Impact of reflected shortwave anisotropy on satellite radiometer measurements of the Earth's energy imbalance

Abstract. The Earth's energy imbalance is the difference between incoming solar radiation and outgoing reflected and emitted radiation from the Earth, and quantifies the current ongoing accumulation of energy in the Earth's climate system. There are indications that the imbalance is growing, and it is important to be able to measure and monitor this quantity to better constrain future changes. The reflected shortwave component of the outgoing radiation depends on surface and atmospheric properties, which leads to strong directional variations associated with the angular geometry relative to the incoming sunlight. This means that a reflected shortwave radiance measurement at a specific point in space and time may differ by an order of magnitude between an assumed isotropic case and a case with more realistic anisotropic reflection. The effect of this anisotropy on global average measurements from wide-field-of-view radiometers has been the topic of some investigation in the past, and results from an earlier study suggest that this effect could potentially lead to substantial systematic biases in the context of the global mean reflected shortwave radiation. Here we simulate wide-field-of-view instruments on satellites in polar, sun-synchronous and precessing orbits, as well as constellations of these types of satellite orbits, with both Lambertian (isotropic) and anisotropic shortwave reflection. We find that both the estimated global annual mean and the estimated interannual trend only exhibit limited sensitivity to whether Lambertian or anisotropic reflection is assumed. With anisotropic reflection, the estimated global annual mean root-mean-square sampling error is at most 0.11 Wm⁻² provided that at least two complementary satellites are used, compared with at most 0.09 Wm⁻² in the case of Lambertian reflection. The magnitude of the difference in the estimated interannual trend is at most 0.07 Wm⁻² per decade, and typically only ∼0.01 Wm⁻² per decade. Analysis of the angular sampling of these satellites reveals that the anisotropic reflection requires sufficient sampling of viewing zenith angle and relative azimuth angle, in addition to the solar zenith angle. However, we conclude that it is possible to choose satellite orbits so that the sampling error is not substantially affected by reflected shortwave anisotropy.