Subdiurnal to Interannual Frequency Analysis of Observed and Modeled Reflected Shortwave Radiation From Earth

Estimates of global top-of-atmosphere radiation on monthly, seasonal, annual, and longer time-scales require estimates of the diurnal variability in both insolation and surface and atmospheric reflection. We compare Earth Polychromatic Imaging Camera (EPIC) and National Institute of Standards and Technology Advanced Radiometer (NISTAR) observations from the Deep Space Climate Observatory (DSCOVR) satellite with Clouds and Earth’s Radiant Energy System (CERES) hourly synoptic fluxes, which are diurnally filled through geostationary observations, and find that their power spectral density functions substantially agree, showing strong relative power at subdiurnal, diurnal, seasonal, and annual time-scales, and power growing from diurnal to seasonal time-scales. Frequency analysis of fluxes from several coupled model intercomparison project 5 model (CMIP5) and CMIP6 models shows that they distribute too much power over periods greater than 1 day but less than one year, indicating that a closer look is needed into how models achieve longer-term stability in reflected shortwave radiation. Model developers can consider using these datasets for time-varying energetic constraints, since tuning parameter choices will impact modeled planetary shortwave radiation across timescales ranging from subdiurnal to decadal. Plain-Language Summary The balance between net incoming solar and outgoing thermal energy exerts a strong influence on the Earth’s climate. The fraction of the incoming solar energy that is reflected back to space is called albedo. Even a slight change in albedo would dwarf the impacts of greenhouse gases, but direct observations indicate that on time-scales longer than a few years, it is remarkably stable and has been for decades. However, this albedo does fluctuate greatly at shorter time-scales, meaning that the underlying causes that ultimately shape albedo interact in a variety of ways to achieve this stability. Using novel data, we present three different observational estimates of how this reflected energy varies at these shorter time-scales, and they all substantially concur. However, a wide variety of climate models do not capture the observed variability, suggesting that further model developments is needed to better represent the underlying contributions to the Earth’s albedo and the causes for its stability to date.