Integrated Water Vapor Content Research Articles

Measurement of the integrated water vapor content in the atmosphere by a radiometric method

An improved version of a two-wavelength radiometric method is described by which the total water vapor amount along an optical path may be determined by the use of a radiometer and a source at the two ends of the optical path. The method requires two transmission measurements: one at 1.14 microm (at the center of an absorption band) and another at 1.06 microm (an atmospheric window). The spectral transmittance is calculated using the FASCODE computer code, convolved with the source, filter, and detector response curves of the transmissometer. Good agreement (1-7%) is obtained with experimental observations of this quantity as a function of total water vapor amount. The method was verified for horizontal paths of up to 10 km.

A global positioning measurement system for regional geodesy in the caribbean

Low cost, portable receivers using signals from satellites of the Global Positioning System (GPS) will enable precision geodetic observations to be made on a large scale. A number of important geophysical questions relating to plate-motion kinematics and dynamics can be addressed with this measurement capability. We describe a plan to design and validate a GPS-based geodetic system, and to demonstrate its capability in California, Mexico and the Caribbean region. The Caribbean program is a prototype for a number of regional geodetic networks to be globally distributed. In 1985, efforts will be concentrated on understanding and minimizing error sources. Two dominant sources of error are uncertainties in the orbit ephemeris of the GPS satellites, and uncertainties in the correction for signal delay due to variable tropospheric water vapor. Orbit ephemeris uncertainties can be minimized by performing simultaneous satellite observations with GPS receivers at known (fiducial) points. Water vapor corrections can be made by performing simultaneous line-of-sight measurements of integrated water vapor content with ground-based water vapor radiometers. Specific experiments to validate both concepts are outlined. Caribbean measurements will begin in late 1985 or early 1986. Key areas of measurement are the northern strike-slip boundary, and the western convergent boundary. Specific measurement plans in both regions are described.

The accuracy of water vapor and cloud liquid determination by dual‐frequency ground‐based microwave radiometry

A dual frequency ground‐based radiometer operating in the 1 to 1.4 cm wavelength range can provide continuous measurements of integrated water vapor and cloud liquid water. Using climatological data, the accuracy of the vapor and liquid determinations is estimated as a function of cloud amount. Limiting factors in the water determination are uncertainties in water vapor absorption coefficients and, during cloudy conditions, uncertainties in cloud temperature. For integrated water vapor content greater than 10 mm, the accuracy of the vapor determination is better than 15% for a wide range of cloudy conditions.

Airborne Infrared Astronomy

Infrared astronomical observations using ground-based equipment are confined to the few ‘windows’ or observation ports allowed by our absorbing atmosphere, the chief absorbing molecules being water vapour and CO2. The majority of such observations have been made using broadband filters defining the photometric J, H, K, L, M, N and O bands. Some narrow band and spectral work (essentially all photometric) has also been carried out, particularly in the 8-14 micron window. From a very few high altitude, cold, dry sites (where the integrated water vapour content of the atmosphere above the site is consistently less than 1mm), it is possible to make observations at 35 and 345 microns. In Australia, the integrated water vapour content above us is typically 15 mm, only rarely dropping below 7 mm, so that long wavelength IR measurements simply cannot be made using ground-based instruments in Australia. Water vapour is fortunately not well mixed in our atmosphere and by raising our observational platform to a high enough altitude we can in fact achieve quite high transmission for all wavelengths from 10 microns to 1 mm, where we link up with microwave observations using essentially radio techniques.

Measurement of Sea Surface Temperature by the NOAA 2 Satellite

Abstract Sea surface temperature (Ts) may be inferred to a useful accuracy from satellite measurements of infrared radiances in the atmospheric windows. However, even in the absence of clouds, the slight opacity produced by water vapor continuum, carbon dioxide, ozone and aerosol absorption causes the temperature “seen” by the satellite to be lower than that measured by ships or aircraft. The difference between Ts and the temperature inferred from infrared radiances when looking vertically through a cloud-free atmosphere is estimated to about ±0.5 K for the 10.5–12.5 μm window of the scanning radiometer of NOAA 2 when data on certain atmospheric variables are available. A simple parametric formula in terms of integrated water vapor content gave about that accuracy while saving considerable computation time. It was found that Ts may be estimated to an accuracy of about ±1 K within about 500 km of the subsatellite track in the Mid-Ocean Dynamics Experiment area in a comparison of satellite data with ship an...

Measurements and interpretation of the microwave spectrum of the terrestrial atmosphere near 1-centimeter wavelength

Solar extinction measurements of the atmospheric absorption spectrum near 1-cm wavelength were made on 10 days during 1964. The 1.35-cm water vapor resonance varied in intensity and shape, and this variation can be correlated with simultaneous radiosonde measurements. The spectrum can be interpreted theoretically in terms of the water vapor content of the atmosphere with an altitude resolution of 15 km or less. The integrated water vapor content of the troposphere can be determined with an accuracy of approximately 5%, including uncertainties in the measurements and the atmospheric model. Theoretically, the integrated liquid water content of heavy clouds can be measured simultaneously with an rms accuracy of approximately 0.004 g/cm2, or 15%.