Modeling urban traffic heat flux in the Community Earth System Model: Formulation and validation for two sites

Vehicular traffic is a major contributor to anthropogenic heat flux (AHF) in urban areas, amplifying urban heat island effects. However, few Earth system models explicitly represent traffic conditions and their associated heat emissions. This study introduces a new urban traffic module into the Community Earth System Model (CESM), enabling interactive simulation of traffic-related heat in urban areas. The module adopts a bottom-up approach to estimate traffic heat flux (Qtraffic) based on time-varying traffic volume and vehicle type distributions, while dynamically responding to meteorological conditions such as snow, rain, and low temperatures. Model validation was performed using observational data from two urban sites: Capitole of Toulouse, France (FR-Capitole), and Manchester, UK (UK-Manchester). At the FR-Capitole site, an annual mean Qtraffic of 22.23 W/m² in 2004 resulted in a simulated annual mean canopy air temperature increase of 0.4K, improving the simulated turbulent heat flux compared to observations. At the UK-Manchester site, the simulation with a yearly mean Qtraffic of 16.27 W/m² showed a 0.25K air temperature increase in 2022. These traffic-induced canopy warming also influenced the indoor environment, contributing to increased air conditioning use in summer and reduced building space heating demand in winter. This new functionality offers potential applications such as simulating traffic-induced AHF and its impacts on the climate system under future climate changes and transport transition scenarios.