Boundary-Layer Development with Buoyancy-Driven Flows over a Built-Up Area in Istanbul

Boundary-layer development is influenced by physical and thermal properties of the underlying surface. Roughness and sensible heat flux of the surface are two essential parameters to define the boundary-layer height. An accurate representation of these parameters provides us to understand the behavior of the layer. Recent studies have proven the need for realistic 3-D building data for surface roughness in large-eddy simulation (LES) models. However, many simulations have been conducted for idealised arrays, such as 2-D street canyons or 3-D cuboid arrays due to the high-computational cost. Furthermore, studies with large domains to capture convective scale eddy and the buoyancy-driven flows are still lacking. In this study, a LES model, PALM(Parallelized Large-Eddy Simulation Model for Atmospheric and Oceanic Flows), was utilized to examine the boundary-layer development over a built-up area in Istanbul, Turkey. Neutral case with no vertical flux of heat and unstable case with upward heat flux simulations were generated in order to investigate the boundary-layer development and the effect of realistic surface roughness on it. For neutral case, Dirichlet conditions (fixed vertical profiles) were specified at the inflow while at the outflow, radiation conditions were applied. For unstable case, constant vertical heat flux from the wall, roof and floor surfaces were included additionally. We created a large numerical domain comprised of a 4-km2 built-up area of Istanbul with 1000 x 1000 x 400 grid points in stream-wise, span-wise and vertical direction, respectively (Fig.1). Fig.2 illustrates the mean horizontal wind-speed (stream-wise) profiles for neutral and unstable cases. Unstable case findings indicated a rapid turbulent mixing caused by buoyancy-driven flows. This mixing increased the boundary-layer height dramatically with the effect of surface heat flux. Although, the wind speeds slowed down due to the surface roughness through downstream, there is a strong updraft motion above 150 m. However, below this level, mixing caused strong downdraft motions. Interaction between convective-mixing layer and surface layer can cause this downdraft motions close the surface. In contrast with the unstable case, neutral case findings indicated a shallow boundary-layer height. Moreover, the wind speed slightly evolved with the changing roughness in whole domain. In addition to the cases mentioned above, the simulation will be generated assigning more realistic: (a) heat flux outputs provided from energy balance model and, (b) inflow boundary conditions coupling with mesoscale model (Weather Research and Forecasting Model). Keywords: Boundary-layer development; heat flux; large-eddy simulation.