Scale-Dependent Transport of Saharan Dust by African Easterly Waves

The scale-dependent transport of Saharan dust aerosols by African easterly waves (AEWs) is examined analytically and numerically. The analytical analysis shows that the meridional and vertical wave transports of dust are modulated by the Doppler-shifted frequency, omega(d), and the wave growth rate, omega(i), both of which are functions of the zonal wave scale. The analytical analysis predicts that the AEW dust transports, which are driven by the Reynolds stresses acting on the mean dust gradients, are largest for the twin limits: omega(d) -> 0, which corresponds to flow near a critical surface, a local effect; and omega(i) -> 0, which corresponds to the slowest growing waves, a global effect. The numerical analysis is carried out with the Weather Research and Forecasting (WRF) model, which is radiatively coupled to the dust field. The model simulations are based on an AEW spectrum consistent with observations. The simulations agree with the theoretical predictions: the slowest growing waves have the strongest transports, which are as much as similar to 40% larger than the transports of the fastest growing wave. Although the transports are highly scale-dependent, largely due to the scale dependence of omega(i), the location of the critical surface and thus the location of the maximum dust transports are not.