Lake-Effect Rainfall over Three East African Lakes

这个项目是对三个非洲五大湖：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。维多利亚湖是圆形的，比坦噶尼喀湖和图尔卡纳湖面积大得多，这两个湖都是狭长的裂谷湖泊。但是，尽管它们在大小、形状和环境方面存在差异，但这三个湖都经历了湖泊效应降雨，即相对于周围环境，湖泊上方的降雨量增加。维多利亚州的湖泊效应尤其强烈，它以夜间雷暴的形式出现，并伴随着强风和巨浪。雷暴给捕鱼船队造成了严重的危险，每年夺走3000到5000人的生命。除了雷暴危险外，湖泊效应雨还因其对湖泊水量平衡的贡献而备受关注，这些湖泊的水位不断下降，导致附近居民生活困难。目前很少有人试图估计湖上降水对湖面水位变化和趋势的贡献，现有的研究也存在相当大的不确定性。该项目的目标是提供湖泊上降雨状况的详细气候学，并研究调节这些状况的气象因素，特别是湖上降水与区域尺度风之间的关系。具体目标是：1)开发关于湖上降雨的定量统计，包括降雨量、日循环、发生的季节性、年际变化以及与流域和/或周围陆地降雨量的关系；2)建立对湖上降雨的气象控制；3)评估湖泊上降雨状况的近期变化，可能是湖温升高等人为变化的结果；以及4)确定这些变化是否与周围集水区或土地的变化相适应。尽管湖泊效应降水是研究的共同主线，但初步结果表明，不同湖泊的降雨增加机制可能不同。维多利亚湖有很大的表面积，位于地形凹陷中，降雨增加可能源于简单的陆风动力学，即当山坡凉爽但湖水仍然温暖(比岸边高出3.5摄氏度)时，夜间湖面上的风汇聚在湖上。坦噶尼喀湖上的降雨增强不那么明显，仅限于雨季，湖面上有两个明显的峰值。这些特征导致PI假设，湖上降水主要受大尺度风和地形的相互作用控制，而不是下坡风和陆风的控制。PI进一步假设，图尔卡纳湖的降雨量既受到维多利亚湖周围发展的斜风的影响，也受到流经图尔卡纳空隙的强大低空急流的影响。作为该项目的一部分，研究了这些机制对湖上降雨量的可变性及其对气候变化的可能反应的影响。这项工作使用了各种观测数据集，包括由PI和其他机构汇编的雨量计数据集，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，强烈的湖面对流造成的危险，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。通过地方组织，例如ACLENet和尼罗河流域倡议，将工作成果传达给利益攸关方。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地表风的详细气候。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优势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.