An Analytic Theory of Near-Surface Relative Humidity Over Land

Prof. Kaighin McColl

Harvard University

Monday November 18, 2024, 2 PM ET

Abstract:

There is no simple explanation for the spatial structure of near-surface relative humidity (RH) over land, despite its importance for human health and ecosystem productivity. The explanation is not obvious: for example, why is the latitudinal profile of RH over land shaped like the letter ‘W’, when both specific humidity and saturation specific humidity essentially decline monotonically from the equator to the poles? I will present a simple theory for RH that shows it is largely controlled by the land surface, particularly soil moisture, and directly answers this question. The theory is then used to address a basic question: do land surfaces get wetter or drier, on average, as the planet warms? It is widely believed that land surfaces rapidly dry, on average. Yet, the same climate models used to project strong aridification show no obvious signs of drastic drying in relevant land surface variables, such as soil water storage or ecosystem productivity. This discrepancy – termed the “aridity paradox” – remains largely unresolved. I will show that the simple theory parsimoniously resolves the aridity paradox, and conclude that there is no reason to expect drier land surfaces, on average, as the planet warms.

Biosketch:

Kaighin McColl is an Assistant Professor at Harvard University, jointly appointed in the Department of Earth and Planetary Sciences and the School of Engineering and Applied Sciences. His group uses theory, models and observations to understand the terrestrial water cycle, and its connection to weather and climate over land. Prior to his appointment in 2018, he was a Ziff Environmental Fellow at Harvard University’s Center for the Environment. He received his Ph.D. from MIT in 2017, funded by an NSF Graduate Research Fellowship; and received bachelor’s degrees in environmental engineering and applied mathematics from the University of Melbourne in 2009. In 2023, he received a Sloan Research Fellowship in Earth System Science.

Webinar:

Event site: https://go.umd.edu/mccoll

Zoom Webinar: https://go.umd.edu/mccollwebinar

Zoom Meeting ID: 961 5278 5333
Zoom password: essic

US Toll: +13017158592
Global call-in numbers: https://umd.zoom.us/u/aMElEpvNu

For IT assistance:
Cazzy Medley: cazzy@umd.edu

Resources:

Seminar schedule & archive: https://go.umd.edu/essicseminar

Seminar Google calendar: https://go.umd.edu/essicseminarcalendar

Seminar recordings on Youtube: https://www.youtube.com/user/ESSICUMD