Dust plume formation in the free troposphere and aerosol size distribution during the Saharan Mineral Dust Experiment in North Africa

Abstract

Dust particles mixed in the free troposphere have longer lifetimes than airborne particles near the surface. Their cumulative radiative impact on earth's meteorological processes and climate might be significant despite their relatively small contribution to total dust abundance. One example is the elevated dust-laden Saharan Air Layer (SAL) over the tropical and subtropical North Atlantic, which cools the sea surface. To understand the formation mechanisms of a dust layer in the free troposphere, this study combines model simulations and dust observations collected during the first stage of the Saharan Mineral Dust Experiment (SAMUM-I), which sampled dust events that extended from Morocco to Portugal, and investigated the spatial distribution and the microphysical, optical, chemical, and radiative properties of Saharan mineral dust. The Weather Research Forecast model coupled with the Chemistry/Aerosol module (WRF-Chem) is employed to reproduce the meteorological environment and spatial and size distributions of dust. The model domain covers northwest Africa and adjacent water with 5 km horizontal grid spacing and 51 vertical layers. The experiments were run from 20 May to 9 June 2006, covering the period of the most intensive dust outbreaks. Comparisons of model results with available airborne and ground-based observations show that WRF-Chem reproduces observed meteorological fields as well as aerosol distribution across the entire region and along the airplane's tracks. Several mechanisms that cause aerosol entrainment into the free troposphere are evaluated and it is found that orographic lifting, and interaction of sea breeze with the continental outflow are key mechanisms that form a surface-detached aerosol plume over the ocean. The model dust emission scheme is tuned to simultaneously fit the observed total optical depth and the ratio of aerosol optical depths generated by fine and coarse dust modes. Comparisons of simulated dust size distributions with airplane and ground-based observations are good for optically important 0.4Á0.7 mm particles, but suggest that more detailed treatment of microphysics in the model is required to capture the full-scale effect of large and very small aerosol particles beyond the above range.

Simulating the Regional Impact of Dust on the Middle East Climate and the Red Sea

“…The particle size distribution strongly affects the spectral extinction and single-scattering albedo, modulating the associated radiative effect. Multiple observational campaigns (Otto et al, 2007;Ryder et al, 2013;Weinzierl et al, 2009) and numerical studies (Khan et al, 2015;Kok et al, 2017) have indicated the presence and importance of coarse mode particles, which significantly contribute to the AOD and single-scattering albedo. In modeling studies, the concentration of coarse particles (with diameter larger than 1-3 lm) is often underestimated and compensated by finer particles (Khan et al, 2015;Kok, 2011).…”

“…Multiple observational campaigns (Otto et al, 2007;Ryder et al, 2013;Weinzierl et al, 2009) and numerical studies (Khan et al, 2015;Kok et al, 2017) have indicated the presence and importance of coarse mode particles, which significantly contribute to the AOD and single-scattering albedo. In modeling studies, the concentration of coarse particles (with diameter larger than 1-3 lm) is often underestimated and compensated by finer particles (Khan et al, 2015;Kok, 2011). The complex shape of the dust particles affects the phase function and angular scattering patterns (Dubovik et al, 2006(Dubovik et al, , 2002.…”

Observations and Cloud‐Resolving Modeling of Haboob Dust Storms Over the Arabian Peninsula

“…The models used by Knippertz et al () and Reinfried et al () did not include a dust component and were focused on simulations of the convection and its associated cold pools. Later, two studies with online dust models complemented that field campaign (Khan et al, ; Solomos et al, ). The African Monsoon Multidisciplinary Analysis campaign (Redelsperger et al, ) was complemented by a modeling study (Cavazos‐Guerra & Todd, ) of 30‐ and 10‐km nested Weather Research and Forecasting‐Chemistry (WRF‐Chem) simulations with parameterized convection and a bulk aerosol scheme.…”

Unexpected vertical structure of the Saharan Air Layer and giant dust particles during AER-D

“…This transition can be summarised as an uplift of the aerosol over the ocean above an altitude of 1-2 km, as opposed to a profile down to the surface over the continent, and it is not observed in winter. The formation of this elevated dust layer is also studied in Khan et al (2015), using WRF-Chem simulations at high resolution, together with the observations of the SAMUM-1 campaign: they highlighted the effect of orographic lifting and the interaction of the continental outflow with the sea breeze as key factors. Mortier et al (2016) have presented nearly a decade of lidar observations at M'Bour, Senegal, a site located on the western African coast at a low latitude (14 • N): their study also highlights a vertical distribution up to ∼ 5 km during summertime dust transport events.…”