The Impact of Meteorology on Aerosol-Cloud Relationship over the Southeast Atlantic Low-Level Clouds (Invited Presentation)

Biomass burning aerosols are episodically transported over the southeast Atlantic low-level clouds. The aerosols are typically carried westward from the adjacent continental Africa by a mid-tropospheric zonal wind that is part of a deep land-based anticyclonic system. Offshore, the presence of the elevated shortwave-absorbing aerosol is associated with an increase in the top-of-atmosphere albedo that can only be explained through a thickening and extension of the underlying low cloud deck. However, meteorological variability associated with the aerosol outflow may confound aerosol effects, so that accurate estimation of the aerosol-cloud relationship becomes a difficult challenge. In this study, we use satellite observations and reanalysis datasets over the southeast Atlantic, as well as high-resolution radiosonde data from St. Helena Island (15.9^oS, 5.6^oW), between September and October (2000-2011), to highlight the impact of the background large-scale dynamics and moisture associated with the aerosol outflow on the low-level clouds. The results show that increases in the aerosol outflow is associated with increases in the 750-500 hPa moisture content. Radiative transfer calculations also show that the net longwave cooling by the moisture reduces the aerosol-layer shortwave warming, by about 30%. In addition, composite analyses show that large-scale dynamical changes during offshore polluted conditions include warm 800 hPa temperature advection off of the continent that reinforces the lower tropospheric stability over the low cloud deck, and a stronger southern African easterly jet (AEJ-S). The AEJ-S, similar to the northern African easterly jet, is a mid-tropospheric zonal wind maximum at approximately 10^oS. Single-particle trajectory model identifies aerosol particles exiting the continent from satellite-derived fire sources do so mostly between 5-15^oS, with more than 60% transported westward, and about 37% recirculated back towards the continent. The AEJ-S is also associated with reduced subsidence above the low-level clouds, between 5-15^oS, so that the aerosol and cloud layers should be more distinctly separated from each other when the AEJ-S is stronger. Hence, the meteorological conditions associated with the aerosol outflow also indicate changes in the low-level clouds, that is separate from the aerosol effect.