Lake-Effect Rainfall over Three East African Lakes

这个项目是对非洲五大湖中的三个湖：维多利亚湖、坦噶尼喀湖和图尔卡纳湖的湖效应降雨的研究。 这些湖泊是东非的重要资源，为坦桑尼亚，肯尼亚和乌干达提供渔业，淡水和水力发电，维多利亚湖也为尼罗河的流动做出了贡献。 维多利亚湖呈圆形，比坦噶尼喀湖和图尔卡纳湖的表面积大得多，后者是狭长的裂谷湖。 但是，尽管它们的大小，形状和设置的差异，所有三个经验湖效应降雨，或降雨量增加了湖泊相对于其周围环境。 湖泊效应在维多利亚上空尤为强烈，在那里它以夜间雷暴的形式伴随着强风和巨浪。 雷暴对捕鱼船队造成了严重的危害，每年造成3000至5000人死亡。除了雷暴危害外，湖泊效应雨还因其对湖泊水平衡的贡献而受到关注，这些湖泊的水位下降导致附近人口的困难。 有几次尝试估计的贡献，在湖泊水位的变化和趋势，现有的研究有相当大的不确定性，这个项目的目标是提供一个详细的气候降雨制度的湖泊，并检查调节这些制度的气象因素，特别注意到湖泊降雨和区域尺度风之间的关系。 具体目标是：（1）开发湖上降雨的定量统计数据，包括量级、日周期、发生的季节性、年际变化以及与集水区和/或周围土地降雨的关系;（2）建立湖上降雨的气象控制; 3）评估湖泊降雨状况的近期变化，可能是人为变化的结果，如湖泊温度升高;虽然湖泊效应降水是研究的共同主线，但初步结果表明，湖上增雨的机制可能因湖泊而异。 在维多利亚湖上，由于它的表面积很大，并且位于地形凹陷中，降雨增强可能是由于简单的陆风动力学，即当山坡凉爽但湖泊仍然温暖时，风在夜间聚集在湖面上（比岸边高3.5摄氏度）。 坦噶尼喀湖的降水增强不太明显，仅限于雨季，湖上有两个明显的最大值。 这些特点导致PI假设，湖上降雨主要是由大尺度风和地形的相互作用，而不是下坡风和陆风控制。PI进一步假设图尔卡纳湖的降雨受到维多利亚湖周围斜坡风和流经图尔卡纳间隙的强烈低空急流的影响。作为该项目的一部分，研究了这些机制对湖上降雨变化及其对气候变化的可能反应的影响。 这项工作是使用各种观测数据集进行的，包括PI和其他人编制的雨量计数据集，全球降水测量使命的卫星数据，以及星载散射仪的地面风。这项工作具有社会意义，因为东非湖泊的重要性，严重的湖上对流造成的危害，以及湖泊效应机制在预测气候变化中湖泊水位未来演变方面的相关性。 通过地方组织，例如ACLENet和尼罗河流域倡议，向利益攸关方通报了工作成果。 该项目还开发研究人员和决策者感兴趣的数据集，包括高空间和时间分辨率的湖上降雨和地面风的详细气候学。 该项目还为一名研究生提供支持和培训。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This project is an examination of lake effect rainfall over three of the African Great Lakes: Lake Victoria, Lake Tanganyika, and Lake Turkana. These lakes are an essential resource for East Africa, providing fisheries, fresh water, and hydroelectric power to Tanzania, Kenya, and Uganda, and Lake Victoria also contributes to the flow of the Nile. Lake Victoria has a round shape and much larger surface area than Tanganyika and Turkana, which are long narrow rift valley lakes. But despite their differences in size, shape, and setting, all three experience lake effect rainfall, or an enhancement of rainfall over the lake relative to its surroundings. The lake effect is particularly intense over Victoria, where it takes the form of nighttime thunderstorms accompanied by strong winds and high waves. The thunderstorms create a severe hazard for the fishing fleet, claiming between three and five thousand lives every year. In addition to the thunderstorm hazard, lake effect rain is of interest for its contribution to the water balance of the lakes, which have suffered declining water levels resulting in hardship for nearby populations. There have been few attempts to estimate the contribution of over-lake rainfall to variability and trends in lake water level, and existing studies have considerable uncertainty.The goal of this project is to provide a detailed climatology of the rainfall regimes over the lakes and to examine the meteorological factors that modulate these regimes, with particular attention to the relationship between over-lake rainfall and regional-scale winds. Specific objectives are to 1) develop quantitative statistics on over-lake rainfall, including magnitude, diurnal cycle, seasonality of occurrence, interannual variability, and relationship to rainfall in the catchment and/or surrounding land; 2) establish the meteorological controls on over-lake rainfall; 3) assess recent changes in the rainfall regime over the lakes, perhaps as a result of anthropogenic changes such as increased lake temperature; and 4) determine whether the changes are commensurate with changes in the surrounding catchment or land.Although lake effect precipitation is the common thread in the research, preliminary results suggest that the mechanisms of over-lake rainfall enhancement may differ from one lake to another. Over Lake Victoria, with its large surface area and siting within a topographic depression, rainfall enhancement could result from simple land breeze dynamics, in which winds converge over the lake at night when the hillslopes cool but the lake is still warm (up to 3.5C warmer than its shores). Rainfall enhancement over Lake Tanganyika is less pronounced and is limited to the wet season, with two distinct maxima over the lake. These characteristics lead the PI to hypothesize that over-lake rainfall is controlled primarily by the interaction of large-scale wind and topography rather than downslope winds and land breezes. The PI further hypothesizes that rainfall over Lake Turkana is influenced by both the slope winds that develop around Lake Victoria and by the strong low-level jet that flows through the Turkana gap. The implications of these mechanisms for the variability of over-lake rainfall and its likely response to climate change is examined as part of the project. The work is conducted using a variety of observational datasets including a rain gauge dataset compiled by the PI and others, satellite data from the Global Precipitation Measurement Mission, and surface winds from spaceborne scatterometers.The work is of societal interest due to the importance of the lakes for East Africa, the hazards posed by severe over-lake convection, and the relevance of the lake effect mechanisms for anticipating the future evolution of lake levels in a changing climate. Results of the work are communicated to stakeholders through local organizations, for instance ACLENet and the Nile Basin Initiative. The project also develops datasets of interest to researchers and decision makers including detailed climatologies of over-lake rainfall and surface winds at high spatial and temporal resolution. The project also provides support and training for a graduate student.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.