Impact of aerosol direct radiative forcing on the radiative budget, surface heat fluxes, and atmospheric dynamics during the heat wave of summer 2003 over western Europe: A modeling study

In this work, an off-line coupling between the chemistry-transport model CHIMERE (associated with an aerosol optical module) and the meteorological model Weather Research and Forecasting (WRF) is used to study (1) the direct radiative forcing of pollution aerosols during the heat wave of summer 2003 over western Europe and (2) the possible feedbacks of this direct radiative forcing on the surface-atmosphere system. Simulations performed for the period 7-15 August 2003 reveal a significant decrease of daily mean solar radiation reaching the surface (F_BOA =-(10-30) W/m²) because of back scattering at the top of the atmosphere (F_TOA =-(1-12) W/m ²) and also absorption of solar radiation by polluted particles (F_atm = + (5-23) W/m²). During daytime, the aerosol surface dimming induces a mean reduction of both sensible (16 W/m²) and latent (21 W/m²) heat fluxes emitted by the terrestrial surface, resulting in a radiative cooling of the air near the surface (up to 2.9 K/d at noon). Simultaneously, the absorption of solar energy by aerosols causes an atmospheric radiative heating within the planetary boundary layer reaching 1.20 K/d at noon. As a consequence, the direct radiative effect of aerosols is shown to reduce both the planetary boundary layer height (up to 30%) and the horizontal wind speed (up to 6%); that may have contributed to favor the particulate pollution during the heat wave of summer 2003.