## 1 BACKGROUND

Wind flow modeling software is widely used for wind resource assessment and mostly based on either the computational fluid dynamics (CFD) approach or the linear WAsP approach. [1] They focus primarily on modeling of the neutrally stratified atmospheric surface-layer (ASL), which typically covers the bottom 10% of the atmospheric boundary-layer (ABL). In the ASL, the logarithmic wind profile is a justified approximation, and the models account for the effects of roughness and topography changes. Atmospheric stability and Coriolis effects are mostly ignored or, like in WAsP, are treated as small perturbations to the neutral background flow that can be added after solving the model equations. In order to decrease the uncertainty of predictions, especially in complex terrain, stability and Coriolis effects on the whole atmospheric boundary layer should be included in such models.

Turbulence within the ABL covers a wide range of scales (from less than a centimeter up to several kilometers[2]). Since the solution of the full Navier–Stokes equations is not computationally feasible, high Reynolds number flows can be based on the solution of the Reynolds-averaged Navier–Stokes (RANS) equations. Recently, Sogachev *et al*.[3] developed an atmospheric model for flows over flat terrain that accounts for stability and Coriolis effects: the energy equation in terms of the potential temperature is solved in parallel to the RANS equations, and a consistent two-equation turbulence model is used to close the equations. Using a two-equation closure method to describe the whole ABL allows the flow to be computed at a much lower computational cost than, e.g., using large-eddy simulations (LES). [4] The aim of the present work is to develop and validate a RANS ABL model framework describing the whole ABL that can be applied for flows over complex terrain. The starting point is the in-house CFD solver EllipSys3D, developed at the Technical University of Denmark's Wind Energy department (Roskilde, Denmark).[5-7] The solver was initially developed for simulating the near-ground surface-layer flow inside a neutrally stratified domain (from now on referred to as the ASL model) and has, under these conditions, been validated against field experiments.[8, 9] To model ABL flows more appropriately, the solver is modified (from now on referred to as the ABL model), and the two-equation turbulence closure from Sogachev *et al*.[3] is used.

To get a better understanding of the physical processes involved in ABL flows and to validate ABL models, more data sets from atmospheric experiments on full scale are necessary. Various experiments that focus on neutral flow over complex terrain are available, e.g., the Askervein Hill experiment,[10] or more recently the Bolund experiment.[8, 11] Existing benchmark literature for non-neutral ABL flows mostly focuses on flat terrain,[12-15] and test cases for complex terrain are scarce. Ross *et al*.[16] present a wind-tunnel study of neutral and stably stratified boundary-flow over a steep hill. The wind-tunnel experiment was designed to represent a realistic non-neutral ABL flow and analyses stability effects over terrain under controlled conditions. Although not real ABL flows at full scale, this test case was chosen to test the applicability and performance of the ABL model for flows over well defined but steep terrain.

The central goal of the present study is to examine how well the ABL model performs in representing neutral and non-neutral ABL flows and to set the starting point to apply the ABL model for flows over complex terrain. In Section 2, the modeling approach is presented, followed by Section 3, where implementation aspects (Section 3.1) and the simulation methodology (Section 3.2) are described. In Section 4, the Monin–Obukhov similarity theory (MOST) is briefly described. Section 5 presents results from four test cases that are used to validate the ABL model.

The simulations are divided in three parts. First, Sections 5.1 and 5.2 focus on neutral ABL flow over rough flat ground. Simulation results are compared against experimental data from the Leipzig wind profile[17] and the Cabauw site.[18] Second, non-neutral ABL flow over rough flat ground is considered in Section 5.3. Experimental data from the second GEWEX (Global Energy and Water cycle EXperiment) ABL Study (GABLS2),[15] which analyses a diurnal cycle in the ABL, is used to validate the ABL model for non-neutral conditions. Additionally, MOST is used to compare simulation results against experimental data from several large-scale field campaigns.[12-14] Third, in Section 5.4, the wind-tunnel experiment analyzing stratified boundary-layer flow over a steep hill[16] is used to assess the applicability of the ABL model for non-neutral flows over terrain. Concluding remarks are given in Section 6.