Decreasing Rainfall to Year 2100 - Role of the Congo Air Boundary (DRY-CAB)

气候变化预计将给地球上许多地区带来相当大的负担，包括半干旱地区，而降雨对沉降农业、水电和生计至关重要。预计到21世纪末将出现两个大的陆地干旱趋势，一个在地中海盆地，另一个在南部非洲。这项建议的重点是南部非洲。初夏时节，非洲热带雨带的南部边缘从刚果盆地向南移动，带来了深对流云，取代了非洲南部亚热带干燥冬季的晴朗天空和稳定大气。气候模型模拟了未来几周降雨开始的延迟和南部非洲初夏30%的干旱。预计安哥拉东南部、赞比亚西部、纳米比亚东北部和南非内陆大部分地区将出现干旱。有几个问题与预计的南部非洲干旱密切相关，需要紧急关注，以便在对变化出现的信心的同时，及时采取适应措施。首先，大气动力学发生在观测极为稀少的区域，特别是在大气层的低几公里处。其次，与稀疏的观测有关，与有人居住的热带和亚热带其他地区相比，我们对动力学的掌握非常差。尽管是第三大热带对流区域，我们对刚果盆地气候系统的了解充其量只是初步的。刚果盆地如何与南亚热带气候相连并可能控制南亚热带气候尚不清楚。与此同时，气候变化正在发生。降雨系统的频率目前正在下降，预计在未来几十年内还会下降。这些系统在靠近刚果盆地南部边界的热带边缘究竟是如何发挥作用的还不得而知。第三，我们对初夏干燥动力学的了解完全取决于数值模型模拟。鉴于我们无法将数值模型与真实的世界进行比较，这种情况降低了在适应措施中应包含初夏干燥的信心。第四，虽然非洲上空的模式能力有了逐步的变化，例如通过作为IMPAL项目的一部分开发的4.5公里对流允许模式，但用来评估旧环流问题的新观点的观测还没有到位。这就推迟了对新的、新兴观点的采纳。在DRY-CAB项目结束时，我们将有办法评估我们用于气候预测的工具是否具有逼真的模拟控制初夏降雨向南发展的动力要素的保真度。这项建议是一项迫切需要的重要计划，通过一项雄心勃勃的观测活动和一套匹配的模型分析，重点关注初夏干燥动态。研究集中在刚果盆地南部边缘的热带深对流和南部干燥稳定的空气之间的界面。这一交界处就是刚果空中边界。我们提出了一个协调和灵活的国际实地计划，以获得CAB的观测迫切需要面对的数值模型，这是我们目前的理解的唯一来源。总体目标是为气候科学带来相应的信心，这是大胆的适应决策所必需的。这项工作发生在世界上两个地方之一，预计未来将发生广泛的干燥。我们期望从观测到的热带雨带边缘演变的动态中获得的见解将在热带其他地区具有价值。

Climate change is expected to impose a considerable burden on many regions of the planet including semi-arid areas and where rainfall is vital for subsidence agriculture, hydropower and livelihoods. There are two large, land-based projected drying trends towards the end of the 21st century, one over the Mediterranean basin and the other over southern Africa. This proposal focuses on southern Africa. In early summer, the southern edge of the African tropical rain belt moves southwards from the Congo Basin bringing with it deep convective clouds which displace the clear skies and stable atmosphere that dominates the dry southern subtropical African winter. Climate models simulate a future delay in rainfall onset of several weeks and an associated drying in the early summer months of 30% over southern Africa. The drying is expected to occur over south eastern Angola, western Zambia, north east Namibia and much of the South African interior. There are several problems bound up with the projected southern African drying that require urgent attention so that confidence in the emergence of the change can be matched with timely adaptation measures. First, the atmospheric dynamics take place in a region with extremely sparse observations, particularly in the lower few kilometres of the atmosphere. Secondly, related to the sparse observations, our grasp of the dynamics is very poor compared with other parts of the inhabited tropics and subtropics. Despite being the third largest region of tropical convection, our understanding of the Congo Basin climate system is, at best, rudimentary. How the Congo Basin connects with and potentially controls the climate of the southern subtropics is not known. Meanwhile, climate change is afoot. The frequency of rain-bearing systems is currently in decline and is projected to decline in future decades. Exactly how these systems function at their tropical edge near the Congo Basin southern boundary is unknown. Third, what we do know about the dynamics of early summer drying depends entirely on numerical model simulations. This situation reduces the confidence with which the early summer drying ought to be embraced in adaptation measures given that we have no means of comparing the numerical models with the real world. Fourth, while there has been a step-change in model capability over Africa, e.g. through the 4.5 km convection-permitting model developed as part of the IMPALA project, observations with which to assess the new view of an old circulation problem, are not in place. This delays the adoption of novel, emerging views. At the end of the DRY-CAB project we will have the means to assess whether the tools we use for climate projection have the fidelity to simulate realistically the dynamical elements that control the progression of rainfall southwards in early summer. This proposal is an urgently needed and vital programme that focuses on the early summer drying dynamics through an ambitious observational campaign and matching suite of model analyses. The research concentrates on the interface between the tropical deep convection at the southern edge of the Congo Basin and the dry, stable air to the south. This juncture is the Congo Air Boundary (CAB). We propose a coordinated and flexible international field programme in order to gain the observations of the CAB urgently needed to confront the numerical models which are the sole source of our current understanding. The overall goal is to bring the matching confidence to the climate science that is required for bold adaptation decision making. The work takes place in one of two places in the world where widespread future drying is projected to occur. We expect that insights gained from the observed dynamics of the evolving edge of the tropical rain belt will be of value in other regions of the tropics.