A scanning Raman lidar for observing the spatio-temporal distribution of water vapor. (December 2016)

• Record Type: Journal Article
• Authors: Yabuki, Masanori; Matsuda, Makoto; Nakamura, Takuji; Hayashi, Taiichi; Tsuda, Toshitaka
• Is Part Of: Journal of atmospheric and solar-terrestrial physics. Volume 150/151(2016)
• Page Start: 21
• Page End: 30
• Publication Date: 2016-12
• Subjects: Scanning Raman lidar -- Water vapor -- Surface layer; Geophysics -- Periodicals; Atmospheric physics -- Periodicals; Géophysique -- Périodiques; Météorologie physique -- Périodiques; Electronic journals; 551.51
• DOI: 10.1016/j.jastp.2016.10.013
• Languages: English
• ISSNs: 1364-6826

Abstract

Abstract: We have constructed a scanning Raman lidar to observe the cross-sectional distribution of the water vapor mixing ratio and aerosols near the Earth's surface, which are difficult to observe when a conventional Raman lidar system is used. The Raman lidar is designed for a nighttime operating system by employing a ultra-violet (UV) laser source and can measure the water vapor mixing ratio at an altitude up to 7 km using vertically pointing observations. The scanning mirror system consists of reflective flat mirrors and a rotational stage. By using a program-controlled rotational stage, a vertical scan can be operated with a speed of 1.5°/s. The beam was pointed at 33 angles over range of 0–48° for the elevation angle with a constant step width of 1.5°. The range-height cross sections of the water vapor and aerosol within a 400 m range can be obtained for 25 min. The lidar signals at each direction were individually smoothed with the moving average to spread proportionally with the distance from the laser-emitting point. The averaged range at a distance of 200 m (400 m) from the lidar was 30.0 m (67.5 m) along the lidar signal in a specific direction. The experimental observations using the scanning lidar were conducted at night in the Shigaraki MU radar observatory located on a plateau with undulating topography and surrounded by forests. The root mean square error (RMSE) between the temporal variations of the water vapor mixing ratio by the scanning Raman lidar and by an in-situ weather sensor equipped with a tethered balloon was 0.17 g/kg at an altitude of 100 m. In cross-sectional measurements taken at altitudes and horizontal distances up to 400 m from the observatory, we found that the water vapor mixing ratio above and within the surface layer varied vertically and horizontally. The spatio-temporal variability of water vapor near the surface seemed to be sensitive to topographic variations as well as the wind field and the temperature gradient over the site. From the wide-range cross-sectional observations of the water vapor mixing ratio and the backscatter ratio of aerosols within a 2000 m range, we can detect small-scale water vapor structures on a horizontal scale of several hundred meters in the atmospheric boundary layer. Highlights: An scanning Raman lidar for observing water vapor mixing ratio (WVMR) was developed. The cross-sectional structure of WVMR was observed within a 400 m range. The WVMR was evaluated by comparing observations between lidar and tethered balloon. The spatio-temporal variability of WVMR in surface layer was investigated.