Collaborative Research: Process Mechanics of Cloudiness Transitions in Subtropical Marine Boundary Layers

云是气候系统的关键组成部分之一，因为它们影响到达地球表面的太阳量和地球辐射回太空的能量。地球上最突出的云是海洋上空低层大气中的薄层云，统称为海洋层积云。气候系统对海洋层积云的覆盖特别敏感，因为它们反射了几乎所有进入海洋表面的阳光，否则这些阳光就会到达海洋表面并使其变暖。随着中纬度的海洋层积云向南向热带移动，它们下面的海洋表面变暖，它们经历了导致云层破裂的结构转变。这一过渡的地点和细节对远洋能源收支有很大影响。虽然确定海洋层积云何时以及如何演变为更破碎状态的一些物理机制是已知的，但具有社会重要性的天气预报和气候模型并不能准确地再现这些云的结构转变。这主要是因为产生这些转变的物理过程的复杂组合还没有被完全理解。北大西洋东部(ENA)的海洋层积云过渡特别重要，因为该地区的云量在过去30年里有所减少，而ENA位于迅速变暖的北极的下游。此外，众所周知，随着北极冰盖面积的减少，该地区的海洋环流对融化水的输入非常敏感。为了更深入地了解海洋层积云转换的过程和相互作用，本研究的重点是调查和了解不同转换阶段驱动力的变化平衡。这些阶段包括单层层积云的斑块、积云的斑块以及包含层积云和积云共存的混合区之间的过渡，这种结构通常被称为“耦合的”层积云。积云耦合层积云存在两种构型：一种是小而随机的积云上升为层积云，另一种是积云表现出覆盖数十公里的广泛的中尺度组织，这种构型被称为海洋边界层对流复合体(MBLCC)。该项目利用了现代高分辨率计算机模拟和新开发的因果框架，在该框架中，层积云转换的关键驱动因素将被用来建立因果网络，以说明潜在过程和相互作用的路径。这个新的框架适用于非线性系统，重要的是，它允许多个变量协同工作，而不仅仅是专注于变量的独立影响。这个项目的研究成果有望解决目前在所有类型的模式中海洋平流层积云的模式表示方面的不足，并为诊断其他大气系统中的相互作用提供新的咒语。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Clouds are one of the key components of the climate system because they influence the amount of sunlight that reaches the Earth’s surface and the amount of energy that the Earth radiates back to space. The most prominent clouds on the planet are thin layered clouds in the lower atmosphere over the oceans, which are known collectively as marine stratocumulus. The climate system is particularly sensitive to the coverage of marine stratocumulus clouds because they reflect almost all the incoming sunlight that would otherwise reach and warm the ocean surface. As marine stratocumuli in the mid-latitudes move south toward the tropics, the ocean surface beneath them warms and they undergo a structural transition that leads to breaks in the clouds. The location, and the details of this transition significantly impact the over-ocean energy budget. While some of the physical mechanisms that determine when and how the marine stratocumulus evolves into a more broken state are known, weather forecast and climate models, which are of societal importance, do not accurately reproduce these cloud structural transitions. This is primarily because the complicated combination of physical processes that produce these transitions are not fully understood. Marine stratocumulus transitions over the Eastern North Atlantic (ENA) are of particular importance because cloud cover over this region has decreased over the past 30-years and the ENA lies downstream of a rapidly warming Arctic. In addition, ocean circulations in the region are known to be sensitive to the input of meltwater as Arctic ice coverage declines. To facilitate a deeper understanding of processes and interactions responsible for marine stratocumulus transitions, the focus of this research is to investigate and understand the shifting balance of driving forces in various transition stages. These stages include transitions between patches of single layer stratocumulus, patches of cumulus, and hybrid regions containing coexisting stratocumulus and cumulus, a structure often referred to as “cumulus-coupled” stratocumulus. Cumulus-coupled stratocumulus present in two configurations: one in which small, random cumulus rise into stratocumulus and another in which the cumulus exhibits extensive mesoscale organization covering tens of kilometers, a configuration referred to as Marine Boundary Layer Convective Complexes (MBLCC). This project capitalizes on modern high-resolution computer simulations and a newly developed causal framework in which key drivers for stratocumulus cloud transitions will be used to build causal webs, illustrating the pathways of underlying processes and interactions. This new framework works for nonlinear systems, and importantly, allows multiple variables to work in concert, beyond concentrating only on independent influences of variables. Research results from this project are expected to address current shortcomings in model representations of marine stratocumulus in models of all types and to provide a new mantra for diagnosing interactions in other atmospheric systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.