Moisture-related processes in the
Earth's atmosphere have important human and economic impacts
because of their relationship to weather and climate. Water vapor
exists in the atmosphere only in small amounts, but it plays a big
role in many important atmospheric processes. For example, water
vapor is a fundamental part of the atmospheric energy budget,
global water cycle, atmospheric chemistry, and transport processes
of pollution. Scientists use the Lidar Atmospheric Sensing
Experiment (LASE) system to obtain comprehensive, remote water
vapor observations for a better understanding of its many roles in
the atmosphere.
 Since
the 1960s, scientists have used lidars (light detection and
ranging) to study the atmosphere. A lidar is an instrument that
uses short pulses of laser light to detect particles or gases in
the atmosphere, like a radar bounces radio waves off rain drops in
clouds. A telescope collects and measures reflected laser
radiation, like a radar dish collects the radar signal, leading to
a profile of the atmosphere's structure along the path of the laser
beam. Researchers can then determine the location, distribution,
and nature of atmospheric particles and molecular species using an
advanced lidar method called the Differential Absorption Lidar
(DIAL) technique.
Different types of lidars measure
different atmospheric properties. Scientists know that different
molecules absorb light only at certain wavelengths. They can then
tune laser pulses to different wavelengths to target the type of
atmospheric molecule they want to study using the DIAL method. To
measure water vapor, aerosols, and clouds, researchers use the LASE
system. Scientists developed LASE as a prototype for a space-borne
DIAL system. Since 1995 researchers have used LASE on six different
major atmospheric field investigations. During the first two field
experiments, LASE operated as an autonomous (controlled by a pilot)
lidar aboard an ER-2 aircraft (fig. 1).
What is LASE?
LASE uses the DIAL technique to
determine vertical profiles of water vapor in the lower atmosphere.
It measures water vapor, clouds, and aerosols by comparing the
absorption and scattering of different laser pulses on these
atmospheric species. Laser beams are pointed out of the aircraft
both upwards and downwards. Researchers operate LASE in the near
infrared region of the electromagnetic spectrum and set one laser
pulse to one wavelength, so water vapor will absorb the radiation,
and another laser pulse to a slightly different wavelength to
measure aerosols and clouds. LASE compares the two laser pulses as
the atmosphere reflects them back to the telescope. Scientists use
differences in the amount of laser light to determine the location
and amount of water vapor, aerosols, and clouds in the atmosphere,
essentially creating an atmospheric map of the area above and below
the aircraft. LASE can measure water vapor distributions from the
ground up to 20 kilometers. An example of simultaneous measurements
of water vapor and aerosol distributions by LASE in the atmosphere
is shown in fig. 2. These data were taken from the ER-2 aircraft as
it flew back from near Bermuda to Wallops Island, Virginia, in July
1996.
Why measure water vapor?
Water vapor is an essential link
in the water (hydrologic) cycle, which includes all forms of water
(gas, liquid, ice, and snow). Also, water vapor is the main
ingredient in cloud formation (which impacts precipitation) and in
severe storm development. It is an invisible gas and acts as a
buffer for the rise and fall of temperatures in the atmosphere.
Once the temperature drops to the dew point, water vapor condenses,
releasing heat, and the temperature rises again. Conversely, as
temperature rises, water evaporates, taking up heat and cooling the
atmosphere. Water vapor, a greenhouse gas, also influences the
Earth's radiation budget by absorbing outgoing longwave energy. In
addition, water vapor channels energy into the atmosphere and is
the main energy source for hurricane development.
Cloud and aerosol measurements
Simultaneous measurements of
clouds and aerosols enhance water vapor investigations conducted
using LASE, which can provide additional important information on
atmospheric structure and movement. Scientists can also infer
meteorological variables from these data, such as the depth of the
mixed layer -- the layer closest to the surface where most of the
pollution is trapped and where many clouds form. Measurement of the
distribution of clouds is important because clouds play a major
role in meteorology, rainfall, and the atmospheric radiation
balance.
Future LASE missions
NASA Langley scientists will
participate in a Department of Energy (DOE) LASE field experiment
on a NASA DC-8 aircraft during November and December of 2000 over
the Southern Great Plains of the United States. The main objective
of this field experiment is to compare the performance of a number
of advanced water vapor sensors for measuring upper atmospheric
humidity -- measurements that are needed for studying the Earth's
radiation budget and climate change.
 For more LASE information, please contact:
NASA Langley Research Center
Office of Public Affairs
Mail Stop 115
Hampton, VA 23681-2199
757-864-6124
Or see the LASE Home Page at:
http://asd-www.larc.nasa.gov/lase/ASDlase.html
for additional information.
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