How can a ground motion sensor improve a weather forecast?
Continuous GPS stations in Yellowstone National Park measure minute ground movements, aiding in monitoring the volcanic system. But by analyzing the GPS signals arriving at the station more carefully, these stations can also monitor weather conditions!
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Scott K. Johnson, Science Communications Associate for UNAVCO.
Continuous GPS stations were initially constructed to measure solid Earth deformation—like motion of tectonic plates, how earthquakes cause ground displacement, and volcano movement due to magmatic activity. The signals from navigation satellites are also sensitive to the conditions they encounter between the satellite and the station. This means that careful corrections are required to make position calculations as precise as possible, but also that other aspects of the environment can be isolated as their own measurement.
For example, GPS stations can measure the amount of water vapor in the atmosphere, as well as the depth of snow cover near the station.
The June 2022 flooding in Yellowstone occurred due to an "atmospheric river" that delivered a large amount of moisture to be wrung out in an intense rainstorm. About 2-3 inches of rain fell in the northern portion of the park on June 13, 2022, but it fell on considerable snow cover that had already saturated the soil as it started to melt. The result was a lot of water with no place to go but into swollen streams and rivers.
This event is easily apparent in the atmospheric water vapor data at continuous GPS station P720, located near Slough Creek Campground in the northeast portion of the park. Water vapor in the atmosphere delays the navigation satellite signal as it travels to the GPS station—the more total water vapor, the greater the delay. By calculating this delay between each station on the ground and each satellite in the sky, a map of atmospheric water vapor can be generated.
The record of total water vapor above GPS station P720 shows high values during June 10–13, followed by a sharp drop on June 13 as a cold front passed through. This is one source of water vapor data that feeds into weather forecast models, which depend on accurate knowledge of atmospheric conditions.
When the satellite signal reaches a GPS station, some signal bounces off the ground before being received by the antenna, which results in signal interference. This, too, can be useful, because the characteristics of that interference change when the distance between the antenna and the surface changes—such as when snow accumulates or a nearby lake level rises.
Stations in open areas—free from signal-blocking trees—are best for this kind of measurement, so it isn't possible at many stations in Yellowstone. As an example, station P360, just west of Yellowstone National Park in Harriman State Park, has produced excellent records of snow depth (https://spotlight.unavco.org/station-pages/p360/p360.html). Using GPS stations in this way can augment the SNOTEL (SNOw TELemetry) network with measurements in additional locations. Apart from tracking water resources, this information can improve flood forecasts for "rain-on-snow" events like the June 2022 flooding in Yellowstone.
It's also possible to measure soil moisture from the reflected signal, although the measurements work a little differently. The distance between the antenna and the surface isn't changing, but the signal is affected by water as it reflects off the ground, causing a measurable shift of the signal's peaks and valleys. Knowing how wet the soil is can also improve flood forecasts, since saturated ground won't be able to absorb much rain.
More generally, GPS stations can be used to measure regional changes in groundwater and surface water based on vertical ground movement. When water resources shrink due to unsustainable water use or drought, the change in mass actually causes Earth's surface to sag or spring upward in that area. This can even be extended to the weight of winter snowpack. For example, vertical movement measured at GPS station P350 in central Idaho compared to nearby snowpack data shows that the station moves up when the snow melts and down when the snow accumulates by about 10 to 20 millimeters (0.4–0.8 inches)—small movements that can be used to calculate the change in snow and water conditions.
The networks of monitoring stations in and around Yellowstone National Park are used by scientists for more than just studying the volcanic system. While the main use of the continuous GPS stations might be to monitor ground movement due to subsurface earthquake and magmatic activity, the data can also be used to better understand environmental conditions and aid weather forecasts. The same instruments that watch for activity underground are also watching the skies!
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