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The direct measurement of Earth’s Energy Imbalance (EEI) from space is one of the most challenging earth observations for climate change research. EEI quantifies the fundamental rate of global heating in response to radiative forcings and feedbacks and is tightly linked to changes in hydrological cycle and the habitability of our planet. Current spaceborne radiometers measure the individual shortwave (Solar irradiance and Earth reflected solar irradiance) and longwave (Earth emitted infrared irradiance) components of Earth’s energy balance with unprecedented stability, but with calibration errors that are too large to determine the absolute magnitude of global mean EEI as the components’ residual. Best estimates of multi-year (2005-2018) EEI are derived from temporal changes in planetary heat content, predominantly ocean heat content, and amount to ∼0.8 Wm-2. To monitor EEI directly from space, we propose an independent approach based on accelerometry that measures non-gravitational accelerations experienced in orbit, such as induced by, e.g., radiation pressure, aerodynamic drag, and variations in spacecraft shape. Considering a spherical spacecraft that absorbs Earth’s and Sun’s outgoing radiation equally well across its surface area, the measured radial accelerations (in Earth radial direction) are proportional to the net radiative flux through the top of the atmosphere at the location of the spacecraft (Figure 3 in [1]). In the late 1970s, it was demonstrated that the accelerometer approach for measuring EEI is overall feasible. Since then, the capability of spaceborne accelerometers has much improved, which indicates it may be feasible to facilitate accuracy levels of 0.1% or 0.3 Wm-2. The proposed observing system suffers from external disturbances (confounding effects) that make the interpretation of radiation flux-induced accelerations challenging. Main confounding effects are aerodynamic drag and thermal thrust effects, for both of which there are potential ways of mitigation via spacecraft shape and spin considerations [1]. We are conducting a feasibility study using state-of-the-art mission design and orbit determination software to inform instrument and mission requirements relevant to the accurate measurement of radial acceleration induced by radiation pressure from Earth and Sun, and due to confounding forces. The approach requires spacecraft(s) of near-spherical shape and well-characterized surface properties to reduce confounding effects. In this context, the sensitivity of radiation-induced and confounding accelerations to variations in spacecraft shape, skin absorptivity, spin frequency, orbital parameters and attitude dynamics are presented and discussed. Reference [1] M. Z. Hakuba et al., "Earth’s Energy Imbalance Measured from Space," in IEEE Transactions on Geoscience and Remote Sensing, vol. 57, no. 1, pp. 32-45, Jan. 2019, doi: 10.1109/TGRS.2018.2851976.
Topic : Theme 1: Earth Energy Balance.
Reference : T1-C18
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