An Examination of Large Eddy Simulation-within-Large Eddy Simulation Framework over Heterogeneous Surface Conditions

In the last four decades since the first introduction of Deardorff (1970), large eddy simulation (LES) has been successfully applied to understand atmospheric boundary layer (ABL) flows. Recently, using the Weather Research and Forecasting (WRF) model, LES is being applied for real-case simulations by means of a framework of LES nested within a mesoscale model (MM). During the downscaling process via nesting, the model resolution may need to cross the terra incognita of Wyngaard (2004), at which the model resolution is comparable to the characteristic length scale of energy containing turbulent eddies. Although a few studies have investigated, the transitional passage between the MM and LES limits has not been fully explored particularly in terms of the ABL structures and processes responding to heterogeneous surface conditions. First, the paired framework of fine LES-within-coarse LES will be evaluated with the single, outer-domain fine LES framework over homogeneous surface conditions but with different model configurations such as grid nesting ratio, one- versus two-way nesting, and/or sub-filter scale turbulence closure schemes. The evaluation will be performed in terms of the temporal evolutions of the mean and variance profiles of potential temperature, water vapor mixing ratio, and wind components as well as the spectra of the variables. Later, over multiscale surface flux heterogeneities in a mesoscale domain prescribed with the inverse Fourier transforms from analyzed surface heat flux spectra, the LES-within-LES framework will be evaluated with the single-domain fine LES. We will first integrate the model with the initial sounding of a fair- weather convective boundary layer condition and later with the sounding favorable for deep moist convection. This presentation will summarize and report the numerical experiment results with the LES-within-LES framework with the reference of those with the single LES, not only over homogeneous surface conditions but also over heterogeneous conditions.