Cloud adjustments from large-scale smoke–circulation interactions affect marine cloud transitions
Diamond, Michael S — Florida State University
Area of research:
Our study combines insights from models at very different scales (regional climate and cloud-resolving) to distinguish between how smoke affects cloud transitions by changing the large-scale environment versus by seeding additional cloud particles and absorbing sunlight near the clouds.
Our discovery that the large-scale circulation effects from smoke strongly affect cloud transitions shows the utility of modeling systems at multiple scales. Although high-resolution models can better capture cloud-scale motions, these large-scale environmental changes would have been impossible to capture in a high-resolution/small-domain model alone.
Smoke from southern Africa blankets the southeast Atlantic from June to October, overlying a major low-altitude cloud transition region. The smoke affects Earth's radiation budget by absorbing and reflecting radiation directly, changing cloud properties via their effect on atmospheric temperature and circulation, and seeding additional cloud droplets when they get mixed into the marine boundary layer. Each of these aerosol radiative effects can be large in magnitude but often has offsetting cooling and warming influences. We investigate these smoke effects on cloud transitions in regional climate and large-eddy simulation (LES) models and compare our results with observations from three recent field campaigns: LASIC (Layered Atlantic Smoke Interactions with Clouds: U.S. Department of Energy Atmospheric Radiation Measurement [ARM[/Atmospheric System Research [ASR]), ORACLES (Observations of Aerosols above Clouds and their Interactions: National Aeronautics and Space Administration [NASA]), and CLARIFY (Cloud-Aerosol-Radiation Interaction and Forcing: UK Meteorological Office). In addition to the expected result of smoke absorption of sunlight increasing temperature in the smoke plume, most of the heating goes toward producing a decrease in large-scale subsidence. This subsidence effect is more important for cloud evolution than the effect of smoke particles serving as cloud condensation nuclei in our case. Such large-scale "semi-direct" effects of smoke absorption would not have been possible to simulate using a small-domain LES model alone.