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
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.
More information about the Multi-angle Imaging SpectroRadiometer is available
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.