# Stable and convective boundary-layer flows in an urban array

## TL;DR

This study investigates how stable and convective stratification affect airflow, turbulence, and surface roughness in an urban environment using wind tunnel experiments, revealing significant modifications in flow dynamics and surface parameters.

## Contribution

It provides new experimental data on the impact of stratification on urban boundary-layer flows, highlighting changes in turbulence, roughness length, and displacement height.

## Key findings

- Stable stratification reduces turbulence and Reynolds stresses.
- Convective stratification increases friction velocity and alters surface roughness.
- Stratification significantly modifies surface and flow parameters in urban-like environments.

## Abstract

In this paper non-neutral approaching flows were employed in a meteorological wind tunnel on a regular urban-like array of rectangular buildings. As far as stable stratification is concerned, results on the flow above and inside the canopy show a clear reduction of the Reynolds stresses and an increment of the Monin-Obukhov length up to 80%. The roughness length and displacement height were also affected, with a reduction up to 35% for the former and an increment up to 12% for the latter. A clear reduction of the turbulence within the canopy was observed. In the convective stratification cases, the friction velocity appears increased by both the effect of roughness and unstable stratification. The increased roughness causes a reduction in the surface stratification, reflected in an increase of the Monin-Obukhov length, which is double over the array compared to the approaching flow. The effect on the aerodynamic roughness length and displacement height are specular to the SBL case, an increase up to 50% of the former and a reduction of the same amount for the latter.