A large gap in our understanding of the world's weather and climate is knowledge of wind speeds and directions in clear air. Instruments on the ground or on balloons and satellite instruments are limited with respect to measuring winds. In fact, direct measurements of global winds are the single most important missing data set at the present time with respect to weather forecasts and climate analyses. A laser wind sensor aboard an Earth observing platform is the strongest candidate to fill this gap.
To
demonstrate technologies for such an instrument, NASA/Marshall and the
GHCC were authorized in 1997 to develop the SPAce Readiness Coherent Lidar
Experiment (SPARCLE), a low-cost demonstration sponsored by NASA's New
Millennium Program. In operation, SPARCLE scans a circular pattern across
the Space Shuttle's ground track, aiming pulses of eye-safe laser light
into the atmosphere and measuring the light that is reflected back to it
by dust and aerosols in the atmosphere. The time between pulse and echo
will (like radar) determine the distance to an object. The shift in the
color of the light will tell how fast the particles are moving along the
laser's line of sight. Combining measurements from viewing ahead to
viewing aft will provide two velocity components, from which the true wind
speed and direction can be calculated in a column from the surface of the
Earth to an altitude of 10 to 20 km. For all its ambition, SPARCLE will
start with a modest size; two "Getaway Specials," cans the size
of a large wastepaper basket, will contain a laser with output power of
only 0.6 watts, the optical system, and the appropriate electronics. Using
a "GAS can" will allow scientists to put SPARCLE's optical and
electronic systems in a pressurized environment without having to
ruggedize them for space. Although it will operate for only about 24
hours, SPARCLE will demonstrate technologies for an operational laser
sounder aboard a future Earth observing platform. The Shuttle mission is
scheduled to fly in 2001.
The
technology chosen for SPARCLE has been amply demonstrated on aircraft in
meteorological studies. The GHCC-led Multi-center Airborne Coherent
Atmospheric Wind Sensor (MACAWS) flies on the NASA DC-8 research aircraft.
MACAWS has a world-class capability to measure fields of winds at several
vertical levels by scanning the laser beam during flight. This permits the
study of atmospheric processes and features that may be inadequately
resolved or missed completely by more conventional techniques, such as
ground-based radar. For example, measurements of the multi-level wind
field near Point Arena, California, in June 1996 (figure below), readily
show the northerly flow in the marine layer, the strong variability in the
cross-shore direction within the marine boundary layer (especially at 150
m above sea level), and the structural changes in the vertical components.
MACAWS is well-suited to study coastal meteorological processes,
especially in regions where complex terrain may affect the siting and
operation of other sensors. Several field experiments with be conducted
with MACAWS in the future, including study of the intensification and
tracking of Atlantic hurricanes in summer 1998, and simulation and
validation of SPARCLE.
Information provided by: http://wwwghcc.msfc.nasa.gov