Collaborative Research: A Precipitation Dipole in Eastern North America: Issues of Space-Time Variability and Physical Mechanisms

合作研究：北美东部的降水偶极子：时空变率和物理机制问题

• 批准号: 0811099
• 负责人: Mathew Barlow
• 金额: $ 17.46万
• 依托单位: University of Massachusetts Lowell
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-08-01 至 2012-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0811099&HistoricalAwards=false
• 关键词: Collaborative; Research; Precipitation; Dipole; Eastern

Recent studies suggest that precipitation over Eastern North America (ENA) exhibits a dipole pattern of wet and dry conditions between the central United States and eastern Canada, with particular prominence at decadal timescales and considerable contribution to local trends ? but which is largely erased in the usual calculations of area-averaged trends. The associated trend toward increased Central US precipitation is not well reproduced in current models and is contrary to the usual expectations of drying in the continental interior with a warmer climate. Decreasing precipitation over eastern Canada is also contrary to model projections of generally wetter conditions nearer the coasts. Several fundamental questions have yet to be addressed: What are the underlying dynamics and key physical mechanisms? How closely is it related to large-scale climate variability? What processes are setting the decadal timescale?What are the intellectual merits of the proposed activity?The goals of this project are to determine the underlying dynamics of the dipole pattern and its connections to large-scale climate variability and trends. The three primary objectives of the proposed work are to: 1) determine the timescales, seasonality, and structure of the dipole pattern, 2) examine the dynamics of the precipitation changes in terms of the hydrologic budget, moisture transport, and storm track variability, and 3) use a hierarchy of models to investigate the influence of large-scale variability on the dipole mode. To investigate the underlying mechanisms, we will test a set of hypotheses on the local forcing of the precipitation via moisture flux and thermodynamically-forced vertical velocity, and on the large-scale controls on the regional circulation via both baroclinic and barotropic response to tropical convection.We will expand our observational data analyses to establish and clarify the link among precipitation variations, circulation anomalies, and boundary forcing that may create a precipitation dipole over ENA. We will then investigate the links to large-scale climate variability and tropical forcing. This is perhaps one of the first attempts to understand and characterize the existence of a precipitation dipole over ENA. Our methodology -- which builds on our ongoing and previous work -- was briefly tested and some preliminary results are presented in the proposal. We will conduct observational analysis of a range of hydrologic and atmospheric variables to determine the structure of the seasonal and spatial pattern. We will analyze the observed hydrologic budget, moisture transport, shifts in the storm tracks, and transient-mean flow interaction to investigate the internal dynamics of the dipole, using both simple compositing and pattern-based analysis techniques such as Principal Component Analysis. We will use multiple estimates of precipitation and atmospheric circulation to alleviate known data quality issues. For dynamical investigations, we will use a range of models of increasing complexity: a global barotropic (one-layer) model linearized about a zonally-varying mean flow, a tropical Gill-Matsuno model with generalized heating, and the NCAR Community Atmospheric Model, which we have modified to allow imposition of convective anomalies.What are the broader impacts of the proposed activity?This collaborative project between the Tufts University and University of Massachusetts builds on mutually synergistic expertise in water cycle research, atmospheric dynamics, and hydrology. This partnership will be further expanded through co-mentoring of a post doctoral fellow, PhD students and involvement of undergraduate and high school students through summer internships. The proposed research addresses several important scientific questions, including the dynamics of large-scale atmospheric influences on hydrology, mechanisms of hydrologic variability at decadal and longer timescales, and trend attribution over different regions of ENA. There is also considerable societal relevance: understanding these regional changes in precipitation and their long-term variations has important implications on how, and to what extent, long-term variations in precipitation over ENA can be predicted and managed. The results are also relevant to agriculture (e.g., winter wheat) and, potentially, to water trade with Canada and increased terrestrial carbon fluxes to stream.

最近的研究表明，北美东部（ENA）的降水在美国中部和加拿大东部之间表现出湿润和干燥条件的偶极模式，在十年时间尺度上特别突出，并对当地趋势有很大贡献？但这在通常的面积平均趋势计算中被很大程度上消除了。当前的模型并未很好地再现美国中部降水增加的相关趋势，并且与气候变暖时大陆内部干燥的通常预期相反。加拿大东部降水量的减少也与模型预测的靠近海岸的地区普遍湿润的情况相反。几个基本问​​题尚待解决：潜在的动力学和关键物理机制是什么？它与大规模气候变化的关系有多密切？哪些过程正在设定十年时间尺度？拟议活动的智力优点是什么？该项目的目标是确定偶极子模式的潜在动态及其与大规模气候变化和趋势的联系。拟议工作的三个主要目标是：1）确定偶极模式的时间尺度、季节性和结构，2）检查水文收支、水分输送和风暴路径变化方面降水变化的动态，3）使用模型层次结构来研究大规模变率对偶极模式的影响。为了研究潜在的机制，我们将测试一系列关于通过水分通量和热力学强迫垂直速度对降水的局部强迫的假设，以及通过对热带对流的斜压和正压响应对区域环流的大规模控制的假设。我们将扩大我们的观测数据分析，以建立和澄清降水变化、环流异常和边界之间的联系 强迫可能会在 ENA 上空产生降水偶极子。然后我们将研究大规模气候变化和热带强迫之间的联系。这可能是理解和表征 ENA 上空降水偶极子存在的首次尝试之一。我们的方法——建立在我们正在进行的和以前的工作的基础上——经过了简短的测试，并在提案中提出了一些初步结果。我们将对一系列水文和大气变量进行观测分析，以确定季节和空间格局的结构。我们将使用简单的合成和基于模式的分析技术（例如主成分分析）来分析观测到的水文预算、水分输送、风暴路径的变化以及瞬态平均流相互作用，以研究偶极子的内部动力学。我们将使用降水和大气环流的多重估计来缓解已知的数据质量问题。对于动力学研究，我们将使用一系列日益复杂的模型：关于区域变化的平均流量线性化的全球正压（单层）模型、具有广义加热的热带吉尔-松野模型以及 NCAR 社区大气模型，我们已对其进行修改以允许施加对流异常。拟议活动的更广泛影响是什么？塔夫茨大学和塔夫茨大学之间的这个合作项目 马萨诸塞州建立在水循环研究、大气动力学和水文学方面相互协同的专业知识的基础上。这种伙伴关系将通过博士后研究员、博士生的共同指导以及本科生和高中生通过暑期实习的参与来进一步扩大。拟议的研究解决了几个重要的科学问题，包括大规模大气对水文影响的动态、十年和更长时间尺度的水文变化机制，以及 ENA 不同区域的趋势归因。还有相当大的社会意义：了解降水的这些区域变化及其长期变化对于如何以及在何种程度上预测和管理 ENA 降水的长期变化具有重要意义。研究结果还与农业（例如冬小麦）有关，并有可能与加拿大的水贸易和增加的陆地碳通量有关。