To what extent does the air flow initialisation of the ENVI-met model affect human heat stress simulated in a common street canyon?

Abstract

This study concerns the effects of the air flow initialisation of the ENVI-met model on simulated human heat stress in a fictive E-W street canyon with different aspect ratios that is typical of Central European cities. Human heat stress is described by near-surface air temperature (Ta), mean radiant temperature (Tmrt) and physiologically equivalent temperature (PET). The numerical simulations, which are performed for a Central European heat wave day in order to consider the increase of severe heat due to regional climate change, are based on the version 4.0 BETA of ENVI-met. The simulation results in terms of grid-related Ta, Tmrt and PET values as well as mean values for both sidewalks of the street canyon are averaged over the period 10-16 CET, because they should be representative of outdoor human heat stress in Central European cities. The simulation results point to the significance of the type of inflow direction in relation to the orientation of the street canyon, i.e. whether it is flowed parallel or across. The type of inflow direction determines the modification of the inflow speed within the street canyon. Due to its physical basis, mean Tmrt does not show a noticeable impact by the inflow conditions. They also influence mean Ta relatively low. However, PET is much more affected by the inflow conditions as it depends on the local wind speed. This impact can reach the magnitude of thermal grassland effects within urban quarters, i.e. it cannot be ignored under a human-biometeorological perspective.