Aircraft measurement campaigns have revealed that super coarse dust (diameter > 10 um) surprisingly accounts for approximately a quarter of aerosols by mass in the atmosphere. However, most global aerosol models either underestimate or do not include super coarse dust abundance. To address this problem, we use brittle fragmentation theory to develop a parameterization for the emitted dust size distribution that includes emission of super coarse dust. We implement this parameterization in the Community Earth System Model (CESM) and find that it brings the model in good agreement with measurements of super coarse dust close to dust source regions. However, the model still underestimates super coarse dust in dust outflow regions. Thus, we conclude that model underestimation of super coarse atmospheric dust is in part due to the underestimation of super coarse dust emission and likely in part due to errors in deposition processes.

The Saharan Air Layer (SAL) has been shown to be an elevated, well-mixed, warm, dry, frequently dusty layer. The structure of the SAL plays an important role in regional climate and in long-range dust transport. A new analysis of aircraft observations 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. We assess the radiative effects of the observed dust and increased water vapor (WV) using a radiative transfer model. 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 11%. The observed WV structure increases the negative surface DRE from dust by 8% and increases atmospheric heating by 17%. These effects are driven by longwave (LW) radiation, whereby WV changes increase the positive TOA LW DRE by 30%, decrease the surface LW DRE by 52% and change the sign of LW atmospheric heating from negative to positive. The observed WV profile 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 a trend of increased WV over the Sahara. This work demonstrates the importance of the upper-SAL WV profile in determining the radiative effect dust.

Traditionally, the Saharan Air Layer (SAL) is defined as an elevated dry layer, frequently containing mineral dust transported from North Africa. The presence and characteristics of the SAL have an impact on tropical Atlantic climate and also on tropical cyclone development. However, recent observations from airborne campaigns in the Eastern Tropical Atlantic have found that under heavier dust loadings, water vapor content is increased in the SAL, rather than decreased as expected. This work will present airborne in-situ profile observations of dust loading and water vapor in the SAL from the AER-D field campaign during August 2015 in the tropical East Atlantic. The radiative impact in the shortwave and longwave spectra of the enhanced water vapor in the SAL will quantified and compared to that from mineral dust in the SAL. Trends in SAL water vapor over recent decades from satellite observations will be presented to assess the representativity of the aircraft data.