Turbulent flow modification in the atmospheric surface layer over a dense city

Area of Science:

• Environmental Fluid Dynamics
• Urban Meteorology
• Atmospheric Science

Background:

• Atmospheric surface layer (ASL) wind dynamics are complex over urban canopies.
• Understanding urban morphology's impact on wind is vital for air quality.
• Existing models often simplify urban canopy layer (UCL) and roughness sublayer (RSL) interactions.

Purpose of the Study:

• Investigate wind profiles and turbulence in urban ASL using building-resolved LES.
• Apply the exponential law to UCLs for wind-speed parameterization.
• Analyze the influence of urban morphology on momentum transport and turbulence structures.

Main Methods:

• Building-resolved large-eddy simulation (LES) of atmospheric surface layer (ASL) winds.
• Parameterization of vertical wind profiles in urban canopy layers (UCLs) using the exponential law.
• Analysis of velocity skewness, turbulence kinetic energy (TKE), and two-point correlations.

Main Results:

• Mean wind-speed profiles parameterized by the exponential law show peak deceleration at the drag center (zero-plane displacement).
• Building heterogeneity amplifies turbulence kinetic energy (TKE) and organized structures (Q2, Q4).
• Upstream wakes enhance intermittent streamwise velocity fluctuations (u"), while excessive planar density (λp ≈ 0.7) hinders momentum transport.

Conclusions:

• Local-scale urban dynamics, including building heterogeneity and upstream wakes, significantly impact ASL wind and turbulence.
• Findings inform theoretical and empirical wall models for urban roughness.
• Improved understanding aids urban planners in enhancing air quality.