Never before have researchers directly measured cloud height from a single satellite, simultaneously measured cloud height and wind, and made these measurements above Earth's polar regions as well as lower latitudes.

Professor Roger Davies and graduate research assistant Akos Horvath of the University of Arizona report first results on cloud wind and height from the polar-orbiting Terra satellite's Multi-angle Imaging SpectroRadiometer (MISR) in the Aug. 1 issue of Geophysical Research Letters.

Simultaneous measurement of cloud height accurate to within 400 meters (about 1,300 feet) and cloud winds accurate to within 3 meters (about 10 feet) per second anywhere over the globe is a potential boon for meteorology, Davies said.

While Terra is a research satellite, not an operational satellite, the success of the radiometer's fully automated multi-angle imaging technique "pioneers the possibility of deploying an operational satellite to gain wind information within the atmosphere, especially over the data-sparse areas of the oceans, for improved weather forecasts," he said.

A specialist in remote sensing of clouds and climate modeling, Davies directs the Radiation, Clouds and Climate Laboratory in the UA atmospheric sciences department. He is co-investigator on the science team that designed and operates MISR.

Horvath, who is working on his doctorate in atmospheric sciences, completed his master's degree on the feasibility of using the Multi-angle Imaging SpectroRadiometer to measure cloud-motion wind. He will brief National Oceanic and Atmospheric Administration scientists this week on the innovative cloud tracking technique.

"The wind retrieval technique definitely worked better than expected," Horvath said. Originally, they had intended to use wind calculations as just a step in getting accurate cloud top height measurements, he added.

"The problem with winds is twofold," Horvath said. "First, there are no wind measurements within the atmosphere over the vast ocean regions, except for over a few islands. The forecasting community really needs data on the wind field above oceans. Satellites are good candidates to fill in that data gap."

"Second, cloud motion winds are useful information for forecasters only if they know wind altitudes. Traditional cloud motion winds can have height errors as large as three kilometers (nearly 2 miles) The MISR height assignment is purely geometric and thus it seems to be more accurate."

Cloud motion wind -- how far a cloud moves through time -- is not exactly the same as real wind, but it is closely related.

Until Terra was launched in December 1999, cloud motion winds were routinely observed by only geostationary satellites -- satellites that orbit above the equator and get their highest resolution images of the area directly below.

As a consequence, satellite information on cloud motion winds has been more accurate nearer the equator than the poles. There typically has been no satellite information on cloud motion winds poleward of latitudes greater than 60 degrees.

Because geostationary satellites measure reflected sunlight in only a single direction, more than a single satellite is needed to measure cloud height, or else researchers must estimate cloud heights using assumed atmospheric temperature profiles or other indirect methods, Davies said.

MISR is a totally new instrument that produces multi-angle imagery. One application of multi-angle imagery is a stereoscopic view of clouds. An array of nine cameras measures reflected sunlight in four colors from nine different directions, covering a 380-kilometer (about 230-mile) wide orbital swath. It takes seven minutes for a given target to be observed at all nine angles.

Coupled with the multi-angle views, this time lapse allows a fast computational algorithm to match solar reflectivity patters from three view angles, then unscramble the measurements to get cloud height and motion.

The data processing methodology was developed through a collaborative effort involving researchers from the University of Arizona, Tucson; the Jet Propulsion Laboratory, Pasadena, Calif.; and University College London.

MISR data on winds may not be very useful to operational forecasting because the instrument covers such a narrow swath of Earth, Davies and Horvath said. Consequently, MISR takes 9 days to cover the entire globe. An operational satellite could feature a wide field-of-view instrument. Or, several small satellites, each carrying three cameras operating in a single color channel could also be cost-effective, they said.

MISR principal investigator David Diner of the Jet Propulsion Laboratory proposed the novel instrument for the global monitoring of clouds, aerosols and planet surface.

"This instrument represents a new way of looking at Earth, and it's exciting to see the data opening up new pathways for geophysical observation and research," he said.

Davies recognized that three of MISR's measurements could be used simultaneously to measure both cloud motion winds and cloud heights. With help from the instrument's data processing team, Horvath analyzed the first data which proved this concept works.

Davies and Hovath are using MISR to measure how much solar radiation clouds reflect from the Earth, which is one of the greatest uncertainties in understanding global climate change. MISR data is needed to learn how clouds and atmospheric particles affect regional and global climate. Climatologists also want accurate measurements of cloud height, for example, Davies said, because changing cloud height could signal changing climate.

Terra is the first of a new generation in NASA Earth Observing System satellites, part of the space agency's Earth Science Enterprise. JPL is managed for NASA by the California Institute of Technology in Pasadena. - By Lori Stiles

[Contact: Roger Davies, Akos Horvath ]

21-Aug-2001