atmospheric conditions. This method is based on an exponential correlation function and generates a series of two-dimensional slices of data which are correlated both in space and in time. These data satisfy a spectrum with a near “-5/3” inertial subrange, suggesting its excellent capability for high Reynolds number atmospheric flows. It is more computationally efficient than other synthetic turbulence generation approaches, such as three-dimensional digital filter methods. A WRF-LES simulation with periodic boundary conditions was conducted to provide prior mean profiles of first and second moments of turbulence for the synthetic turbulence generation method, and the results of the periodic case were also used to evaluate the inflow case. The inflow case generated similar turbulence structures to those of the periodic case after a short adjustment distance. The inflow case yielded a mean velocity profile and second-moment profiles that agreed well with those generated using periodic boundary conditions, after a short adjustment distance. For the range of the integral length scales of the inflow turbulence (±40 %), its effect on the mean velocity profiles is negligible, whereas its influence on the second-moment profiles is more visible, in particular for the smallest integral length scales, e.g. those with the friction velocity of less than 4 % error of the reference data at x/H=7. This implementation enables a WRF-LES simulation of a horizontally inhomogeneous case with non-repeated surface land-use patterns and can be extended so as to conduct a multi-scale seamless nesting simulation from a meso-scale domain with a kilometre-scale resolution down to LES domains with metre-scale resolutions.
Highlights
Atmospheric boundary layer flow involves a wide range of scales of eddies, from quasi-two-dimensional structures at the mesoscales to three-dimensional turbulence at the microscale (Muñoz-Esparza et al, 2015)
This study focuses on an implementation of this synthetic inflow turbulence generator (Xie and Castro, 2008) in the idealised Weather Research and Forecasting (WRF)-large eddy simulation (LES) (v3.6.1) model under neutral atmospheric conditions
This suggests that the synthetic turbulence generated at the inlet can develop into realistic turbulence with wellconfigured structures from an adjustment distance downwind of about x/H = 5–10
Summary
Atmospheric boundary layer flow involves a wide range of scales of eddies, from quasi-two-dimensional structures at the mesoscales to three-dimensional turbulence (normally with higher Reynolds number, i.e. Re ∼ 108–109) at the microscale (Muñoz-Esparza et al, 2015). The use of periodic boundary conditions implicitly assumes that atmospheric fields and the underlying land use have repeated periodic features This assumption may be unrealistic for real landscapes where land-use patterns and the atmospheric phenomena coupled to them can be very heterogeneous. Such periodic WRFLES simulations are restricted to studies of the atmospheric boundary layer flow with a single domain We implement a welltested synthetic turbulence inflow scheme (Xie and Castro, 2008) in the WRF-LES model (v.3.6.1), in which the mesoscale model could provide the mean flow information as the input of the synthetic turbulence inflow scheme This scheme provides a step towards enabling WRF’s capability of nesting micro-scale turbulent flows within realistic meso-scale meteorological fields
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