Abstract
An analytical analysis is combined with numerical modeling simulations in order to expose the physical and dynamical processes that control the zonal-mean transport of Saharan mineral dust aerosols during the incipient growth phase of African easterly waves. The analytical analysis provides the theoretical basis for understanding and predicting how the waves and background flow combine to affect the zonal-mean eddy transports of dust. The analytically derived transport equations―which are valid for any wave field, irrespective of its spatial or temporal scale―predict that the eddy transports of dust are largest where the maximum in the background dust gradients coincide with a critical surface, i.e., where the Doppler-shifted frequency of the wave field vanishes. Linear simulations of the eddy dust transports are conducted using a mechanistic version of the Weather Research and Forecasting (WRF) model coupled to an interactive dust model. The simulations show that the eddy dust transports are directed down the background dust gradients and that the meridional transports of dust dominate over the vertical transports. The numerical simulations confirm the theoretical predictions. The predictions are used to explain recent statistical analyses of reanalysis data for dust-coupled African easterly waves.
Highlights
The myriad wind systems that occur over North Africa during summertime cause Saharan mineral dust aerosols to episodically erupt from the surface to form plumes that span the synoptic-scale (Figure 1) [1,2]
The dust-modified energy balance, in turn, affects the thermal driving of the atmospheric circulation, which includes two of the most prominent synoptic-scale circulations over North Africa: the African easterly jet (AEJ) and African easterly waves (AEWs), which together form a system that is central to the weather and climate over North Africa
How do the zonal-mean distributions of zonal wind and dust combine to affect the transports of dust by AEWs? Second, is there a region where the eddy dust transports are optimized, and, if so, what are the controlling factors?
Summary
The myriad wind systems that occur over North Africa during summertime cause Saharan mineral dust aerosols to episodically erupt from the surface to form plumes that span the synoptic-scale (Figure 1) [1,2]. Studies of the Observational, analytical, and modeling studies have shown that the structure and evolution of dust-modified AEJ, for example, have utilized a wide variety of models and data sets, yet there is one the AEJ-AEW system is modulated by the radiative and microphysical effects of dust. The WRF-dust model simulations, which were averaged coupled to an online dust model to examine the effects of Saharan dust on the structure, location, over a single summer season, showed that the dust-modified heating field strengthens the AEJ, shifts and energetics of the AEJ-AEW system. We combine a theoretical framework with numerical simulations, which together expose the physical and dynamical processes that control the zonal-mean transport of Saharan mineral dust aerosols during the incipient growth phase of AEWs. Our analysis is framed around two questions. How do the zonal-mean distributions of zonal wind and dust combine to affect the transports of dust by AEWs? Second, is there a region where the eddy dust transports are optimized, and, if so, what are the controlling factors?
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