Collaborative Research: Variations and Trends in Fall Precipitation over the Central United States: Issues of Physical Mechanisms, Circulation Anomalies and Boundary Forcing

气候模型中有一个广泛的共识，即变暖的世界将导致美国大部分亚热带地区气候干燥。然而，观测证据表明，在过去几十年中，美国各地的总降水量和溪流流量都有所增加，其中美国中部的秋季增加幅度最大。在目前的气候模式中，一个明显的秋季干燥偏差可能会使观测到的降水趋势的起源的识别变得复杂。大多数耦合气候模型大大低估了密西西比盆地的降水在秋季，限制了我们的能力，巧妙地预测未来的降水变化或属性最近观察到的趋势，人为的起源。观察到的秋季降水趋势和气候模型中的干燥偏差之间的这种明显不一致促使主要研究人员（PI）更好地理解和确定产生秋季降水趋势和变化的主导机制。美国冬夏季大气环流和地面气候之间的关系一直是许多观测和模拟研究的主题。对秋季降水的关注相对较少，限制了我们对秋季气候时空变化和可预测性的认识。这项研究的一个关键目标是了解美国中部秋季降水的长期趋势和十年变化。为了实现这一目标，PI将集中在三个广泛的问题：（a）什么机制是重要的，以产生趋势和十年变化的秋季降水在整个美国中部，以及如何以及他们代表在当前一代的气候模式？(b)为什么这些机制在秋季特别占主导地位，而在其他季节却不占主导地位？(c)秋季降水的年代际变化与太平洋或大西洋海表温度（SST）之间的物理联系能否确定？PI将开始通过扩展他们正在进行的观测数据分析和文献中的现有结果，以进一步建立秋季降水变化，环流异常和边界强迫之间的关联联系。然后，他们将试图确定可能的物理机制，可以解释所观察到的相关性和关联性的联系。这项研究的一个主要成果将是更好地了解和确定主要的大气过程，负责秋季降水的空间连贯的趋势和十年变化，以及它们如何不同于其他季节。这项研究将解决有关的问题，在季节，年际和十年的时间尺度上的秋季降水变化和趋势的起源和性质。现有的研究大多只考虑冬、夏季降水的变化。在今后的任何气候变化评估中，例如政府间气候变化专门委员会（气专委）第四次评估报告中，都没有单独考虑秋季过渡季节。这项研究的结果将提供新的见解，解释为什么主要在美国中部的秋季观察到降水大量增加，以及为什么目前的气候模型无法捕捉到这一趋势。塔夫茨大学和哥伦比亚大学之间的这种合作伙伴关系建立在水循环研究，大气动力学和水文学的相互协同的专业知识。这种伙伴关系将通过博士生的共同建议和本科生通过暑期实习的参与得到进一步加强。研究所将整合这项研究的结果，开发一个关于美国降水变化的互动多媒体教育模块，作为我们的双层次（高年级本科生和一年级研究生）环境信号处理课程的三周教学工具。他们将在国家会议和档案期刊上介绍他们的研究结果，并以各种媒体形式发表他们的研究结果，以便记者、教师和公众随时可以查阅。

There is a broad consensus among climate models that a warming world will lead to a drier climate over most of the subtropical United States. Yet, observational evidence suggests that total precipitation and stream flow have increased across the United States over the last several decades with the largest increases generally observed in fall across the central United States. Identification of origins for the observed precipitation trends may be complicated by an apparent fall dry bias in current climate models. Most coupled climate models significantly underestimate precipitation over the Mississippi basin during fall, limiting our ability to skillfully predict future changes in precipitation or attribute the recently observed trends to anthropogenic origins. This apparent inconsistency between observed trends in fall precipitation and the dry bias in climate models motivates the Principal Investigators (PIs) to better understand and identify the dominant mechanisms that produce trends and variations in fall precipitation. The relationship between atmospheric circulations and surface climate over the United States in winter and summer has been the subject of many observational and modeling studies. Relatively little attention has been paid to fall precipitation, limiting our knowledge of the space-time variations and predictability of fall climate. A key goal of this research is to understand the long-term trend and the decadal variability of fall precipitation over the central United States. To achieve this goal, the PIs will focus on three broad questions: (a) What mechanisms are important to produce trends and decadal variations in fall precipitation across the central United States and how well are they represented in current generation climate models? (b) In what ways and why these mechanisms are particularly dominant in fall and not in other seasons? (c) Can the physical linkages between decadal variations in fall precipitation and Pacific or Atlantic Sea Surface Temperature (SST) be identified? The PIs will begin by expanding their ongoing observational data analyses and existing results from the literature to further establish the associational link among fall precipitation variations, circulation anomalies, and boundary forcing. Then, they will attempt to identify possible physical mechanisms that can explain the observed correlation and associational links. A main outcome of this research will be a better understanding and identification of the dominant atmospheric processes responsible for the spatially coherent trends and decadal variations in fall precipitation and how they differ from other seasons. This research would address questions related to origin and nature of fall precipitation variability and trends at the seasonal, inter-annual, and decadal time scales. Most of the existing studies consider precipitation variations in winter and summer seasons only. The fall transition season was not considered separately in any of the future climate change assessments, such as those by the Fourth Assessment Report of the Intergovernmental Panels on Climate Change (IPCC). Results from this research will provide new insight on why a large increase in precipitation is primarily observed in the central United States in fall and why current climate models are unable to capture this trend. This collaborative partnership between the Tufts University and Columbia University builds on mutually synergistic expertise in water cycle research, atmospheric dynamics, and hydrology. This partnership will be further strengthened through co-advising of PhD students and involvement of undergraduate students through summer internships. The PIs will integrate findings from this research to develop an interactive multimedia education module on Precipitation Variations over the United States to be used as a three-week teaching instrument for our dual-level (senior undergraduates and first year graduate students) course on Environmental Signal Processing. They will present their results in national conferences and archival journals and publish their findings in diverse media formats so they will be readily available to journalists, teachers and the general public.