SAN FRANCISCO — As Hurricane Blanca traveled across the eastern Pacific Ocean in early June whipping up winds of over 200 kilometers per hour on its way toward Mexico’s Baja peninsula, meteorologists at the National Hurricane Center in Miami used information drawn from space-based altimeters to forecast the storm’s varying intensity levels.
Blanca was classified as a Category 4 hurricane on June 3, but weakened to a Category 1 hurricane during the following day and a half when the storm stopped moving and churned the ocean so much that cold water from its depth moved close to the surface. On June 6, Blanca became a Category 4 hurricane again after moving away from the area of cold water. Then, it lost strength a second time as it approached Mexico’s Pacific Coast.
“During the last day a half before it made landfall June 8, Blanca traveled over water in the low 20 degrees Celsius,” said Mark DeMaria, technology and science branch chief at the National Weather Service’s National Hurricane Center. “So [Blanca] weakened quite a bit as it approached the coast and made landfall as a tropical storm.”
Hurricanes are fueled by warm, moist air rising over the tropical and subtropical oceans. Because warm water expands, meteorologists use space-based altimeters that measure sea surface height to create maps showing where heat is stored in the oceans. For Hurricane Blanca, that knowledge of the water’s heat content along the storm’s path helped forecasters anticipate how the storm’s intensity would continue to change. “The complex interaction with the ocean made Hurricane Blanca an especially challenging forecast, but the knowledge of the subsurface ocean from altimeters is an important piece of the puzzle for improving intensity forecasts,” DeMaria said.
Hurricane intensity is only one of the jobs performed by space-based altimeters like the one onboard Jason-3, a satellite built by prime contractor Thales Alenia Space that the U.S. National Oceanic and Atmospheric Administration, Europe’s Eumetsat meteorological satellite organization, NASA and the French space agency, CNES, were planning to launch July 22 until engineers detected contamination in one of four spacecraft thrusters. Jason-3 is now scheduled to launch in early August onboard a Space X Falcon 9 rocket from Vandenberg Air Force Base in California.
Researchers also use space-based altimeters to observe sea surface winds, wave conditions, circulation patterns and rising sea levels. Jason-3 is the fourth satellite in a series of altimetry missions dating back to the 1992 launch of the TOPEX/Poseidon satellite, which was followed by Jason-1 in 2001 and Jason-2 in 2008.
For hurricane Blanca forecasts, meteorologists used data from altimeters flying on Jason-2 and the Saral/AltiKa satellite launched in 2013 by CNES and the Indian Space Research Organisation. Meteorologists prefer to rely on data drawn from two space-based altimeters to ensure there are no gaps in global coverage. If Jason-2 failed for some reason before Jason-3 was able to supply data, hurricane intensity forecasts would be less accurate, DeMaria said.
Although Jason-2 was designed to last only five years, the seven-year-old spacecraft remains healthy. “The anticipated short delay in the Jason 3 launch will not impact any of the critical systems of the satellite, including battery life or fuel for orbit insertion and attitude control,” NOAA spokesman John Leslie said June 10 by email.
Jason-3’s primary instrument, the Poseidon-3B altimeter built by Thales Alenia Space, is designed to measure sea level height with an accuracy of at least 3.3 centimeters by measuring how long it takes C- and Ku-band radar waves to bounce off the water’s surface and return to the spacecraft antenna. To achieve that level of precision, Jason-3 must know its exact position, which it obtains with the help of a CNES microwave tracking system, called Doppler Orbitography and Radiopositioning Integrated by Satellite, and two instruments from NASA’s Jet Propulsion Laboratory in Pasadena, California: a satellite laser ranging reflector and GPS receiver. JPL also built Jason-3’s Advanced Microwave Radiometer, an instrument designed to measure radiation from Earth’s surface at three different frequencies. By comparing the results, researchers can measure water vapor in the troposphere and use that information to correct the altimeter’s signal.
Jason-3 is scheduled to launch into low Earth orbit at an altitude of 1,336 kilometers inclined at 66 degrees relative to the equator and travel about 60 seconds behind Jason-2. Jason-3 mission planners hope the new satellite will operate alongside Jason-2 for at least six months while mission officials calibrate Jason-3 instruments and validate the data.