Abstract
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<p><span style="font-size: 12.000000pt; font-family: 'CMR12'">The Southeast Atlantic is the home to one of the largest semi-permanent stratocumulus decks on Earth, and the prevailing anticyclonic circulation over southern Africa transports the global majority of the shortwave absorbing smoke over the stratocumulus deck during the southern African biomass burning season. This dissertation studies the behavior of the low-level marine clouds over the remote southeast Atlantic, as a function of the light-absorbing smoke. The dissertation primarily relies on measurements gathered at the Ascension Island (8</span><span style="font-size: 8.000000pt; font-family: 'CMSY8'; vertical-align: 4.000000pt">◦ </span><span style="font-size: 12.000000pt; font-family: 'CMR12'">S, 15</span><span style="font-size: 8.000000pt; font-family: 'CMSY8'; vertical-align: 4.000000pt">◦ </span><span style="font-size: 12.000000pt; font-family: 'CMR12'">W) during the 2016 and 2017 southern African biomass burning seasons (July–October), as part of the Layered Atlantic Smoke Interactions with Clouds (LASIC) field campaign.
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<p>Field measurements at Ascension indicate that the highest amounts of sunlight- absorbing smoke occur annually within the marine boundary layer during August. Chapter 2 discusses a diurnal analysis of August data composited into the upper and lower terciles of near-surface black carbon mass concentrations. The diurnal mean liquid water paths and low-cloud fractions reduce when more smoke is present. This is consistent with previous studies highlighting a boundary layer semi-direct effect but represents its first documentation for this region. On more smoke-free days, the diurnal cycle in cloudiness includes a nighttime maximum in cloud water path and rain, an afternoon cloudiness minimum, and a secondary late-afternoon increase in cumulus occurrence and rain. The afternoon low-cloud fraction minimum is more pronounced on days with a smokier boundary layer. The cloud liquid water paths are most reduced, relative to cleaner conditions, at night. An exception to the reduction in cloudiness is found at mid-morning, when the boundary layer deepens and liquid water paths increase with increased smoke loading in the boundary layer. Precipitation frequency and intensity are nevertheless reduced across the full diurnal cycle when smoke is present.</p>
<p>The data support a novel view that a radiatively-enhanced decoupling persisting throughout the night is key to understanding the changes in the cloud diurnal cycle when more smoke is present. Under these conditions, the nighttime stratiform cloud layer does not always recouple to the sub-cloud layer, and the decoupling maintains more moisture within the sub-cloud layer. After sunrise, enhanced shortwave ab-sorption in a smokier boundary layer can drive a vertical ascent that momentarily couples the sub-cloud layer to the cloud layer, deepening the boundary layer and ventilating moisture throughout. After noon, shortwave absorption within smokier boundary layers again reduces the upper-level stratiform cloud and the sub-cloud relative humidity, discouraging further cumulus development and again strengthening a decoupling that lasts longer into the night.</p>
<p>Accompanying changes in synoptic-scale meteorological conditions can complicate the interpretation of the impacts of the aerosol perturbations. Easterlies and northeasterlies in the lower free-troposphere are more frequent when more smoke is present in August, consistent with a more direct westward transport of smoke from the continent. Cloud top inversions are weaker, and the wind shear is stronger across the cloud tops when more smoke is present. Turbulent mixing across the cloud-top inversion is hypothesized to contribute to the longevity of the boundary layer smoke episodes observed at Ascension during August. A detailed case study focuses on a 2017 mid-August smoke event that was also well sampled by complementing aircraft campaigns.</p>
<p>As a result of the seasonal shift in the strength and vertical location of the Southern African Easterly Jet, the vertical distribution of the biomass burning smoke arriving at Ascension shifts from predominantly in the boundary layer in July, to both in the boundary layer and free-troposphere in August, to predominately in the free-troposphere in September and October. Low-cloud observations over Ascension indicate an altered seasonal cycle in low-cloud cover and cloud types when more absorbing aerosol is present in the atmosphere. In such altered seasonal cycle, low-cloud fractions are reduced in July–August, with an increase in the occurrence of surface-forced cumuliform clouds and a decrease in the top-driven stratiform cloud occurrence, whereas the opposite is observed in October. I note that during September, the change in cloudiness is muted, and the trend in low-cloud type changes still follows that of July and August.</p>
<p>The free-troposphere is better-mixed when the aerosol optical depth (AOD) is higher during September. Enhanced water vapor mixing ratios often colocate with the elevated smoke layer. The less stable free-troposphere does not appear to be advected from the continent. Radiative transfer calculations of example profiles suggest, instead, that pronounced longwave cooling from water vapor during transit can help destabilize the free-troposphere by generating a negative buoyancy at the top of the layer.</p>
<p>When the atmosphere is more smoky during September, the marine boundary layer is clearly more humid and shallower with higher occurrence of surface-forced cumuliform clouds and enhanced precipitation in the afternoon. Weaker winds and reduced surface fluxes are observed under these conditions. At a larger scale, 800 hPa subsidence and surface pressure are reduced, and a cyclonic anomaly in the surface winds appears over the region, when more smoke is present. These changes in the large-scale features are consistent with the weaker winds and the reduced surface fluxes indicated by the local measurements, suggesting a possible connection between the smoke radiative influence and a change in the large-scale circulation.</p>
<p>The results of this dissertation provide a detailed depiction of the interactions between the smoke aerosol, marine boundary layer clouds, and the atmospheric thermodynamic structure, using observations collected over the remote southeast Atlantic during the southern African biomass burning seasons of 2016 and 2017. Useful insights are provided on the process-level understanding of the impacts of absorbing aerosols on the behaviors of low-level clouds that reside in a typically decoupled subtropical marine boundary layer. These process-level insights can serve as a simulation target phenomenology for models to develop confidence in simulating more challenging scenarios for which observations may not yet exist, and ultimately, in future climate projections.</p>