Filling in the gaps: New ways to pull data from uncharted skies
Two pioneering sets of airborne platforms, both developed largely at NCAR and UCAR, made meteorological history in 2006. A constellation of six satellites began gathering round-the-clock weather data in April from previously inaccessible parts of the globe. And as they sailed across the Atlantic in late summer, a flotilla of instrument-toting balloons canvassed the poorly observed regions where many hurricanes form.
Balloons that mean business
On the crystal-clear morning of 28 August 2006, a three-story-high weather balloon rose from the arid plains surrounding Zinder, Niger. Beneath the balloon hung a gondola packed with weather instruments and transmitters. It was the first research deployment of a new observing platform called a driftsonde, with several more launched over the next month. Rising to the lower stratosphere, each driftsonde spent 6 to 18 days moving westward across the tropical Atlantic Ocean. Along the way, each one dropped dozens of parachute-borne instrument packages (dropsondes), with the timing specified by scientists at a Paris control center.
The driftsondes were built and deployed through a collaboration among NCAR’s Earth Observing Laboratory, the French space agency (CNES), and France’s National Center for Scientific Research (CNRS). In all, nearly 300 dropsondes gathered and transmitted data from critical regions that serve as breeding grounds for some of the worst U.S. hurricanes. “It would take a fleet of research aircraft to gather the same data,” says Philippe Drobinski, the project’s scientific co-lead from the CNRS.
The launches were in conjunction with the French-led African Monsoon Multidisciplinary Analysis, a long-term effort to study the weather and climate over West Africa. They also supported the 10-year THORPEX program of the World Meteorological Organization, dedicated to improving forecasts of high-impact weather.
To build the new system, scientists, engineers, and machinists on both sides of the Atlantic had to overcome many hurdles. The entire setup had to be robust enough to endure the intense sunlight of the high, thin atmosphere, not to mention days of extreme stratospheric cold averaging 62° Celsius (–80° Fahrenheit). “Try letting your car sit at minus 80 for 14 days, and then try to start it,” says David Parsons, the NCAR lead on the driftsonde project.
As far back as the 1970s, NCAR ballooning experts explored the driftsonde concept. However, the gondolas were large and heavy, requiring expensive balloons to lift them. Battery technology and communications links were also problematic. The dream was revived for THORPEX, with the driftsonde concept now more practical thanks to highly precise GPS (Global Positioning System) tracking and low-cost, lightweight instruments capable of operating reliably with very low power in cold, high-altitude air.
NCAR engineers and machinists came up with an instrument package roughly the size of a bicyclist’s water bottle but weighing only about 140 grams (5 ounces). That’s less than half the weight of the NCAR-designed dropsondes deployed by the hundreds in the last decade from “hurricane hunter” aircraft. However, these NOAA and U.S. Air Force flights don’t extend to the eastern Atlantic, where some of the strongest and longest-lived hurricanes are born.
Driftsonde data for this region found unusually deep layers of moist air wrapping around the east sides of hurricanes-to-be Florence and Gordon, just below the strong southerly flow of easterly waves. “This suggests that variations in the fluxes of heat and moisture from the ocean associated with easterly waves play a critical role during the spin-up of these storms,” says Parsons.
Scientists hope to use driftsondes in two multi-institutional projects during the latter half of 2008, one based in the Antarctic and the other in eastern Asia. Another goal is to improve the driftsondes’ success rate. Of the eight African launches in 2006, one was thwarted by an electrical storm and another by a balloon failure. Still, says Parsons, “it was a successful proof of concept in many ways. The partnerships we’ve developed will open up a new realm of observations.”
A COSMIC success
Since the 1960s, satellites have peered into Earth’s weather and climate, gathering many types of data beyond the iconic images that are now standard on television weathercasts. Despite their global reach and other strengths, satellites have their limits as well. Clouds block the view of some satellite sensors; others are limited in their accuracy or vertical resolution.
COSMIC—the Constellation Observing System for Meteorology, Ionosphere and Climate—hits a sweet spot few other observing platforms can match. Launched in April 2006, the $100 million, six-satellite system is a Taiwan-U.S. collaboration led by Taiwan’s National Space Organization (NSPO) and UCAR. U.S. support comes from NSF, NASA, NOAA, and the U.S. Navy and Air Force.
COSMIC is designed to generate about 2,500 vertical profiles of temperature, water vapor, and electron density per day, all freely available for weather prediction, climate monitoring, and space-weather research. That’s nearly twice the number of soundings available from the global weather-balloon network of radiosondes that has served as a mainstay of weather observing for 60 years. The twice-daily radiosonde launches, though critical to weather forecasting, omit the 70% of Earth covered by oceans, whereas COSMIC provides global coverage.
The roots of COSMIC go back more than four decades. Scientists at the Jet Propulsion Laboratory and Stanford University measured the atmospheres of other planets in the early 1960s using a technique called radio occultation, in which the bending and slowing of radio signals was analyzed to infer the atmospheric properties behind the distortion. Decades later, JPL scientists proposed applying the technique to Earth’s own atmosphere. In a 1995 experiment called GPS-MET, UCAR teamed with JPL and other partners to piggyback a GPS receiver onto a commercial spacecraft.
“GPS-MET was incredibly successful,” says UCAR president Richard Anthes, one of the principal investigators. “It proved the concept of obtaining high-quality atmospheric soundings of Earth through GPS.”
Each of COSMIC’s six satellites sports a receiver that intercepts GPS signals as they pass through Earth’s atmosphere (see graphic). The signals are slowed and bent by changes in air density as temperature, pressure, and moisture vary along the signals’ paths. By measuring
how long it takes a signal to reach a COSMIC receiver, scientists can infer the refractivity of the atmosphere and produce vertical profiles of temperature and water vapor from near the surface up to as high as 40 kilometers (24 miles). Two other instruments aboard each satellite measure the number of electrons and the nighttime emissions of photons up to the orbital height of 800 km (500 mi); these data help space physicists to analyze the upper atmosphere and monitor the effects of disruptive solar storms.
Thanks to advances in GPS signal tracking, COSMIC is more reliable than its satellite predecessors in probing the atmosphere down to its lowest kilometer, the boundary layer, where strong vertical gradients in temperature and moisture often resist satellite-based monitoring. One test forecast showed that adding COSMIC data allowed computer models to correctly simulate the birth of 2006’s Hurricane Ernesto in the Caribbean Sea several days in advance. Models that omitted COSMIC data failed to generate the storm.
COSMIC may also shed light on the comings and goings of the El Niño–Southern Oscillation, which remains hard to predict. Robert Kursinski (University of Arizona) found a correlation between ENSO and moisture data drawn from a German satellite called CHAMP that uses a JPL-built radio occultation instrument. Kursinski hopes to fund a postdoctoral researcher to carry out a more extensive analysis using COSMIC’s six-satellite network. “I suspect the COSMIC data will be significantly better beause of denser coverage and the ability to probe the boundary layer,” says Kursinski.
More than 500 scientists from around the world now access data from COSMIC soundings. Everyday weather prediction is getting a boost as well. At the European Centre for Medium-Range Weather Forecasts, the inclusion of COSMIC data reduced errors in temperature predictions in the lower stratosphere by up to 11%, with benefits extending up to 10 days out. By May 2007, U.S. and British forecast centers had brought COSMIC data into their day-to-day operations, with Canada and France expected to follow suit by 2008.
Filling in the gaps | Tiny troubles in the air | Coding the atmosphere | A magnetic team |