Multiscale interaction between a cluster of buildings and the ABL developing over a real terrain

- The atmospheric boundary layer influences the turbulence level at building scale.
- The influence of the atmospheric boundary layer (ABL) depends on its stability.
- The urban heat island intensity is similar for a neutral and a convective ABL.
- Rough wall functions are mostly not able to reproduce the roughness layer in an ABL.
- Synthetic turbulence inflow generators can reproduce ABL over complex topographies.

A nested Large Eddy Simulation (LES) is used to compute the multiscale interaction between the Atmospheric Boundary Layer (ABL) and an existing cluster of buildings over the real topography. A Neutral ABL (NBL) and a Convective ABL (CBL) are simulated. As the ABLs develop over a no-flat terrain with heterogeneous roughness elements, standard ABL-LES with cyclic boundary condition cannot be used. Instead, a synthetic inflow generator is used to generate realistic turbulent NBL and CBL over the real terrain. The outer domain of the nested-LES is dedicated to solve the ABL flow, with the building parametrized with a rough wall model, whereas the inner domain computes the flow through the cluster of buildings. Both LES are connected with a one-way downscale nesting procedure. The effects of the NBL and the CBL are clearly visible at building scale. The horizontal turbulent flux and the vertical turbulent flux are enhanced in the CBL compared to the NBL. The temperature difference between the air within the cluster of buildings and the rural surrounding is similar for both ABL condition, even if the turbulent mixing is higher in the CLB case. We further show that the critical part of the nested simulation is the ability of the ABL-LES to properly compute the roughness layer near the ground. Current rough wall models used to parametrize urban area are not able to reproduce such layer, which can negatively impact the flow inside the building-resolved LES.