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

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.