Stratocumulus Cloud Clearings: Statistics from Satellites, Reanalysis Models, and Airborne Measurements

Over the decades of research on stratocumulus clouds conducted off of the western coast of the United States, one feature that has not received sufficient attention is large-scale clearings that are easily observed in satellite imagery and often exceed 100 km in width. The synergy of data from satellite remote sensors (the GOES imager and MODIS), MERRA-2 reanalysis products, and airborne in-situ measurements were used in this study to provide insights on clearing frequency and spatial distribution, diurnal characteristics, and environmental parameters influencing their evolution. Clearing episodes were analyzed for the summer months (June - August) between 2009 and 2018 for the spatial domain bordered by 115°-135° W and 30°-50° N. The relative percentage of total days in the summer season having clearings ranged from 15.2%-48.9% between 2009 and 2018 with a mean ± standard deviation of 33.3% ± 10.9%. August typically had the least number of events among the summer months (2-17), whereas July usually had the most (6-21), followed by June (4-16). Various dimensional characteristics of cloud clearings were examined. Using different MERRA-2 products, large-scale characteristics of a dynamic and thermodynamic nature were contrasted between clearing and non-clearing days. In both cases (clearing and non-clearing), the cross-coast gradient both in sea-level pressure and 850 hPa height are the highest in northern California and Oregon; however, due to the displacement of the Pacific high towards the northeast on clearing days, these gradients are much more profound in clearing cases as compared to non-clearing cases. This results in a more northeasterly flow (~2-5 m s^-1 increase in wind speed) at the 850 hPa level, which promotes off-shore flow in northern California. Moreover, a Gradient Boosted Regression Tree (GBRT) model approach was implemented to investigate the impact of environmental parameters on the evolution of clearing events. The results of this study have implications for modeling of fog and boundary layer clouds.