Radiative Effects of Saharan dust layer on the marine atmosphere

Millions of tonnes of dust annually are advected from Africa across the Atlantic Ocean. Very significant impacts on marine atmospheric structure develop including well-mixed Saharan aerosol layers (SAL) by mechanisms not fully understood. In this research, the marine structure was investigated using the UKMO LEM based on FENNEC aircraft-based observations. Surprisingly, FENNEC observations showed that water vapour had significantly elevated levels in all the dusty SAL layers. LEM simulations with offline radiative heating showed that water vapour radiative effects were dominant in influencing the marine atmospheric structure rather than dust. Well-mixed potential temperature layers are created in approximately eight hours most effectively by longwave radiation interacting with elevated water vapour of the SAL. Other factors had little or no impact, such as shortwave radiation or wind shear. Once formed, the layers could last for long periods of a week or more even with radiative interactions disabled. Differential CAPE/CIN analyses indicated stability was enhanced below the SAL and strong CAPE throughout the SAL. A new bin resolved hybrid Lagrangian-Eulerian 1-D transport model including 1st-order turbulence closure forced by ECMWF reanalysis was developed to estimate the levels of dust across the Atlantic. Transported dust was found to be most sensitive to fall velocity with strong dependence on kinematic viscosity, dust density, refractive index (through determining dust mass from observations) and somewhat on particle shape. Dust amounts predicted in the Caribbean were not enough to maintain well-mixed layers, however, water vapour radiative effects were, for the cases studied. This research found radiative heating rates of elevated water vapour, beyond background levels, was dominant in determining the impacted SAL marine potential temperature structures. This work will ensure water vapour is targeted as key to the SAL structures, which will be important for future work understanding dust transport and downwind weather impacts of SALs.