Saharan Dust Transport during the Incipient Growth Phase of African Easterly Waves

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.

The Discovery of African Dust Transport to the Western Hemisphere and the Saharan Air Layer: A History

“…Some dust might also be injected directly into the boundary layer along the coast of Africa (Reid et al, 2003). The complexity of these processes presents a challenge to our understanding of the life-cycle of dust emissions and transport from Africa (Knipertz and Todd, 2012) and the relationship to easterly waves (Grogan and Thorncroft, 2019;Nathan et al, 2019).…”

“…This is important in assessing the impact on tropical cyclones since the injection of the hot dry SAL air into tropical cyclones is believed to play an important role in the weakening of tropical cyclones (e.g., Braun et al, 2012;Dunion and Velden, 2004;Dunion, 2011;Reed et al, 2019;). Indeed, the generation of many dust storms in West Africa is closely linked to the genesis of easterly waves in that region (Bercos-Hickey et al, 2020;Nathan et al, 2019). There are efforts underway to assess these impacts in a systematic way through models (e.g., Strong et al, 2018).…”

Formation of a low‐level barrier jet and its modulation by dust radiative forcing over the Hexi Corridor in Central China on March 17, 2010

“…This forcing includes the effects from dust–radiation and dust–cloud interactions. Dust–radiation interaction, which depends on longwave emission and the absorption and scattering of both shortwave and longwave radiation, is central to the thermal structure and circulation of the atmosphere (Bercos‐Hickey et al ., 2017; 2020; Chen et al ., 2010; 2015; Grogan and Thorncroft, 2019; Heinold et al ., 2008; Hosseinpour and Wilcox, 2014; Nathan et al ., 2017; 2019; Rémy et al ., 2015). During daytime, the dust–radiation interaction warms the upper part of the dust plume, which is dominated by the shortwave effect, and warms or cools it below, which is affected by both the shortwave and longwave effects (Alizadeh‐Choobari et al ., 2012; Chen et al ., 2010).…”

Passive versus Active Transport of Saharan Dust Aerosols by African Easterly Waves

“…The transport of dust by AEWs is a manifestation of the coupling between dust and waves. The coupling has been demonstrated in both observational and modeling studies [23,25,[28][29][30][31]. Carlson and Prospero [25], for example, analyzed dust and wind data acquired during the 1969 Barbados Oceanographic and Meteorological Experiment and made comparisons between the positions and movement of Saharan dust pulses and easterly wave activity over the tropical Atlantic.…”

“…These studies, however, did not address the factors that control where the transports are maximized. This has been addressed, in part, by Nathan et al [31], who developed a theory that exposes the physical-dynamical interactions that control the zonal-mean transport of Saharan mineral dust by linearly unstable AEWs. The theory, which was developed from a conservation equation for dust, predicts that the AEW transports of dust will be largest where the maximum in the background dust gradients coincide with a critical surface.…”