The influence of using a multi-layer urban canopy scheme and detailed land use in very high-resolution NWP

This study evaluates the performance of the Harmonie-AROME Numerical Weather Prediction (NWP) model in reproducing real atmospheric conditions over Paris during an intense heatwave in the summer of 2022. A series of very-high-resolution simulations are conducted at different spatial resolutions and incorporate diverse land use and urban morphology datasets.

The simulations utilized ECMWF operational forecasts at 9 km resolution as boundary conditions for the operational 2.5 km runs, with additional one-way nested domains at 500 m and 200 m resolutions. To assess the impact of urban areas, the Town Energy Balance (TEB) urban canopy parameterization (Masson et al., 2000) was implemented, comparing its single-layer and newly developed multi-layer configurations. Additionally, improvements were introduced to better account for the influence of urban structures on turbulent vertical diffusion.

To investigate the impact of different urban morphology datasets, simulations were performed with:

• ECOCLIMAP-SG land use dataset at 300 m resolution, which classifies urban areas into 10 categories based on the WUDAPT Local Climate Zones (LCZ) classification.
• Geoclimate urban morphology dataset at 100 m resolution, derived from OpenStreetMap (OSM) data (Bernard et al., 2022), utilizing a random forest technique to estimate missing building heights and generate a more realistic representation of urban geometries.

Comparison with observations reveals that category-based land use datasets (ECOCLIMAP-SG) struggle to capture temperature variability in heterogeneous urban areas, even at higher resolutions. In contrast, the OSM-based dataset better represents city heterogeneity and horizontal variability, demonstrating its suitability for high-resolution urban simulations.

At 200 m resolution, results indicate that category-based land use datasets are not adequate for resolving neighborhood-scale variations within a city. Furthermore, employing the multi-layer TEB model significantly improves the simulation of air temperature in high-density districts and wind speed within the whole urban canopy boundary layer, emphasizing the need for vertically coupled urban canopy parameterizations within the planetary boundary layer (PBL) scheme.