The Impact of Meteorology on Smoke and Low-level Clouds over the Southeast Atlantic

In this dissertation, we use radiosondes and satellite observation, reanalysis datasets, as well as radiative and trajectory models to document the relationship between the low-level clouds, smoke and meteorology over the southeast Atlantic. The southeast Atlantic presents a natural environment with one of the world’s largest marine low-level clouds, occurring along with the largest consumption of biomass fire over the adjacent southern African continent. This combination results in an extensive region of above-cloud biomass burning aerosols (predominantly smoke) over the marine low-level clouds, whereby the elevated smoke could lead to the stabilization of the lower troposphere, reduction of the cloud-top entrainment, and the build-up of water vapor within the boundary layer, which may eventually lead to increases in cloud fraction and decreases in cloud-top heights, in a process called semi-direct aerosol effect. The smokes are transported at a preferred altitude (∼750hPa – 550hPa) by a background easterly winds between July and October. During the same period, strong surface winds and ocean-influenced cold surface temperature characterize the meteorology within the boundary layer. The marine low-level cloud region is also associated with strong climatological subsidence above it, and cloud-top temperature inversion layer. The meteorological variations occurring above and below the low-level clouds are capable of influencing the cloud properties, and therefore may confound with the aerosol effects, making the separation of the aerosol and meteorological influences, on the low-level cloud, a very difficult challenge. We address this problem by identifying the dynamical and thermodynamical variations above the low-level clouds during the the peak aerosol months (July–October). Specifically, three areas are explored in this dissertation: the convolution of the dynamical and moisture effects with shortwave-absorbing aerosols over the low-level clouds; the role of the mid-tropospheric easterly-transporting system on both the elevated smoke and the low-level cloud environments; and the synoptic-scale sensitivity of the low-level clouds to a range of interacting meteorological conditions, with and without the presence of the elevated smoke. First, the analysis of the radiosondes at St. Helena Island, a small island located approximately 1800 km offshore of continental Africa (15.9◦S, 5.6◦W), shows the presence of mid-tropospheric moisture within the smoke layer, and above the low-level clouds. The smoke layer has previously been associated with hot and dry layer, perhaps in analogy to the Saharan air layer. The mid-tropospheric moisture over the south-east Atlantic has not been previously documented, and it occurs more than 70% of the time between September and October. During the same months, as the aerosol loading increases, the amount of the mid-tropospheric moisture also increases. The result of the radiative transfer calculations shows that the mid-tropospheric moisture-induced longwave cooling accounts for about 30% of the aerosol-induced shortwave warming, within the smoke layer. At the cloud top, increased mid-tropospheric moisture could increase cloud-top heights by enhancing the downwelling longwave radiation at the cloud top, and potentially reducing the turbulent mixing within the boundary layer, which may ultimately reduce the cloud fraction. This process essentially opposes the cloud-thickening semi-direct aerosol effect due to the elevated smoke alone. Dynamical changes occurring during the polluted condition over the southeast Atlantic include enhanced above-cloud warm horizontal temperature advection, and reduced subsidence. The flow is from the warmer adjacent continent air, and the warm advection directly above the marine low-level cloud is also capable of the increases in cloudiness through its influence on stability, that is separate from the stability associated with semi-direct aerosol effect. In addition, though a previous modeling study focussing on the southeast Atlantic has similarly identified the association between the elevated shortwave-absorbing aerosols (smoke) and the reduced subsidence, it remains unclear if the reduced subsidence is a response of the elevated aerosols alone, or an associated change in the above-cloud meteorological features, or both. Here, we show that during the September–October period, the southeast Atlantic is characterized by the Southern African Easterly Jet (AEJ-S), a region of maximum mid-tropospheric easterly winds responsible for the westward transport of both the aerosols and the moisture above the low-level clouds. The AEJ-S can be thought of as a southern counterpart of a better-known northern African easterly jet, and using the wealth of information available for the northern African easterly jet, we show that the AEJ-S is associated with induced upward motion directly below the jet, between 5◦S-15◦S. The AEJ-S-induced upward motion strengthens the large-scale ascent over the land, and therefore enhances the efficiency of trajectory-derived aerosol lifting emissions into the free-troposphere. Over ocean, the induced upward motion also reduces the large-scale subsidence over the marine low-level cloud. All else equal, the reduction of the offshore subsidence can be associated with reduction in nearby cloud fraction and increases in cloud-top heights, between 5◦S-15◦S. Between the effects of AEJ-S and the elevated aerosols on subsidence, the AEJ-S can be associated with 16% reduction in subsidence compared to 7% reduction by the elevated smoke-aerosols, between 5◦S-15◦S. By isolating the independent effects of selected above-cloud and surface-based meteorological influences during July–October period, it is shown that increases in lower tropospheric stability (LTS, defined as θ800hP a − θ1000hP a ), cold surface advection and warm 800 hPa horizontal temperature advection, all lead to increases in marine low-level cloudiness over the southeast Atlantic. In addition, the mid-tropospheric moisture is shown to independently result in reduction of cloud fraction, especially in the near- coastal region. Enhanced subsidence independently leads to increased cloudiness north of 20◦S, but decreased cloudiness south of 20◦S. The subsidence-induced reduction in cloudiness, south of 20◦S, contributes significantly to counteracting the stability-induced increases in cloudiness over the same region. When aerosol is present north of 20◦S, the effect of LTS on the low-level cloud is increased by about 13% over the region where the elevated aerosols are more likely separated from the underlying clouds, and by about 32% when the aerosols are more likely in contact with the underlying cloud layer. The results of this dissertation provide a detailed, useful meteorological context, highlighting new information, for the several multi-national upcoming field campaigns planned over the southeast Atlantic region. It also provides a useful understanding of the background dynamics and basic physical connections between the aerosols, the low-level clouds and the meteorology over the southeast Atlantic, that can be used in future studies involving aerosol-cloud-radiation interactions, and ultimately in numerical climate models to improve the estimation of climate sensitivity to subtropical low-level clouds