Downwelling Surface Solar Irradiance

Downwelling surface solar irradiance is the shortwave solar radiative flux reaching a horizontal surface from the sun and sky, measured in watts per square meter and integrated over the solar spectrum. It is a primary term in the ocean surface energy budget, driving the heating that sets sea-surface temperature, mixed-layer depth, and the light field available for photosynthesis. Satellite estimates derive it from geostationary and polar imagers by retrieving cloud and aerosol properties, and reanalyses such as ERA5 provide it on global grids. Clear-sky noon values reach roughly 1,000 W/m2 at the surface, falling sharply under cloud.