Shallow stratocumulus clouds created below the base of the Saharan Air Layer by the action of Saharan dust

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Shallow stratocumulus clouds created below the base of the Saharan Air Layer by the action of Saharan dust

Pavel Kishcha, Arlindo da Silva, Boris Starobinets, Pinhas Alpert

Pp 62-66


Previous studies showed that, over the global ocean, there is no noticeable hemispheric asymmetry in cloud fraction (CF). This cloud symmetry contributes to the balance in solar radiation reaching the sea surface in the Northern and Southern hemispheres. By contrast, over the tropical Atlantic, the CF hemispheric asymmetry takes place. The tropical Atlantic (30N – 30S) is frequently affected by Saharan dust intrusions. Based on MODIS cloud fraction data during a ten-year study period, we found that these dust intrusions contribute to significant cloud cover up to 0.8 – 0.9 along the Saharan Air Layer (SAL). The area of SAL with significant CF is characterized by limited precipitation, indicating that clouds along the SAL are not developed enough. Meteorological conditions below the temperature inversion at the SAL base include high atmospheric humidity and the presence of large amounts of settling dust particles together with marine aerosols. Being below the temperature inversion and acting as efficient CCN, Saharan dust particles coated with sea-salt contribute to the formation of shallow stratocumulus clouds. Significant cloud fraction under the base of SAL, together with clouds over the Atlantic Inter-tropical Convergence Zone, contributes to the 20% hemispheric CF asymmetry between the tropical North and South Atlantic. Saharan dust is also the major contributor to the pronounced hemispheric aerosol asymmetry over the tropical Atlantic, based on MERRAero aerosol reanalysis data. These two factors could lead to an imbalance in strong solar radiation (which reaches the sea surface between the tropical North and South Atlantic), affecting climate formation in the tropical Atlantic.

Keywords: Shallow stratocumulus clouds; Desert dust; Saharan air layer; Sahara; Atlantic Ocean