Sampling the Earth's energy imbalance with the Earth Climate Observatory (ECO) constellation - insights regarding shortwave anisotropy

The Earth’s energy imbalance (EEI), i.e. the difference between incoming solar radiation and outgoing reflected and emitted radiation, is the one quantity that ultimately controls the evolution of our climate system. Despite its importance, the exact magnitude of the energy imbalance is not well known, and because it is a small net difference of about 1 Wm⁻² between two large fluxes (approximately 340 Wm⁻²), it is difficult to measure directly. There has recently been a renewed interest in using wide-field-of-view radiometers on board satellites to measure the outgoing radiation and hence deduce the global annual mean energy imbalance, for example as part of the EE12 candidate Earth Climate Observatory (ECO) mission.

A potential issue with wide-field-of-view radiometers, which has been the source of some concern, is the effect of anisotropic radiation, particularly anisotropic surface reflection of incoming sunlight. A wide-field-of-view radiometer does not distinguish the direction of incoming radiation, and earlier results have indicated that shortwave anisotropy could lead to substantial systematic biases in the global mean.

We simulate wide-field-of-view satellite measurements from satellites in polar, sun-synchronous and precessing orbits, as well as constellations of these orbits, and investigate how such measurements can be used to correctly determine the global annual mean imbalance. We present the results of ongoing work concerning different orbits, and how they affect the estimated global annual mean EEI, with a focus on e.g. the shortwave component and a comparison between isotropic and anisotropic shortwave reflection.