Feedback between dust particles and atmospheric processes over West Africa during dust episodes in March 2006 and June 2007

摘要

We used the comprehensive model system COSMO-ART to quantify the impact ofmineral dust on the radiative fluxes, the temperature and the feedbackbetween dust particles and their emissions. We simulated two dust stormsover West Africa in March 2006 and in June 2007. Simulations with andwithout coupling of the actual dust concentration with the radiative fluxesand the thermodynamics were carried out for each case. The model results forthe 2006 case were compared with observations of the AMMA campaign.At the surface the shortwave radiative effect of mineral dust can bedescribed by a linear relation between the changes in net surface radiationand the aerosol optical depth (AOD). For an AOD at 450 nm of 1 the averageshortwave radiation reduction amounts ?140 W m?2 during noon. Thelongwave radiative effect of mineral dust is nonlinear, with an averageincrease of +70 W m?2 for an AOD (450 nm) of 1. At the top of theatmosphere the effect of the dust layer with an AOD of 1 on radiative fluxesis not as significant as at the surface. It is slightly positive for theshortwave and approximately 26 W m?2 for the longwave radiation.The height range and the extension of the dust layer determine the effect ofdust particles on the 2 m temperature. When the dust layer is attached tothe surface and lasts for several days it leads to an increase of thesurface temperature even during daytime. In case of an elevated dust layerthere is a decrease in 2 m temperature of up to 4 K during noon.It is shown, that the temperature changes caused by mineral dust may resultin horizontal temperature gradients which also modify near surface winds.Since surface wind thresholds decide the uptake of dust from the surface, afeedback on total emission fluxes is established. The coupled model providesan increase in the total emission fluxes of dust particles by about 16%during the dust storm in March 2006 and 25% during the dust episode inJune 2007.