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

这是一项自20世纪80年代中期以来五大湖地表温度变暖趋势的研究。在此期间，五大湖的地表温度在所有季节都呈上升趋势，尤其是在夏季，休伦湖和密歇根湖的夏季地表温度上升了4摄氏度。这一升温速度超过每十年1.3摄氏度，比同期区域地表气温趋势高出10倍。湖泊变暖有许多后果，包括由于蒸发导致的湖泊水位下降，与降雨变化相比，蒸发在历史上一直是二阶效应，但最近由于温度升高，在量级上已变得相当。关于快速变暖的普遍假设是，冬季冰盖的减少导致春季分层提前和整个夏季供暖日数的增加，从而解释了暖季温度的大趋势。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测到的变暖。在这个项目中进行的研究验证了一个假设，即温度升高被雾和低云覆盖的反馈放大，其中湖泊温度升高导致雾和低云减少，这反过来又使湖泊表面暴露于增加的日照。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来检验。本研究的预期结果是：1)确定导致五大湖观测到的水面温度升高的各种因素；2)估算未来水面温度的升高；3)评估蒸发增加导致的当前和未来湖泊水位下降；4)全球变暖引起的五大湖行为变化对区域气候影响的识别。这项工作将产生更广泛的影响，而不仅仅是解决科学问题。湖泊温度升高对五大湖的生态系统和渔业造成了许多不利的后果，包括过度的初级生产、藻华以及由于过量有机物分解而导致的缺氧。这些影响与“死区”和鱼类死亡有关。主要研究人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（包括沿海社区，水资源和冷水鱼工作组）分享他们的发现。此外，该项目还支持和培养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支持。

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.