15 May 2019

The Effects of Surface Longwave Spectral Emissivity on Atmospheric Circulation and Convection over the Sahara and Sahel

Desert spectral emissivity matters.

Change of simulated rainfall climatology due to inclusion of surface spectral emissivity in Sahara and Sahel
Science

We improved the fidelity of surface-atmosphere longwave radiation coupling over Sahara and Sahel region in the climate model. We showed such improvement can affect simulated climate, regional and beyond. 

Impact

Current earth system models all used blackbody or graybody assumption for surface longwave radiation. We showed that what really matters is actually the spectral surface emissivity, not broadband surface emissivity.  Contradictory to traditional wisdom, we showed that, by just specifying realistic surface spectral emissivity in Sahara and Sahel alone, it can affect simulated regional climate and beyond. Thus, this study calls for a more faithful representation of surface-atmosphere longwave coupling.

Summary

This study quantifies the impact of the inclusion of realistic surface spectral emissivity in the Sahara and Sahel on the simulated local climate and beyond. The surface emissivity in these regions can be as low as 0.6-0.7 over the infrared window band while close to unity in other spectral bands, but such spectral dependence has been ignored in current climate models. Realistic surface spectral emissivities over the Sahara and Sahel are incorporated into the Community Earth System Model (CESM) version 1.1.1, while treatments of surface emissivity for the rest of the globe remain unchanged. Both the modified and standard CESM are then forced with prescribed climatological SSTs and fixed present-day forcings for 35-year simulations. The outputs from the last 30 years are analyzed. Compared to the standard CESM, the modified CESM has warmer surface air temperature, as well as a warmer and wetter planetary boundary layer over the Sahara and Sahel. The modified CESM thus favors more convection in these regions and has more convective rainfall, especially in the Sahara. The moisture convergence induced by such inclusion of surface spectral emissivity also contributes to the differences in simulated precipitation in the Sahel and the region south to it. Compared to observations, inclusion of surface spectral emissivity reduces surface temperature biases in the Sahara and precipitation biases in the Gulf of Guinea but exacerbates the wet biases in the Sahara. Such realistic representation of surface spectral emissivity can help unmask other factors contributing to regional biases in the original CESM.

Contact
Xianglei Huang
University of Michigan