Mobile water vapor Raman lidar for heavy rain forecasting: system description and validation

Abstract

Abstract. To improve the lead time and accuracy of predictions of localized heavy rainfall, which can cause extensive damage in urban areas in Japan, we developed a mobile Raman lidar (RL) system for measuring the vertical distribution of the water vapor mixing ratio (w) in the lower troposphere. The RL was installed in a small trailer for easy deployment to the upwind side of potential rainfall areas to monitor the inflow of moist air before rainfall events. We describe the lidar system and present validation results obtained by comparing the RL-measured data with collocated radiosonde, Global Navigation Satellite System (GNSS), and high-resolution objective analysis data. The comparison results showed that RL-derived w agreed within 10 % with values obtained by radiosonde at altitude ranges between 0.14 and 1.5 km in the daytime and between 0.14 and 5–6 km at night in the absence of low clouds; the vertical resolution of the RL measurements was 75–150 m, their temporal resolution was less than 20 min, and the measurement uncertainty was less than 30 %. RL-derived precipitable water vapor values were similar to or slightly lower than those obtained by GNSS at night, when the maximum height of RL measurements exceeded 5 km. The RL-derived w values were at most 1 g/kg (25 %) larger than local analysis data. Four months of continuous operation of the RL system demonstrated its utility for monitoring water vapor distributions for heavy rain forecasting.