Lake-Effect Rainfall over Three East African Lakes

该项目旨在检查非洲三大湖（维多利亚湖、坦噶尼喀湖和图尔卡纳湖）的湖泊效应降雨。 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，维多利亚湖还为尼罗河的流量做出了贡献。 维多利亚湖呈圆形，表面积比坦噶尼喀湖和图尔卡纳湖大得多，这两个湖都是狭长的裂谷湖。 但是，尽管它们的大小、形状和环境有所不同，但这三个区域都经历了湖泊效应降雨，或者说湖泊降雨量相对于周围环境有所增强。 维多利亚州的湖泊效应尤其强烈，表现为夜间雷暴并伴有强风和巨浪。 雷暴对捕鱼船队造成严重危害，每年夺去三到五千人的生命。除了雷暴灾害之外，湖泊效应降雨也因其对湖泊水平衡的贡献而受到关注，湖泊水位下降导致附近居民生活困难。 很少有尝试估计湖面降雨对湖泊水位变化和趋势的贡献，并且现有的研究具有相当大的不确定性。该项目的目标是提供湖泊降雨状况的详细气候学，并研究调节这些状况的气象因素，特别关注湖面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，包括大小、昼夜周期、发生的季节性、年际变化以及与流域和/或周围土地降雨的关系； 2）建立湖面降雨气象控制； 3) 评估湖泊降雨状况的近期变化，这可能是由于人为变化（例如湖泊温度升高）造成的； 4) 确定这些变化是否与周围流域或土地的变化相称。虽然湖泊效应降水是研究的共同点，但初步结果表明，湖上降雨增强的机制可能因湖泊而异。 维多利亚湖面积较大，且位于地形洼地内，降雨量的增加可能是由于简单的陆风动力学造成的，即夜间山坡变凉但湖水仍然温暖（比湖岸温度高出 3.5 摄氏度）时，风会聚集在湖面上。 坦噶尼喀湖的降雨量增加不太明显，并且仅限于雨季，湖上有两个明显的最大值。 这些特征使 PI 推测湖上降雨主要是由大规模风和地形的相互作用控制的，而不是下坡风和陆风。 PI 进一步假设图尔卡纳湖的降雨量受到维多利亚湖周围形成的斜坡风和流过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的影响及其对气候变化的可能反应。 这项工作使用各种观测数据集进行，包括由 PI 和其他人编制的雨量计数据集、来自全球降水测量任务的卫星数据以及来自星载散射计的表面风。由于湖泊对东非的重要性、严重的湖上对流造成的危害以及湖泊效应机制与预测气候变化中湖泊水位未来演变的相关性，这项工作具有社会意义。 工作结果通过当地组织（例如 ACLENet 和尼罗河流域倡议）传达给利益相关者。 该项目还开发了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表面风的详细气候学。 该项目还为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智力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.