The Role of Water Vapor in Midlatitude Stormtracks and the Global Circulation

首席研究员和他的合作者最近对等熵环流的研究表明，中纬度地区向极地的物质输送同样分为对流层上层的“干分支”和“湿分支”，后者与温暖潮湿的亚热带空气的低空向极地流动有关，这些空气在对流层上层的风暴路径内上升。该项目将开发一个新的框架，以便在地方和全球尺度上更好地评估降水与中纬度天气系统之间的关系，并将评估全球环流如何受到不同气候条件的影响。它寻求更好地理解(i)是什么控制着这些干湿分支之间环流的分配，（ii）这种分配如何影响环流的能量和动量输送，（iii）在单个风暴的水平上有什么影响，以及（iv）未来这种情况如何由于自然气候变率或人为温室气体排放而发生变化。这里的方法是基于分析在干燥等熵面（即恒定势温面）或湿润等熵面（即恒定等效势温面）上的纬向平均环流。研究内容包括湿润过程在中纬度天气系统动力学中的作用的理论发展，斜压生命周期和全球大气的数值模拟，以及基于再分析数据集的当前环流综合研究。还将开发一套诊断工具，以评估数值模式中的等熵环流，并提供给更广泛的社区。拟议研究的更广泛影响在于，它有可能提高我们预测气候变化如何影响亚热带和中纬度天气，以及改变全球降雨分布的能力。政府间气候变化专门委员会（IPCC）使用的不同全球气候模型将被仔细评估，并与再分析数据进行比较。这既表明了改进这些模型的可能方法，也增加了我们对它们精确模拟气候系统和水文循环能力的信心。

A recent study of the isentropic circulations by the principal investigator and his collaborators has shown that the poleward mass transport in the mid-latitudes is equally split between a "dry branch" in the upper troposphere, and a "moist branch", associated with a low level poleward flow of warm, moist subtropical air that ascends in the upper troposphere within the stormtracks.This project will develop a new framework to better assess the relationship between precipitation and mid-latitude weather systems on both the local and global scales, and will evaluate how the global circulation is affected by different climatic conditions. It seeks a better understanding of (i) what controls the partitioning of the circulation between these dry and moist branches, (ii) how does this partitioning affect the energy and momentum transport by the circulation, (iii) what are the implications at the level of the individual storm, and (iv) how this might change in the future either as a result of natural climate variability or as a consequence of anthropogenic greenhouse gas emissions. The approach here is based on analyzing the zonal mean circulation averaged on either dry isentropes, i.e., surfaces of constant potential temperature, or moist isentropes, i.e., surfaces of constant equivalent potential temperature. The research includes theoretical development to assess the role of moist processes on the dynamics of mid-latitude weather systems, numerical simulations of baroclinic life-cycle and of the global atmosphere, and a comprehensive study of the current circulation based on reanalysis datasets. A set of diagnostic tools to assess the isentropic circulation in numerical models will also be developed and made available to the broader community.Broader impacts of the proposed research are in the potential to improve our ability to predict how climate change can affect subtropical and mid-latitude weather, and modify the global distribution of rainfall. Different global climate models used by the Intergovernmental Panel on Climate Change (IPCC) will be carefully evaluated and compared to reanalysis data. This should both indicate possible ways to improve these models, as well as increase our confidence in their ability to accurately simulate the climate system and the hydrological cycle.