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

云是决定地球系统传入和传出能量之间平衡的关键组成部分，因为它们反射传入的太阳光并吸收/发射热辐射能量。了解控制海洋上空云层的过程特别重要，因为黑暗的海洋表面吸收太阳能和热辐射能量，而白云反射的太阳能比其热能发射的更多，这导致海洋表面可用能量的净减少。非洲海岸外的副热带东南大西洋的大气条件有利于在大范围内形成持久的低云。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，低云可以沿着一条大约数百至数千公里长的南北方向的过渡线迅速消散。过渡线从非洲海岸开始，以大约10米/S的速度向西移动。一次回滚可能会使多达100万平方公里的低云区域消散。这些低云需要几个小时或更长的时间才能重生。向西移动的云回滚在4-5-6月有一个明显的年度高峰，5月份平均有20次回滚事件。这种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。这个项目将使用观测和数值模拟相结合的方法来彻底检查这些云层回滚。框架假设是来自云层外的非局地机制，如来自邻近大陆的重力波和温带气旋的逼近，是造成低层海洋云突然中断的关键因素。在多云的一天，海洋表面吸收的太阳能大约是晴朗时的一半。这些云层回滚类似于移除遮阳物，使海洋暴露在更多的阳光下。海洋表面云量的突然变化会对气候、海洋生态和生物地球化学产生影响。气候模型显示，东南大西洋上空的云层存在巨大而持久的误差，这可能部分是因为无法捕捉到云层中的这些快速回落。该项目的更广泛影响还包括对两名研究生和几名本科生进行天气和全球大气动力学及气候研究方面的培训。

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.