Saharan and Asian dust: similarities and differences determined by CALIPSO, AERONET, and a coupled climate-aerosol microphysical model

Abstract

Abstract. This study compares the properties of atmospheric dust from the Saharan deserts and the Asian deserts using data from CALIPSO and AERONET during 2006 and 2007 along with simulations using a coupled climatemicrophysical sectional model. Saharan deserts are largely south of 30 • N, while Asian ones are primarily north of 30 • N, hence they experience different meteorological regimes. Saharan dust lifting occurs all year long, primarily due to subtropical weather systems. However, Asian dust is lifted mostly in spring when mid-latitude frontal systems lead to high winds. Rainfall is more abundant over Asia during the dust lifting events, leading to greater local dust removal than over the Sahara. However, most dust removal is due to sedimentation. Despite the different meteorological regimes, the same dust lifting schemes work in models for Asian and Saharan dust. The magnitudes of dust lifted in Africa and Asia differ significantly over the year. In our model the yearly horizontal dust flux just downwind of the African dust source is about 1088 Tg (10 • S-40 • N, 10 • W) and from the Asian dust source it is about 355 Tg (25 • N-55 • N, 105 • E) in 2007, which is comparable to previous studies. We find the difference in dust flux is mainly due to the larger area over which dust is lifted in Africa than Asia. However, Africa also has stronger winds in some seasons. Once lifted, the Saharan dust layers generally move toward the west and descend in altitude from about 7 km to the surface over several days in the cases studied. Asian dust often has multiple layers (two layers in the cases studied) during transport largely to the east. One layer stays well above boundary layer during transport and shows little descent, while the other, lower, layer descends with time. This observation contrasts with studies suggesting the descent of Correspondence to: L. Su (sul@colorado.edu) Saharan dust is due to sedimentation of the particles, and suggests instead it is dominated by meteorology. We find the size distributions of Asian and African dust are similar when the dust is lifted, but the mode size can differ and secondary size modes can develop probably due to differences in vertical wind velocities during transport. The single scattering albedo of African and Asian dust does differ, due primarily to the imaginary parts of the refractive indexes being different, which in turn is likely due to different dust composition. This study is a step towards a global understanding of dust and its properties.

Dust vertical profile impact on global radiative forcing estimation using a coupled chemical-transport–radiative-transfer model

“…5b. As presented by previous study (Su and Toon, 2011), two evident dust layers with large values of dust VRAOD are seen at different altitudes. One is located over the Gobi desert (90-110 • E), with the high dust VRAOD extending from the surface to 800 hPa.…”

“…The maximum of dust VRAOD are consistent from surface to 600 hPa with the maximum of more than 0.015 (see Fig. 5a) since the Saharan dust source region is primarily located in a deep layer of SAL (Su and Toon, 2011). Moreover, the SAL shows a peculiar vertical profile characterized by a large aerosol concentration in the mid-troposphere, which is significantly different from the non-dust source regions.…”

“…Some of the general circulation models and RTMs simply assume a prescribed vertical weighting profile of extinction based on "climatology" that does not vary in space and time (Haywood et al, 1999;Gu et al, 2006;McComiskey et al, 2008). However, dust vertical distributions can be modified by numerous dynamical processes and can be quite different over dust source regions and downwind areas as documented by a number of previous studies (e.g., Karyampudi et al, 1999;Su and Toon, 2011). Recently, though many current general circulation models with an online calculation of radiative effects for dust and other aerosols have already include horizontal and vertical aerosol distribution computed by the model (Koffi et al, 2012;Myhre et al, 2013), the results are quite sensitive to the performance of dust simulations, especially the vertical distribution of dust.…”

“…The Low-level jets (LLJs) and cold pools of mesoscale convective systems also play important roles in affecting diurnal and seasonal characteristics of Saharan dust emission, dust devils and dusty plumes, as well as transport (Koch and Renno, 2005;Washington et al, 2006;Schepanski et al, 2009;Knippertz and Todd, 2012). In addition, the deep convection and strong winds embedded in the trades (northeasterly flow) would result in Saharan dust lifting occurring all year long (Su and Toon, 2011). The Taklimakan and Gobi desert are the major constitutors of the Asian dust belt (Uno et al, 2005).…”

“…The Taklimakan and Gobi desert are the major constitutors of the Asian dust belt (Uno et al, 2005). The seasonal variation of dust outbreaks in Asia is associated with the seasonal modulation of the wind speed (Su and Toon, 2011). The soil conditions and mid-latitude cyclones accompanied by strong surface winds are related to the occurrences of dust storms over the Asian source region (Zhao and Zhao, 2006).…”