October 1991

AIRS: The Atmospheric Infrared Sounder

Moustafa T. Chahine

A major objective of the study of global change is the development of accurate long-term data sets of the Earth's climate system. To accomplish this, a wide range of observations will be carried out by the NASA Earth Observing System (EOS). The Atmospheric Infrared Sounder (AIRS) is a facility instrument selected by NASA to fly on the first polar orbiting platform, EOS-A1. The same platform will also carry the NOAA operational Advanced Microwave Sounding Unit (AMSU-A) and the Microwave Humidity Sounder (MHS). The AIRS/AMSU/MHS system will provide both new measurements not previously achievable and measurements with a greater degree of accuracy and resolution than are currently available.

Observing atmospheric winds with a Doppler lidar

Most lidar systems used for atmospheric probing (see the associated articles in this issue) measure the intensity of laser radiation backscattered from the atmosphere to provide information on parameters such as aerosol structure, atmospheric density, or trace species concentration. Doppler lidars are designed to measure the frequency as well as the intensity of backscattered radiation. By comparing the frequency of the backscattered radiation to that of the transmitted laser pulse, the frequency change due to the motion of the scatterers (Doppler shift) can be computed and used to infer the component of scatterer velocity along the line of sight of the lidar. Since the particles that effectively scatter laser light are very small (less than a few micrometers in diameter), they move with the wind; hence, measurement of the mean velocity of a volume of scatterers provides a measurement of the mean radial wind speed at the location of the scattering volume.

Atmospheric moisture structure revealed by Raman lidar

Water vapor is an important constituent of the atmosphere. The release of latent heat as water changes state from vapor to liquid to ice is one of the dominant drivers of atmospheric circulation. Water vapor is also the most active infrared (IR) molecule in the atmosphere. Thus, water vapor will play a major role in any global warming triggered by increasing carbon dioxide. The importance of water vapor in climate processes was recently re-emphasized at a workshop held in Easton, Md., just one year ago.

The JPL MkIV interferometer

The Jet Propulsion Laboratory MkIV interferometer is a high resolution Fourier Transform Infra-Red (FTIR) spectrometer, designed to remotely sense the atmospheric composition. This instrument made ground-based observations from McMurdo, Antarctica, in September and October 1986, flew on board the NASA DC-8 aircraft in the polar AAOE and AASE campaigns of 1987, and 1989, and made observations from high altitude (39 km) research balloons in 1989, 1990, and 1991. In the winter of 1991/92, it will participate in another airborne Arctic campaign (AASE II).

New spectroscopic instrumentation for measurement of stratospheric trace species by remote sensing of scattered skylight

Measurement of the column abundance of atmospheric molecular species can be accomplished from the ground or an aircraft by spectroscopic observation of light either from the sun or moon directly or from the zenith scattered sky when the air mass factor is large for photons passing through the stratosphere (conditions of low sun/moon elevation). The first twilight scattered sky measurements of NO2 were reported in 1973; the technique was greatly improved and expanded by Noxon and coworkers. This short paper discusses the new generation of instrumentation now under construction at the NOAA Aeronomy Laboratory to measure (at least) NO2, O3, NO3, BrO, OCIO, and SO2. The scientific aspects are discussed elsewhere.

The High Resolution Doppler Imager

The distributions of most chemical species in the stratosphere are affected by both dynamical and chemical processes. Conversely, the distribution of certain photochemical species, such as ozone, can influence the radiative budget of the stratosphere, affecting temperatures and motions. Satellite remote observations of the stratosphere to date have provided only temperature and constituent measurements. The horizontal winds on a global basis have been deduced from temperature fields by using the thermal wind relationships, which relate the vertical shear of the geostrophic wind components to horizontal temperature gradients.

Optical standards: What takes so long?

About this time, ISO/TC172/SC1 will be meeting in Tokyo to finalize several major optical standards documents, including one on how to indicate optical element attributes on drawings. This document has been undergoing writing and revision for over 10 years and, in spite of substantial pressure, it is not obvious that agreement on its final form will be reached at this meeting. Why is it so difficult to write optical standards such as these?

Flashing bulbs and smoke rings

While asking young people why the sky is blue (OPN, August 1991), I found that several of them wanted to know why a light bulb works. This is hardly a sophisticated optical phenomenon, yet it can be a mystery to youngsters who may not be aware of filaments or electrical currents. More importantly, the question can provoke a nice set of experiments* that are both entertaining and enlightening (if you'll pardon the pun). For these experiments, you will need the following supplies: 1 flashlight, 2 or more flashlight bulbs, 2-3 D cell batteries, 19V battery, 1 candle, stranded #22 insulated wire, masking tape, and needlenose pliers. In this sequence of experiments, we show first how a bulb works, then demonstrate the simple principles behind why it works, and finally demonstrate these principles directly with a bulb.

Ozone and aerosol measurements with an airborne lidar system

Ozone (O3) is an important component of atmospheric pollution. It indirectly controls the chemistry of the troposphere (atmospheric region typically below about 10 km in altitude) and directly contributes to the greenhouse effect by being a radiatively active gas. In the lower stratosphere (atmospheric region typically between 10-30 km in altitude), O3 is the principal absorber of solar ultraviolet radiation, and because of this, it protects life at the surface from harmful ultraviolet radiation. In addition, it controls the temperature in the lower stratosphere and is important in determining the chemistry of this region.

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