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

Abstract

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-yr 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.

Direct impact of solar farm deployment on surface longwave radiation

“…This study provides some guidelines for how large-scale deployment of solar farms should be parameterized in the longwave radiation scheme of the climate models: (1) ideally the diurnally dependent LST differences between solar farm and adjacent land grids should be captured in such parameterization: solar farm as an entity, has a lower LST than surrounding land grids at noon but similar LST at midnight; (2) the solar farm surface emissivity should be different from surrounding land grids. While an overly dominant majority of climate models assume blackbody surface in their atmosphere model and broadband graybody surface in their land model, recent studies have shown the improvements when spectrally dependent surface emissivity was incorporated into the climate model, for global climate and for the regional climate over Sahara desert (Huang et al 2018, Chen et al 2019. Riverola et al (2018) suggests the c-Si solar cell has its emissivity as large as 0.75 over the atmospheric window region.…”

Sensitivity of Arctic Surface Temperature to Including a Comprehensive Ocean Interior Reflectance to the Ocean Surface Albedo Within the Fully Coupled CESM2

“…As in previous studies (Chen et al., 2019, 2020; Gu et al., 2021; Huang et al., 2018), we chose a relatively coarse atmospheric horizontal resolution of 1.9° latitude by 2.5° longitude for computational efficiency. Each run simulates the 35‐year period of monthly mean from 1980 to 2014, and the last 30 years of simulations (1985–2014) are analyzed, discarding the first 5 years as a model spin‐up.…”

Ground‐Based Far Infrared Emissivity Measurements Using the Absolute Radiance Interferometer

“…The obtained measurements supplement the few high spectral resolution measurements in the FIR that currently exist. Retrieved emissivities from this work are made publicly available and are compared to theoretical simulated emissivity estimates (e.g., Huang et al., 2016) for various physical materials, including ice and water, that are often used in climate model sensitivity studies (Chen et al., 2019; Huang et al., 2018).…”