Impacts of High-Resolution Coupling of Solar Radiation Between Atmospheric and Cryospheric Components in Earth System Models

Earth system models (ESMs) often exchange solar fluxes and albedos between components using only two spectral bands (visible (VIS) and near-infrared (NIR)). In an effort to predict the albedo of cryospheric surfaces, which varies significantly through the NIR region, models often attempt to repartition these spectrally coarse incident solar fluxes into higher resolutions using prescribed, time-invariant weights. Here, we increase the resolution of solar fluxes and albedos exchanged between the atmosphere and snow-covered land surfaces within a fully coupled ESM from two bands to eight (one VIS and seven NIR bands). The exchange of higher resolution solar fluxes at the surface allows the surface models to dynamically weight the mean NIR albedo in response to time-varying atmospheric conditions. Diagnostic experiments within a fully coupled ESM show that the induced forcing on surface absorption caused by using the dynamic high resolution NIR insolation rather than prescribed weights ranges between -1.90-4.73 Wm^-2. This forcing is strongly modulated by atmospheric humidity, as the presence of water vapor absorbs NIR radiation, thus changing the spectral distribution of NIR radiation at the surface, which cannot be captured with fixed weights. We find low/high humidity generally increases/reduces surface absorption. Regional climate responses over snow-covered surfaces are consistent with the applied forcing both in sign and magnitude. Replacing the coarse two-band surface albedo with an eight-band albedo better captures the steep drop of snow reflectance at longer NIR wavelengths, reducing the solar warming rate in the lower atmosphere. These advances provide a foundation for implementing a high resolution, spectrally consistent coupling of solar radiative fluxes across components within ESMs, demonstrating that increasing the spectral resolution of radiative processes yields a more physically realistic representation of albedo, surface absorption, and atmospheric absorption.