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

云是决定地球系统中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和白色云层之间存在强烈的对比，前者吸收太阳能和热辐射能，而后者反射的太阳能多于其热能发射，导致海洋表面可用的能量净减少。非洲海岸外的亚热带东南大西洋的大气条件有利于大面积地区长时间持续低云的发展。最近利用卫星观测的一项发现显示了一种云回滚现象，其中低云可以沿着沿着一条长度为数百至数千公里的大致南北向的过渡线迅速消散。过渡线从沿着非洲海岸开始，以大约10米/秒的速度向西移动。一个面积达100万平方公里的低云可能会在一次回滚中消失。这些低云改革需要几个小时或更长的时间。向西移动的云回滚在4 - 5 - 6月有一个明显的年度峰值，5月份平均有20次回滚事件。这种类型的云量转变的性质无法解释，因为影响云的典型过程似乎不适用于这种回滚现象。这个项目将彻底检查这些云回滚使用的观测和数值模拟相结合。框架假说认为，源自云面之外的非局部机制，如来自邻近大陆的重力波和热带气旋的接近，是造成这种海洋低云突然中断的关键因素。在多云的日子里，海洋表面吸收的太阳能大约是天空晴朗时的一半。这些云的回滚类似于移除遮阳板，使海洋暴露在更多的阳光下。海洋表面云量的突然变化对气候、海洋生态和海洋地球化学都有影响。气候模型在东南大西洋的云量上显示出巨大而持续的误差，这可能部分是由于无法捕捉到云层中的这些快速回滚。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.