TROPOSPHERIC WATER-VAPOR DERIVED FROM SOLAR SPECTROMETER, RADIOMETER,AND GPS MEASUREMENTS

Tropospheric water vapor is of central interest in a large variety of geoscientific fields, such as geodesy, geodynamics, climate research, and meteorology. A new instrumental approach to ground-based mapping o f tropospheric water vapor has been developed. It utilizes high-resolu tion absorption measurements in the near-infrared region by means of a solar spectrometer (SSM). The processing algorithm for retrieval of t he precipitable water vapor (PW) is based on a line-by-line calculatio n of the observed solar spectrum in a narrow wavelength interval (1 nm ) using a simple absorption model of the troposphere. To prove the fea sibility and accuracy potential of the new technique; we carried out a 30-day field experiment. Simultaneous measurements of colocated SSM, water vapor radiometers (WVR) and Global Positioning System (GPS) rece ivers were performed, exploiting absorption, emission and refraction p roperties of water vapor, respectively. A comparison of the three diff erent techniques demonstrated the potential of solar spectrometry for precise and absolute determination of PW without meteorological a prio ri information. Apart from apparent systematic errors of the GPS measu rements, a good agreement between the SSM and WVR results within their individual accuracy limits was observed. The PW standard deviations o f the techniques were determined to 0.37 mm for the WVR, 0.75 mm for t he SSM, and 1.40 mm for the GPS retrievals. The independence of SSM fr om external calibration by radiosondes and the high potential for furt her development may qualify this new technique to contribute to develo ping an error budget for other techniques, such as GPS meteorology.