Mechanisms for the Seasonal Transition of Precipitation Organization in the Southeastern United States: Current and Future Climate

美国东南部(-------------------------------------------------------------------------The US)丰富的水资源来自多种降水形式，从与锋面天气系统有关的大范围降水到与邻近大西洋有关的海风对流，再到热带气旋、中尺度对流系统和下午的雷暴。这些降水类型在重要方面存在差异，包括它们的可预报性和对区域气候变化的敏感性，因此了解它们的行为和对美国东南部平均降水和降水变化的相对贡献是可取的。PIS将降水云系的简单特征发展为中尺度降水特征(MPF)，包括至少100公里长的连续降水区域，以及较小和寿命较短的孤立降水特征(IPF)。尽管分类方案很简单，但它已被证明对理解美国东南部降水的地理和时间行为是有效的。PIS先前研究的一个主要结果是，虽然MPF(包括锋面降水和中尺度对流系统)没有表现出强烈的季节性周期，但IPFS的季节周期的特点是在5月至6月期间出现相对突然的春季，并集中在佛罗里达州、墨西哥湾邻近的海岸线和大西洋海岸以北至哈特拉斯角。美国东南部的IPF降雨开始让人想起西非和东亚等季风区突然过渡到雨季，这些地区得到了比美国东南部更广泛的研究。本文利用观测数据和数值模式模拟相结合的方法探索了春季IPF的爆发。观测数据来自国家马赛克和多传感器定量降水估计(NMQ)数据集，这是一个从覆盖几乎整个美国大陆的NEXRAD雷达站网络构建的降水数据集。PIS开发了一种自动降水组织分类算法来识别MPF和IPF，他们将该算法应用于11年(2002-2012年)每小时的NMQ数据，以获取春季IPF爆发的统计数据。这些数据将与北美区域再分析一起使用，以评估对流可用势能和对流抑制等热力学因素以及环流变化(如北美副热带高压和该区域低空急流的季节性移动)对爆发的贡献。另一个目标是了解IPF季节性如何在气候变化中发生变化。PI假设IPF应在较温暖的气候中较早出现，因为相关的热力学条件可能会在今年较早的时候出现。利用天气研究和预报(WRF)模式研究了IPF对背景变暖的敏感性，该模式用于模拟给定边界气象条件的区域区域的天气和气候。模拟使用伪全球变暖(PGW)方法，其中气象条件来自业务气象分析，以模拟近年来有记录以来的季节性转变，然后使用边界条件重复模拟，在边界条件中添加未来和当前气候模拟之间的差异，以表示预期的气候变化。美国东南部地区降水类型的季节转换具有实用和科学意义，因为决策者依赖天气和气候预报进行农业规划，并为与降水有关的灾害和城市发展提供指导。私人投资机构通过各种场所，包括在公共图书馆的演讲，与从罗利到外银行的当地社区接触。这项计划的成果，部分透过以世界天气预报基金为基地的天气预报教学实验室，应用于私人督导员所教授的课程，该实验室既有教育用途，亦可提高学生在天气预报方面的成就。

-------------------------------------------------------------------------The Southeastern United States (SE US) owes its ample water resources to many forms of precipitation, from the large-scale precipitation associated with frontal weather systems to sea-breeze convection associated with the adjacent Atlantic, to tropical cyclones, mesoscale convective systems and afternoon thunderstorms. These precipitation types differ in important ways including their predictability and their sensitivity to changes in regional climate, thus an understanding of their behavior and relative contributions to mean precipitation and precipitation change over the SE US is desirable. The PIs have developed a simple characterization of precipitating cloud systems into mesoscale precipitation features (MPFs), which consist of a contiguous region of precipitation of at least 100 km length, and smaller and shorter-lived isolated precipitation features (IPFs). Despite the simplicity of the classification scheme, it has proven effective for understanding the geographical and temporal behavior of SE US precipitation. A key result of the PIs' previous research is that while MPFs (which include frontal precipitation and mesoscale convective systems) do not exhibit a strong seasonal cycle, the seasonal cycle of IPFs is characterized by a relatively abrupt spring onset occurring between May and June and concentrated over Florida, the adjacent coastline of the Gulf of Mexico, and the Atlantic coast as far north as Cape Hatteras. The onset of IPF rainfall over the SE US is somewhat reminiscent of the abrupt transition to the rainy season in monsoon regions such as West Africa and East Asia, which have been more extensively studied than the SE US. Work conducted here explores the springtime IPF onset using a combination of observational data and numerical model simulations. Observational data come from the National Mosaic and Multi-sensor Quantitative Precipitation Estimation (NMQ) dataset, a precipitation dataset constructed from the NEXRAD network of radar stations that covers nearly all of the continental US. The PIs have developed an automated precipitation organization classification algorithm to identify MPFs and IPFs, and they apply the algorithm to 11 years (2002-2012) of NMQ data on an hourly basis to capture statistics of the spring IPF onset. These data will be used in conjunction with the North American Regional Reanalysis to assess contributions to the onset from thermodynamic factors such as convective available potential energy and convective inhibition, and circulation changes such as the seasonal movement of the North American subtropical high (NASH) and low-level jets in the region. A further goal is to understand how IPF seasonality may change in a changing climate. The PIs hypothesize that the IPF onset should occur earlier in a warmer climate, as the relevant thermodynamic conditions would likely occur earlier in the year. Research on IPF sensitivity to background warming is conducted using the Weather Research and Forecasting (WRF) model, which is used to simulate weather and climate over a regional domain given meteorological conditions at the boundaries. The simulations use the pseudo global warming (PGW) method, in which meteorological conditions are taken from operational meteorological analysis to simulate the seasonal transition in recent years of record, and simulations are then repeated using boundary conditions to which differences between future and present-day climate simulations are added to represent anticipated climate change. The seasonal transition of precipitation types over the SE US is of interest for practical as well as scientific reasons, as decision makers rely on weather and climate forecasts for agricultural planning as well as guidance on precipitation-related hazards and urban development. The PIs engage with local communities from Raleigh to the Outer Banks through venues including presentations at public libraries. Results of the project are used in courses taught by the PIs, in part through a WRF-based teaching laboratory for weather forecasting, which serves an educational purpose and also enhances the success of students pursuing careers in weather prediction.