Abstract/Summary

The Saharan Air Layer (SAL) is known as an elevated, well-mixed, warm, dry, frequently dusty layer and plays an important role in regional climate and dust transport. A new analysis of aircraft observations of haboob-driven dust events shows that although increased dustiness in the SAL is associated with drier conditions in the lower-SAL as expected, dustiness is also associated with increased moisture in the upper-SAL, likely originating from cold pool outflows driving the dust uplift. We assess the radiative effects of the observed dust and increased water vapor (WV). The observed WV in the upper-SAL affects the top-of-atmosphere (TOA) direct radiative effect (DRE), while lower-SAL WV affects the surface DRE and column atmospheric heating. TOA DRE is negative for dust-only, while including both the observed dust and WV reduces the magnitude of the negative TOA DRE by 17% (3.0±0.8 Wm-2) when AOD>0.6. The observed WV structure increases the magnitude of the negative surface DRE from dust by 8% (5.1±0.8 Wm-2) and increases atmospheric heating by 17% (8.0±0.6 Wm-2). These effects are driven by longwave (LW) radiation, whereby WV changes increase the positive TOA LW DRE, decrease the surface LW DRE and change the sign of LW atmospheric heating from negative to positive. WV leads to enhanced cooling in the moist upper-SAL and heating in the dry lower-SAL under dustier conditions. Increased WV in the SAL is consistent with other studies demonstrating increasing Saharan WV. This work demonstrates the importance of upper-SAL WV in determining the radiative effect of dust.