Collaborative Research: Mechanisms Governing Synoptic-scale, Rapid Cloud Dissipation in Subtropical Marine Low Clouds

Clouds are a key component in determining the balance between incoming and outgoing energy in the Earth system as they reflect incoming sunlight and absorb/emit thermal radiative energy. Understanding the processes that control the clouds over the ocean is particularly important because of the strong contrast between the dark ocean surface, which absorbs both solar and thermal radiative energy, and white clouds, which reflect more solar energy than their thermal energy emission leading to a net reduction of energy available to the ocean surface. The subtropical southeast Atlantic Ocean off the African coast has atmospheric conditions that are favorable for the development of long-lasting persistent low clouds over large areas. A recent discovery using satellite observations shows a cloud rollback phenomenon wherein low clouds can rapidly dissipate along a roughly north-south orientated transition line hundreds to thousands of kilometers in length. The transition line starts along the African coast and moves westward at about 10 m/s. An area of low clouds up to 1 million square kilometers may dissipate away in a single rollback. It takes several hours or longer for these low clouds to reform. The westward-moving cloud rollbacks have a distinct annual peak in April-May-June with an average of 20 rollback events during May. The nature of this type of cloudiness transition defies some explanation since typical processes that affect clouds do not appear to be applicable to this rollback phenomenon. This project will thoroughly examine these cloud rollbacks using a combination of observations and numerical modeling. The framing hypothesis is that the non-local mechanisms originating outside of the cloud deck, such as gravity waves originated from adjacent continent and the approaching of extratropical cyclones, are the key factors for such abrupt disruptions of low marine clouds.

On a cloudy the day, the solar energy absorbed at the ocean surface is roughly one-half what it is when the sky is clear. These cloud rollbacks are akin to removing a sun shade, exposing the ocean to substantially more sunlight. Abrupt changes in cloudiness over the ocean surface have implications for climate, marine ecology, and biogeochemistry. Climate models exhibit large and persistent errors in cloud cover over the southeast Atlantic, which may be in part due to incapable of capturing these rapid rollbacks in clouds. Broader impacts of the project also include training of two graduate students and several undergraduate students in synoptic and global atmospheric dynamics and climate research.