Effects of Saharan Dust on the Linear and Nonlinear Dynamics of African Easterly Waves

The direct radiative effects of Saharan mineral dust aerosols on the dynamics of African easterly waves (AEWs) are examined using the Weather Research and Forecasting (WRF) model coupled to an online dust aerosol model. The coupled model is governed by the Advanced Research WRF dynamical core and continuity equations for twelve dust particle sizes that represent the spectrum of mineral dust aerosols observed in the atmosphere. Zonally averaged basic states are chosen consistent with summertime conditions over North Africa: the basic state wind corresponds to an unstable African easterly jet (AEJ) and the basic state dust field is placed north of the AEJ axis. Linear and nonlinear simulations are then carried out to show the effects of dust-radiative feedbacks on the spatial-temporal properties of AEWs. Linear simulations show: (1) dust increases the growth rate of the most unstable AEW by 13%; (2) dust-modified local energetics is largest between the jet axis and the latitudinal maximum in the dust distribution. An analytically derived expression for the local generation of eddy energy by the dust field explicitly shows that eddy generation is primarily controlled by the transmissivity and spatial gradients of the zonal-mean dust, and the wave Doppler-shifted frequency. Nonlinear simulations show: (1) dust increases the peak amplitudes of the most unstable AEW by 22%; (2) dust shortens the time needed to reach the peak amplitude. These changes to the peak amplitude and the subsequent wave driving on the zonal-mean flow produces significant changes to the location of critical latitudes for the stabilized AEJ, which previous studies have shown are ideal locations for tropical cyclogenesis.