Role of Low-Level Clouds in the Accelerated Warming of the Great Lakes - A Dual Observational and Regional Modeling Assessment

这是对20世纪80年代中期以来五大湖表面温度变暖趋势的研究。在此期间，五大湖的表面温度在所有季节都呈上升趋势，特别是在夏季，休伦湖和密歇根湖的夏季表面温度上升了4摄氏度。这一升温速度超过了每十年1.3摄氏度，比同期区域地表气温趋势高出10倍。湖泊变暖有一系列后果，包括由于蒸发导致的湖泊水位下降，与降雨量变化相比，这在历史上一直是二阶效应，但最近由于气温升高，其规模变得与之相当。关于快速变暖的普遍假设是，冬季冰盖减少导致春季层结提前，整个夏季取暖天数增加，从而解释了暖季气温的大趋势。但是，如果没有额外的放大机制，冰盖减少的影响可能不足以解释观察到的变暖。在这个项目中进行的研究测试了这样一个假设，即温度上升被涉及雾和低云量的反馈放大，在这种情况下，温暖的湖温导致雾和低云的减少，这反过来又使湖面暴露在更大的日照下。雾和低云变化的影响，以及其他变暖机制，将使用一套数值模拟模拟来研究。这项研究的预期结果是：1)确定导致所有五大湖观测到的水面温度上升的各种因素；2)估计未来的水面温度上升；3)评估蒸发增加将导致当前和未来的湖平面下降；4)确定全球变暖引起的五大湖行为变化对区域气候的影响。这项工作将产生超出所涉及的科学问题的更广泛的影响。湖泊温度变暖对五大湖生态系统和渔业造成了一些不可估量的后果，包括初级生产力过高、藻类大量繁殖，以及随之而来的随着过量有机物分解而导致的缺氧。这些影响与“死亡区”和鱼类死亡有关。首席调查员是威斯康星州气候变化影响倡议(WICCI)的积极成员，并承诺与WICCI的相关工作组分享他们的调查结果，包括沿海社区、水资源和冷水鱼类工作组。此外，该项目还支持和培训了两名研究生和一名博士后，从而为该领域的下一代研究人员提供了条件。

This is a study of the warming trends in surface temperatures of the Great Lakes since the mid-1980s. Great Lakes surface temperature trends have been upward during this period in all seasons, especially in summer, and the summer surface temperatures in lakes Huron and Michigan have increased by 4 degrees Celsius. This rate of warming, in excess of 1.3C per decade, is greater by a factor of 10 than regional surface air temperature trends over the same period. The warming of the lakes has a number of consequences, including a reduction in lake level due to evaporation, which historically has been a second-order effect compared to rainfall variations but has recently become comparable in magnitude due to the higher temperatures. The prevailing hypothesis for the rapid warming is that reduced winter ice cover leads to earlier springtime stratification and an increase in the number of heating days through the summer, thereby accounting for the large trend in warm season temperature. But the impact reduced ice cover may not be sufficient to account for the observed warming without additional amplifying mechanisms. Research conducted in this project tests the hypothesis that the temperature increase is amplified by feedbacks involving fog and low cloud cover, in which warmer lake temperatures lead to reductions in fog and low cloud, which in turn expose lake surfaces to increased insolation. The impact of changes in fog and low cloud, as well as other warming mechanisms, will be examined using a suite of numerical modeling simulations. Expected outcomes of the research are 1) the identification of various factors that are driving the observed water-surface temperature increases in all of the Great Lakes; 2) estimation of future water-surface temperature increases; 3) assessment of current and future lake-level decreases which will result from increased evaporation; 4) identification of regional climate impacts stemming from global warming induced changes in the Great Lakes' behavior.The work will have broader impacts beyond the scientific questions addressed. Warming lake temperatures have a number of undersirable consequences for the Great Lakes ecosystem and fisheries, including excessive primary production, algal blooms, and consequent hypoxia as excess organic matter decomposes. These effects are associated with "dead zones" and fish kills. The Principal Investigators are active members of the Wisconsin Initiative on Climate Change Impacts (WICCI) and pledge to share their findings with relevant working groups of WICCI including the Coastal Communities, Water Resources, and Coldwater Fish Working Groups. In addition, the project supports and trains two graduate students and a postdoc, thereby providing for the next generation of researchers in this area.