Collaborative Research: Dynamic and Thermodynamic Mechanisms of Heat Extremes in the Northern Hemisphere

我们通常认为热浪只是令人不快或不便，但它们也可能是致命的。例如，1995年的热浪导致芝加哥600多人死亡，2003年的欧洲热浪夺走了5万多人的生命。热浪还会导致农作物歉收、野火、干旱和糟糕的空气质量，从而造成间接危害。在过去的几十年里，伴随着全球平均气温的上升，热浪频率急剧增加，随着气温的继续上升，预计还会进一步增加。气温升高也会导致更多的“同时热浪”，也就是同时热浪覆盖了夏季半球大部分的陆地表面。引发热浪的天气模式很容易识别，通常是停滞不前的高压系统伴随着晴朗的天空和下沉的空气。但需要更有洞察力的分析来理解这些天气模式的形成原因、它们保持多长时间，以及它们内部的最高温度受到地表干燥或其他局部影响的程度。另一个考虑因素是，热浪通常被认为是极端炎热天气的长期存在，但高湿度对人类健康的影响可能与高温一样大。与传统的热浪相比，湿热浪的气象学是相对未被开发的。这里进行的研究考虑了动力和热力过程对热浪的贡献，同时考虑了干湿事件。一个关键的考虑因素是干燥土壤延长和加强热浪的程度，这防止了陆地表面的蒸发冷却。另一个是不同的物理机制导致干燥和潮湿事件的程度，其中潮湿事件被定义为湿球温度(WBT)的极端。WBT是在给定环境温度和湿度的情况下，通过蒸发冷却可以达到的最低温度。它是人类热应激的一个很好的指标，因为我们依赖汗液的蒸发来避免高温下的热应激。其中一个分析工具是因果效应网络(CENS)，这是一种借用信息论和机器学习的图形技术，通过滞后回归建立因果关系。使用CENS的初步工作表明，初夏的热浪可能会导致降水减少，从而使土壤干燥，并促进额外的热浪。这项工作结合了对当前和预测未来气候的气候模型模拟的观测和输出，并设计了模型模拟来进一步测试和检查物理机制。鉴于上文提到的热浪的严重后果，这项工作具有实用和科学意义。通过东北城市气候风险联盟(CCRUN)与关注热浪影响的利益攸关方进行接触，其中一名研究人员发挥领导作用。与对气候影响感兴趣的学生的互动是通过首席研究员作为哥伦比亚大学授予气候与社会硕士学位计划的主任来实现的。该项目还涉及一个以本科生为主的机构(PUI)，并将结果纳入本科课堂教学。此外，该奖项还支持两名研究生。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。

We commonly think of heat waves as merely unpleasant or inconvenient, but they can also be deadly. For instance a 1995 heat wave resulted in over 600 deaths in Chicago, and the European heat wave of 2003 claimed more than 50,000 lives. Heat waves also cause indirect harm by contributing to crop failures, wildfires, droughts, and poor air quality. Heat wave frequency has increased dramatically over the past decades, accompanying the rise of global mean temperature, and further increases are expected as temperature continues to rise. Warmer temperatures also lead to more "concurrent heat waves", meaning simultaneous heat waves that cover a substantial fraction of the summer hemisphere land surface.Weather patterns leading to heat waves are easy to identify, commonly involving stagnant high-pressure systems accompanied by clear skies and subsiding air. But more perceptive analysis is required to understand why those weather patterns form, how long they remain in place, and the extent to which maximum temperatures within them are affected by the dryness of the surface or other local effects. A further consideration is that heat waves are typically identified as prolonged periods of extremely hot weather, but high humidity can matter as much as high temperature for human health effects. Compared to traditional heat waves the meteorology of humid heat waves is relatively unexplored.Research conducted here considers the contributions of dynamic and thermodynamic processes to heat waves considering both dry and humid events. A key consideration is the extent to which heat waves are prolonged and intensified by dry soil, which prevents evaporative cooling of the land surface. Another is the extent to which distinct physical mechanisms lead to dry and humid events, where humid events are defined as extremes of the wet bulb temperature (WBT). WBT is the lowest temperature that can be achieved through evaporative cooling given ambient temperature and moisture. It is a good indicator of human heat stress as we rely on evaporation of perspiration to avoid heat stress at high temperatures.One analysis tool is causal effect networks (CENs), a graphical technique borrowed from information theory and machine learning in which causality is established through lagged regression. Preliminary work using CENs suggests that an early summer heat wave can lead to reduced precipitation, which dries the soil and promotes additional heat waves. The work is performed using a combination of observations and output from climate model simulations of present-day and projected future climate, and model simulations are devised to further test and examine physical mechanisms.The work is of practical as well as scientific interest given the severe consequences of heat waves as noted above. Outreach to stakeholders concerned with heat wave impacts is performed through the Consortium for Climate Risk in the Urban Northeast (CCRUN), where one of the researchers has a leadership role. Interactions with students interested in climate impacts is enabled through the lead researcher's role as Director of the Masters degree granting Climate and Society program at Columbia University. The project also involves a Primarily Undergraduate Institution (PUI), and results are incorporated into undergraduate classroom teaching. In addition, two graduate students are supported through this award.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.