Processes determining stratospheric water vapour

大多数空气进入温度较低的热带对流层顶区域的平流层，通过冷冻干燥产生的脱水将水蒸气浓度降低到非常小的值。尽管平流层水蒸气的浓度很低，但由于它在平流层臭氧化学中的作用以及对对流层辐射平衡的影响，它在化学-气候系统中是重要的。模式对过去和未来变化的模拟依赖于对热带对流层顶区域温度分布的正确模拟，以及对空气包裹从对流层向平流层移动过程中所采取的路径进行采样。这方面的重要方面包括年循环和热带对流层顶温度的纵向变化，也许还有季节内和较短时间尺度的变化。该项目的合作伙伴将是剑桥大学和英国气象局。对于用于气候预测的英国气象局地球系统模型来说，改进对平流层水汽的模拟仍然是一个挑战。在平流层下层的水汽分布和对流层顶温度之间有很强的联系，对流层顶温度反过来又决定了水汽，因此可能存在正反馈，这可能会显著增强其他相关过程的模式表示中适度误差的影响。该项目将以剑桥大学和其他地方最近的工作为基础：(a)利用轨迹技术来检查对流层顶温度与平流层水汽之间的年度、年际和长期联系；(b)利用离线辐射计算和简单动力学模型的结合，研究了热带对流层顶区域水汽和温度之间的辐射耦合。该项目的重点将是分析由英国气象局统一模型（UM）模拟的月、年、年际和更长时间尺度上的水蒸气变化，并将这些变化与相应的温度和运输变化联系起来。（这种分析的一个组成部分将是因为已经为UM提供的轨迹代码。）这些结果将与对最近真实大气历史的相应分析进行比较（其中一些已经在科学出版物中记录在案）。在后期阶段，该项目将考虑热带对流层顶温度和太平洋水汽浓度之间的双向耦合，并评估对这些量的长期变化的模式预测可能产生的影响。在访问英国气象局期间，学生将在新的UKESM1地球系统模型的长期历史和情景模拟中研究这些过程，该模型将支持未来的气候和臭氧评估。学生项目的工作将为学生提供一个在科学领域做出贡献的机会，这既是英国气象局地球系统建模工作的基本兴趣，也是真正的实际兴趣。

Most air enters the stratosphere in the tropical tropopause region, where temperatures are low, and the resulting dehydration through freeze-drying reduces water vapour concentrations to very small values. Notwithstanding the very low concentrations, stratospheric water vapour is important in the chemistry-climate system through its role in stratospheric ozone chemistry and also through its effects on the radiative balance of the troposphere. Model simulation of past and future changes depend on correct simulation of both the temperature distribution in the tropical tropopause region and the pathways taken by air parcels as they sample this distribution in moving from troposphere to stratosphere. Important aspects of this include both the annual cycle and the longitudinal variation in tropical tropopause temperatures and perhaps variation on intraseasonal and shorter timescales.The co-operating partners in this project will be the University of Cambridge and the Met Office. Improving simulation of stratospheric water vapour remains a challenge for Met Office Earth System Models that are used for climate prediction. There are strong links between the water vapour distribution in the lower stratosphere and the tropopause temperatures which in turn determine water vapour, so positive feedbacks are possible that may significantly enhance the effects of modest errors in model representation of other relevant processes. The project will build on recent work in Cambridge and elsewhere that (a) has exploited trajectory techniques to examinethe annual, interannual and longer-term links between tropopause temperatures and stratospheric water vapour and (b) has investigated the radiative coupling between water vapour and temperatures in the tropical tropopause region using a combination of offline radiative calculations and simple dynamical models. The focus of the project will be to analyse the variations of water vapour on monthly, annual, interannual and longer timescales simulated by the Met Office Unified Model (UM) and link these to the corresponding temperature and transport variations. (One component of this analysis would beuse of a trajectory code which is already available for the UM.) The results will be compared against corresponding analysis of the recent history of the real atmosphere (some of which is already on record in scientific publications). In its later stages the project will consider the two-way coupling between tropical tropopause temperatures and water vapour concentrations in the UM and assess the possible implications for model predictions of long-term changes in these quantities. During visits to the Met Office the student will investigate these processes in long historical and scenario simulations of the new UKESM1 earth system model that will support future climate and ozone assessments. The work in the studentship project will provide an opportunity for the student to make a contribution in a scientific area that is both offundamental interest and of real practical interest to the Met Office earth-system modelling effort.