Radio occultation experts from around the globe gathered at UCAR's Center Green facility for the second annual COSMIC Users Workshop. (Photos by Carlye Calvin.)
by Bob Henson
With more than 750,000 profiles gathered in less than two years, the atmospheric sounding system known as COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) has proven it's no slouch. The six-satellite network is also a bargain compared to most satellite missions, with a total cost of around $100 million. Taiwan, where the mission is known as FORMOSAT-3, is the major sponsor, contributing around $80 million, with a number of U.S. agencies partnering to fund the remaining $20 million.
Of course, gathering lots of data affordably doesn't guarantee that the data are any good. At the second annual COSMIC data users workshop, held at UCAR in October, more than 100 participants from around the world showed that the system is not only economical and productive but also beneficial to daily forecasting as well as to climate research. Operational weather prediction centers in France, the United Kingdom, and the United States have all shown measurable gains in the skill of their daily numerical model forecasts through incorporating COSMIC data. The soundings will soon be used operationally in Canada, Japan, Taiwan, and other countries.
Even as workshop attendees celebrated the success of COSMIC so far, there was a touch of anxiety in the air. Two of the six COSMIC satellites are now limited to 50–66% of their capacity due to solar power issues, and the hardware now in orbit can't be repaired. Moreover, there is no funding as yet to sustain or replace COSMIC once the system reaches the end of its anticipated five-year lifespan in 2011. The budget crunch could become critical as soon as next April, when the current funding cycle ends, according to the U.S. COSMIC Interagency Working Group. However, according to Ying-Hwa "Bill" Kuo, director of the COSMIC Project Office in UOP, "We anticipate that U.S. agencies and Taiwan will jointly fund the continued operation of COSMIC through its mission life so that we can derive the maximum scientific benefit."
Financial concerns didn't dim the glow of accomplishment at the October workshop. Presenters examined the fast-growing roster of COSMIC achievements as well as a range of strategies for keeping the system, or something similar, going.
"COSMIC has already made a huge impact," says Kuo. More than 600 researchers from 42 countries are registered users of COSMIC data, which are freely available to users in all countries. The data include electron counts in the ionosphere and atmospheric soundings of temperature, moisture, and pressure in the troposphere and stratosphere. The latter are gleaned through the radio occultation (RO) technique, whereby GPS signals are intercepted and analyzed for effects induced by the atmosphere along their paths.
Some 80% of COSMIC soundings are available within three hours of collection. They're directly improving global analyses of the atmosphere, especially above the oceans, polar regions, and other hard-to-sample areas. Moreover, since so many soundings are collected around the clock and around the globe, COSMIC provides a three-dimensional picture of the diurnal cycle in all types of weather. For example, COSMIC is adept at tracking the height of the inversion capping the boundary layer, and thus monitoring its depth, thanks to techniques developed by UCAR's Sergey Sokolovskiy and colleagues.
Ying-Hwa "Bill" Kuo.
Because COSMIC soundings are relatively bias-free, they've also proven to be a useful benchmark for correcting soundings that are derived from satellite-borne microwave and infrared sensors, which sample large areas at high horizontal resolution but with less precision than COSMIC. "Correcting satellite biases has always been a challenge," says Kuo.
High-altitude researchers are finding COSMIC data a boon as well, with the data on electron counts providing a new window on three-dimensional structures and associated dynamics in the ionosphere. For example, Taiwan's Jann-Yenq Liu (National Central University) reported at the workshop on a new feature, plasma caves, uncovered through analysis of COSMIC data. The caves are most evident by day beneath two crests of ionization located on either side of the magnetic equator.
The low-bias nature of RO systems, and their global round-the-clock reach, make them especially important for long-term climate monitoring, noted NCAR's Kevin Trenberth. With several major U.S. satellite programs on hold or cancelled, and others scaled back, "the satellite record is in jeopardy," said Trenberth. "The risk of gaps is now considered likely. This is where RO observations can play a major role. We really need to continue these measurements, but at the moment they're not guaranteed." A strong consensus from the workshop participants is that a GPS RO constellation, as demonstrated by COSMIC, should be maintained operationally to support global weather forecasting, climate monitoring, and space weather forecasting. This includes keeping COSMIC operating through its mission life as well as launching follow-on missions to continue the constellation.
What next for RO?
COSMIC wasn't the first RO-based system to sample Earth's atmosphere, but it's the largest to date by far. Designed to gather as many as 2,500 soundings daily, COSMIC has been retrieving 1,600 to 1,800 over the last few months. A number of smaller-scale RO missions are slated for launch in the next several years. However, unless these are coordinated and provide real-time data, they won't be able to sustain the global coverage and operational impact now being achieved by COSMIC. Several options were discussed at the workshop, including:
- A FORMOSAT-3/COSMIC follow-on mission. If Taiwan can secure interest and support from international collaborators, it hopes to replace COSMIC in 2012 with a constellation at least twice as large, with 12 satellites that could gather 10,000 soundings per day.
- A system that would piggyback on the next cycle of Iridium communications satellites. With up to 66 potential receivers in space, such a network could generate as many as 66,000 soundings daily by 2013.
- A public-private partnership concept dubbed CICERO (Community Initiative for Continuing Earth Radio Occultation). Start-up costs for CICERO would be funded by private interests and recouped through user fees. "Why should a number-one community priority be perennially blocked when it is fully within our power to achieve it ourselves?" asked Thomas Yunck (GeoOptics). He envisions up to 24,000 CICERO soundings a day as soon as 2011.
- A "poor man's constellation" that would be pulled together through coordination of the many smaller systems now on the drawing board.
As COSMIC principals explore these options over the coming months and years, they face the more immediate challenge of how to keep the current system rolling for the next three or so years. "We designed COSMIC as a science mission rather than an operational mission," says Kuo. "It has enough fuel to last five years, but it wasn't built with an absolute requirement to last that long." He notes that operational weather and climate satellites are typically five to ten times more expensive. There's also work to be done in figuring out how best to assimilate COSMIC data into operational systems, as the results vary somewhat from country to country.
New RO-based techniques and applications continue to emerge. For example, by intercepting GPS beams that glance off the sea surface, an RO system could deduce ocean height, which would complement data now gathered by satellite-borne altimeters. In another development, Robert Kursinski and colleagues at the University of Arizona are exploring how a next-generation
RO system using much higher occultation frequencies could maximize the information derived from the RO technique. Such a system could probe water vapor absorption lines and simultaneously profile water vapor, ozone, and temperature from near the surface to the mesopause, independent of other observations, models, or analyses. Other constituents could be detected as well, says Kursinski. With the help of two WB-57 platforms from NASA, NSF is supporting an aircraft-to-aircraft test of the proposed system, dubbed ATOMMS (Active Temperature, Ozone, and Moisture Microwave Spectrometer).
"ATOMMS is a natural extension of GPS RO," says Kursinski. "The result, we hope, will be a global climate monitoring system with the high vertical resolution, all-weather sampling, and absolute accuracy of RO. Our goal is to find out how the climate is actually behaving and evolving, completely independent of models."