NASA gives pole-to-pole view of cloud heights, winds
Posted: August 21, 2001

An artist's concept of Terra spacecraft. Photo: NASA/JPL
Scientists for the first time ever can simultaneously measure the height and motion of clouds over Earth from pole to pole, which may improve weather forecasts.

Never before have researchers directly measured cloud heights from a single satellite, simultaneously measured cloud heights and winds, and done this above Earth's polar regions as well as lower latitudes.

Professor Roger Davies and graduate research assistant Akos Horvath of the University of Arizona, Tucson, report first results on cloud winds and heights from NASA's polar-orbiting Terra satellite's Multi-angle Imaging SpectroRadiometer in the Aug. 1 issue of Geophysical Research Letters.

Simultaneous measurement of cloud heights 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.

Davies directs the Radiation, Clouds and Climate Laboratory in the University of Arizona's atmospheric sciences department. He is co-investigator on the science team that designed and operates the Multi-angle Imaging SpectroRadiometer.

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 the National Oceanic and Atmospheric Administration this week on the innovative cloud-tracking technique. "The wind-retrieval technique definitely worked better than expected," Horvath said. Originally, the researchers had intended to use wind calculations as just a step in getting accurate cloud- top height measurements, he added.

Until Terra was launched in December 1999, cloud-motion winds were routinely observed by only geostationary satellites. These 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.

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

The Multi-angle Imaging SpectroRadiometer is a totally new instrument that produces multi-angle imagery, one application of which is a stereoscopic view of clouds. An array of nine cameras measures reflected sunlight in four colors from nine different directions, covering an orbital swath 380-kilometers (about 230-miles) wide. 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 mathematical formula to match solar reflectivity patterns 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; NASA's Jet Propulsion Laboratory, Pasadena, Calif.; and University College, London.

Multi-angle Imaging SpectroRadiometer 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, the instrument takes nine days to cover the entire globe. A future 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.

The Multi-angle Imaging SpectroRadiometer's principal investigator, Dr. David J. Diner, of JPL, proposed the novel instrument for the global monitoring of clouds, aerosols and the 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 the radiometer's camera angles 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 that proved this concept works.

Davies and Horvath are using the Multi-angle Imaging SpectroRadiometer to measure how much solar radiation clouds reflect from Earth, which is one of the greatest uncertainties in understanding global climate change. The data are 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 NASA's Earth Science Enterprise. JPL is managed for NASA by the California Institute of Technology in Pasadena.