William B. Waltman

Ground‐based measurements of water vapor in the middle atmosphere

We present measurements of the middle atmospheric water vapor mixing ratio profile obtained using the ground-based Naval Research Laboratory water vapor millimeter-wave spectrometer (WVMS) instrument at the Jet Propulsion Laboratory Table Mountain Observatory. The measurements cover a period of 262 days from January 23, 1992, to October 13, 1992. During this campaign it was possible to retrieve useful daily mixing ratio profiles for 186 days. We thus have a nearly continuous record of water vapor mixing ratios for altitudes from ≈35 to 75 km. The retrievals are obtained using the optimal estimation method. Details of the error analysis are presented, and a technique is introduced that reduces baseline effects and helps to estimate the baseline error. The high-altitude (≳65 km) data show a sharp rise prior to the expected maximum near the summer solstice and a gradual decline in the following months. The mixing ratios generally peak between 55 and 65 km, at which point the mixing ratios are 6–7 parts per million by volume. The highest peaks occur in January, May, and October.

The Millimeter Wave Atmospheric Sounder (MAS): a shuttle-based remote sensing experiment

The Millimeter Wave Atmospheric Sounder (MAS) will be launched in the spring of 1992 as part of the ATLAS 1 (Atmospheric Laboratory for Application and Science) mission. Using passive limb-scanning millimeter-wave radiometry, it will sense the thermal emission produced by ozone at 184 GHz, water vapor at 183 GHz, chlorine monoxide at 204 GHz, and oxygen (for retrieval of temperature and pressure) at 60 GHz. From these observations, concentration profiles of these gases throughout the middle atmosphere will be made. The fundamentals of the measurements, the design of the radiometers, and the approaches used for the data analysis are described. >

Measurements of water vapor in the middle atmosphere and implications for mesospheric transport

We present data obtained during more than 3 years of nearly continuous measurements of middle atmospheric water vapor. The data are obtained from ground-based measurements at 22 GHz taken at two sites, one in each hemisphere, using the Naval Research Laboratory water vapor millimeter-wave spectrometer (WVMS). With the construction of a second instrument, it has been possible to maintain continuous monitoring from both sites since January 1994. The measurements from both instruments show significant seasonal variability. There is a clear annual cycle, with the water vapor above ∼60 km increasing in summer and decreasing in winter. The observed amplitude of the annual oscillation is larger at 45.0°S than at 34.4°N, a result which is qualitatively consistent with the higher latitude of the southern hemisphere site. There is also an indication of a semiannual cycle, particularly at altitudes near 80 km. The annual cycle is consistent with transport due primarily to advection, while the weaker semiannual cycle may be indicative of the effect of gravity waves on diffusive transport.

Measurements of O3, H2O and ClO in the Middle Atmosphere Using the Millimeter-Wave Atmospheric Sounder (MAS)

The Millimeter-Wave Atmospheric Sounder (MAS) is a shuttle-based limb-sounding instrument designed for global spectroscopic studies of O3, and constituents important in O3 photochemistry, in the middle atmosphere. It is part of the NASAs Atmospheric Laboratory for Applications and Science (ATLAS) spacelab shuttle mission. This paper presents an overview of the instrument, operation, and data analysis. In addition, as an example of the results, we present zonal average retrievals for O3, H2O and ClO obtained in ATLAS 1. The MAS O3 and H2O measurements are shown to agree well with simultaneous observations made with the UARS MLS instrument.

A comparative study of mesospheric water vapor measurements from the ground-based water vapor millimeter-wave spectrometer and space-based instruments

We compare water vapor measurements from the Naval Research Laboratory ground-based Water Vapor Millimeter-wave Spectrometer (WVMS) instruments with measurements taken by five space-based instruments. For coincident measurements the retrievals from all of the instruments show qualitatively similar altitude profiles. The retrieved mixing ratios from most instruments generally differ from an average calculated using retrievals from all of the instruments by <1 ppmv at most altitudes from 40 km to 80 km. Comparisons with the Microwave Limb Sounder (MLS) and the Halogen Occultation Experiment (HALOE) allow for the validation of observed temporal variations. The observed variations show similar annual and semiannual cycles. A comparison of several years of data from HALOE and WVMS also shows that the instruments are detecting similar interannual variations. A regression analysis of the WVMS and HALOE data sets shows that the observed variability is consistent within the estimated errors in the mesosphere and that in the upper stratosphere, where the natural variability is small, there is a positive correlation between the WVMS and the HALOE data.

Cryogenic Servo-Controlled Infrared Fabry-Perot Spectrometer

High resolution infrared spectroscopy obtains fundamental information about the kinematics, composition, and energetics of astronomical infrared sources because it can sort out typical Doppler velocity components and separate adjacent lines from various species. In important cases the instrument having the maximum possible sensitivity for detection is a cryogenic Fabry-Perot spectrometer because it rejects noise from the bright and often flickering background without injecting any radiation. Such an instrument need only be limited by the performance of the best detectors. Coupled to a two-dimensional array, a cryogenic Fabry-Perot can produce spectroscopic infrared images of high quality. We have developed a prototype cryogenic infrared Fabry-Perot spectrometer and have operated it successfully on-site. The system uses a unique optical servo control for reliable sensing of the interferometric surfaces, and electromagnetic voice coil displacement drivers for the large motions needed for flexible operation. The present etalons and detector are optimized for the 4 to 5 Am band. We present a description of the instrument, recent astro-physical results obtained at a telescope, and a summary of development plans. Such a system can be operated from space out to submillimeter wavelengths.

Ground-based monitoring of water vapor in the middle atmosphere: the NRL water-vapor millimeter-wave spectrometer

A novel mm-wave radiometer system specifically designed for measuring water vapor in the stratosphere is presented. The instrument, which is based on an HEMT front-end amplifier, is described in detail. The data retrieval scheme and the results of an extensive instrument data simulation study are also presented. The devices principal features are its capability to conduct measurements of the water vapor profile simultaneously from 25-75-km altitude, with excellent long-term relative precision, and semiautomatically at a remote site.

A Cooled Optically Servo-Controlled Infrared Fabry-Perot Spectrometer

A cooled optically servo-controlled infrared Fabry-Perot spectrometer has been constructed. The control system maintains both alignment and spacing of a tunable infrared etalon. An annular portion of the IR etalon is coated for control use at visible wavelengths. Correct spacing is determined by reference to an associated visible etalon of fixed and slightly different spacing. The spacing of the IR etalon then varies approximately linearly with the output wavelength of a visible control monochromator. The full range of the mono-chromator, typically 500 to 700 nm, corresponds to a change in etalon spacing which can be as little as a small fraction of an IR order or as much as several orders. The movable plate of the IR etalon is positioned by an electromagnetic drive system with a mechanical range of about a millimeter. The instrument is planned to accommodate up to three IR etalons which can be scanned synchronously by tuning the output of a single control mono-chromator. A single-etalon version of the spectrometer has been operated at the Cassegrain focus of the KPNO 1.3m telescope.