A Theory for the Effects of Dust-Radiative Heating on the Linear and Weakly Nonlinear Dynamics of African Easterly Waves

Abstract:

Winds over West Africa loft vast plumes of Saharan mineral dust aerosols into the atmosphere. The plumes affect the radiative balance, causing changes to the thermal and dynamical structure of the atmosphere. How these dust-induced changes of the atmosphere affect the growth, propagation and subsequent evolution of African easterly waves (AEWs) remains poorly understood. Because AEWs often serve as seeds for the development of tropical storms, understanding how dust affects their evolution is of both practical and theoretical importance. In this study, a quasigeostrophic model is used to construct a theory that explicitly describes how dust-radiative-dynamical feedbacks affect the linear and weakly nonlinear dynamics of AEWs. The model consists of coupled equations for potential vorticity, temperature and dust concentration. The radiative-dust heating rate accounts for both shortwave and longwave radiative transfer. The source of dust is due to surface emission, which depends on surface wind; the sinks of dust are due to sedimentation and dry deposition. An asymptotic analysis yields analytical expressions for the propagation and growth characteristics of the model’s AEWs, as well as their weakly nonlinear evolution and feedback on the zonal-mean field. The analysis shows the importance of the meridional and vertical gradients of the basic state dust distribution to the linear and weakly nonlinear dynamics of the AEWs. Idealized examples are used to highlight how the dust-radiative feedbacks affect the propagation, group velocity, growth, structure, wave fluxes and weakly nonlinear evolution of AEWs. The clarity of the expressions connecting dust aerosols to the dynamics of AEWs provides an important interpretive tool for analyzing results obtained from comprehensive model simulations of AEWs, such as those produced by the Weather Research and Forecasting (WRF) model.