How radiative transfer assumptions shape the large-scale atmospheric circulation and its response to warming

Radiative transfer lies at the heart of Earth's climate system, governing the fundamental energy balance that drives atmospheric circulation and the hydrological cycle. Yet idealized climate models often use gray radiation schemes, which ignore the spectral nature of light. These schemes are easy to use and simple to understand, but this simplicity comes at a cost: gray radiation fundamentally distorts the large-scale atmospheric circulation and its response to climate change. 

Using an idealized aquaplanet GCM with a hierarchy of radiation schemes, I show that gray radiation produces a tropopause that is too low, a subtropical jet that is displaced equatorward, and a Hadley Cell that is too weak. Under warming, gray radiation underestimates tropical upper-tropospheric amplification and produces unrealistic changes in jet structure and Hadley Cell strength.

I then introduce the “Simple Spectral Model” (SSM), a radiation scheme which represents the spectral nature of greenhouse gas absorption using simple, analytic fits. This scheme is simple and easy to understand (like gray radiation), but faithfully represents the spectral nature of radiative transfer. I show that this scheme alleviates the significant circulation biases associated with gray radiation, and provides a more accurate picture of the response of the large-scale atmospheric circulation to warming. This work demonstrates that radiative transfer is not merely a "detail" in climate modeling, but that it fundamentally shapes the atmospheric circulation.