Tropospheric water vapor derived from solar spectrometer, radiometer, and GPS measurements

Abstract

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 of tropospheric water vapor has been developed. It utilizes high‐resolution absorption measurements in the near‐infrared region by means of a solar spectrometer (SSM). The processing algorithm for retrieval of the precipitable water vapor (PW) is based on a line‐by‐line calculation of the observed solar spectrum in a narrow wavelength interval (1 nm) using a simple absorption model of the troposphere. To prove the feasibility 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) receivers were performed, exploiting absorption, emission and refraction properties of water vapor, respectively. A comparison of the three different techniques demonstrated the potential of solar spectrometry for precise and absolute determinaton of PW without meteorological a priori information. Apart from apparent systematic errors of the GPS measurements, a good agreement between the SSM and WVR results within their individual accuracy limits was observed. The PW standard deviations of the techniques were determined to 0.37 mm for the WVR, 0.75 mm for the SSM, and 1.40 mm for the GPS retrievals. The independence of SSM from external calibration by radiosondes and the high potential for further development may qualify this new technique to contribute to developing an error budget for other techniques, such as GPS meteorology.