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A refined forecast for satellite displacements

A refined forecast for satellite displacements

3 May 2024

Radiation reflected from storm systems can change the trajectory of instruments orbiting Earth. Weather models can tell us by how much.

An illustration of two red, black, and yellow trapezoidal satellites hovering over the surface of Earth.
The GRACE-FO mission, composed of two satellites that orbit the planet together, uses precise measurements of the distance between its two crafts to calculate changes to Earth’s gravity. Credit: NASA/JPL-Caltech

Tropical storm Bongoyo traversed the Indian Ocean in December 2020 without much fanfare. Despite being classified as a severe tropical storm, the weather system didn’t cross any inhabited land and thus posed no threat. Hundreds of kilometers above, though, solar radiation reflected from Bongoyo’s clouds pushed the two Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) satellites slightly off course, about 20 μm away from Earth during their two minutes of overpass. It’s not much, but for a satellite that precisely records variations in Earth’s gravity, every force matters.

The standard correction applied to GRACE-FO data to account for radiative forcing comes from the Clouds and the Earth’s Radiant Energy System project, which provides a monthly mean radiation measurement. But a new study in the Journal of Geophysical Research: Atmospheres demonstrates that numerical weather-prediction models can provide more precise estimates. Sanam Motlaghzadeh, who led the study, says that the standard practice of using a monthly climatological average simplifies the calculations, but it means that short-term events that provide an extra radiative push, like Bongoyo, are not well accounted for.

GRACE-FO is composed of a pair of orbiting satellites that circle Earth every 90 minutes. Separated by roughly 220 km, one satellite follows the other, and microwave signals are sent between the two to measure small changes in their distance. Those slight changes are mainly driven by differences in the gravitational pull from Earth that result from variations in the distribution of mass on Earth’s surface. The largest influence on gravity measurements comes from variations in topography and rock density, but those signals are mostly unchanging over time.

With a well-established baseline gravity map of Earth, the changes in gravity measured by GRACE-FO are used to estimate the movement of water and thinning of ice sheets. “The most important product of this GRACE-FO mission is the terrestrial water storage,” says Motlaghzadeh. Those data are used across disciplines like climate studies, agriculture, and disaster management. GRACE-FO’s predecessor, GRACE, for example, measured the loss of more than 50 trillion liters of groundwater in the Colorado River Basin over a decade of drought from 2004 to 2013.

The big difference between using climatological data and numerical weather-prediction models to estimate radiative forcing is the time scale. The weather model supplies data every hour rather than averaged over a month and thus provides a resolution that had not previously been integrated with geodetic models. Looking forward, Motlaghzadeh plans to investigate how calculations of radiative forcing affect downstream estimates of water volume derived from the GRACE-FO gravity data. She says that other satellite missions, like Sentinel, could also benefit from weather models’ more precise estimates of radiative flux. (S. Motlaghzadeh et al., J. Geophys. Res. Atmos. 129, e2023JD040009, 2024.)

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