Abrupt cloud clearing of marine stratocumulus in the subtropical southeast Atlantic

Abstract

We document rapid and abrupt clearings of large portions of the subtropical marine low cloud deck that have implications for the global radiation balance and climate sensitivity. Over the southeast Atlantic, large areas of stratocumulus are quickly eroded, yielding partial or complete clearing along sharp transitions hundreds to thousands of kilometers in length that move westward at 8 to 12 meters per second and travel as far as 1000+ kilometers from the African coast. The westward-moving cloudiness reductions have an annual peak in occurrence in the period from April through June. The cloud erosion boundaries reduce cloud at ≈10-kilometer scale in less than 15 minutes, move approximately perpendicular to the mean flow, and are often accompanied by small-scale wave features. Observations suggest that the cloud erosion is caused by atmospheric gravity waves.

Stratocumulus cloud clearings: statistics from satellites, reanalysis models, and airborne measurements

“…The shallowing of the MBL near the coast of California is also notable with enhanced gradients on clearing days. The aforementioned MBL shallowing is believed to be a crucial element in development of the coastal jet off the coast of California (Zemba and Friehe, 1987;Parish, 2000). Previous studies (Beardsley et al, 1987;Edwards et al, 2001;Parish, 2000;Zuidema et al, 2009) also reported MBL height adjustment in the vicinity of the coast due to hydraulic adaptation to coastal topography, thermally driven circulation, and geostrophic adjustment in the cross-coast direction in response to the contrast in surface heating between ocean and land.…”

“…|v| was calculated by multiplying 40 km by the slope of the linear fit of v versus distance from cloud edge, where negative (positive) x values represent distance away from the edge on the clear (cloud) side. The results reveal that the horizontal wind shear was strongest somewhere between mid-cloud and This can be attributed to turbulent transport of the momentum (Zemba and Friehe, 1987) to the surface and the consequent drop in CLLJ wind speeds in the clear column. In addition, Fig.…”

“…Due to the dis- placement of the Pacific high towards the northeastern part of the study region on clearing days, these gradients are much more profound on clearing days as compared to non-clearing days. The zonal pressure gradient is the main parameter controlling the intensity and occurrence of Californian CLLJs (Zemba and Friehe, 1987;Parish, 2000;Lima et al, 2018). The probability of CLLJ incidents is most likely greater on clearing days as a response to the enhanced pressure gradients near the coast.…”

Cloud Cover in the Australian Region: Development and Validation of a Cloud Masking, Classification and Optical Depth Retrieval Algorithm for the Advanced Himawari Imager

“…As a result some originally cumulative cells (with narrower size spectrum) are classified as stratiform with wider size spectrum (Hess et al, 1998). Due to the critical effects of subtropical marine stratocumulus on Earth's energy balance (Wood, 2012;Yuter et al, 2018), satellite data, such as shown in sub-region R1, will play an important role in the still intensive researches in understanding their formation and dynamics.…”

Spatiotemporal dynamics of fog and low clouds in the Namib unveiled with ground and space-based observations

“…On seasonal scales, the spatiotemporal FLC occurrence indicates a connection to the southeastern Atlantic stratocumulus cloud deck. As such, process knowledge from studies on the heavily investigated stratocumulus clouds in this region (e.g., Adebiyi and Zuidema, 2018;Andersen and Cermak, 2015;Diamond et al, 2018;Fuchs et al, 2017Fuchs et al, , 2018Gordon et al, 2018;Painemal et al, 2014;Yuter et al, 2018) may be applicable to Namib-region FLC, and vice versa.…”