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

云是决定地球系统输入和输出能量平衡的关键组成部分，因为它们反射入射的阳光并吸收/发射热辐射能量。了解控制海洋上空云层的过程尤为重要，因为深色海洋表面吸收太阳能和热辐射能量，而白色海洋表面反射的太阳能多于其释放的热能，导致海洋表面可用能量的净减少。非洲海岸外的亚热带东南大西洋的大气条件有利于在大面积上空形成持久持续的低云。最近利用卫星观测的一项发现显示了一种云回滚现象，在这种现象中，低云可以沿着一条长度为数百至数千公里的大致南北方向的过渡线迅速消散。过渡线从非洲海岸开始，以10米/秒的速度向西移动。一个面积达100万平方公里的低云区域可能在一次回滚中消散。这些低云需要几个小时或更长时间才能重新形成。向西移动的云回滚在4 - 5 - 6月有明显的年度高峰，5月平均有20次回滚事件。这种类型的云量转换的性质难以解释，因为影响云的典型过程似乎不适用于这种回滚现象。该项目将使用观测和数值模拟相结合的方法彻底检查这些云回滚。框架假说认为，源自云甲板外的非局地机制，如来自邻近大陆的重力波和温带气旋的逼近，是这种低空海洋云突然中断的关键因素。在阴天，海洋表面吸收的太阳能大约是晴天时的一半。这些云的回滚类似于移除遮阳篷，使海洋暴露在更多的阳光下。海洋表面云量的突变对气候、海洋生态和生物地球化学都有影响。气候模式在东南大西洋上空的云量上显示出大而持久的误差，部分原因可能是无法捕捉到云层的这些快速回滚。该项目的更广泛影响还包括培训天气和全球大气动力学与气候研究方面的两名研究生和几名本科生。

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.