Water vapor lidar observations for forecasting localized heavy rainfall in Tokyo metropolitan area

In recent years, the frequency occurrence of localized heavy rainfall that can cause extensive damages, has been increasing in urban areas of Japan. For early prediction of heavy rainfall, numerical weather prediction model is employed using conventional meteorological observation data. However, the lead time (period of time between the issuance of a forecast and the occurrence of the rainfall) and the accuracy of the prediction are limited because partly of the coarse temporal and spatial resolutions of the observations on the water vapor distribution.

To improve them, we have developed a mobile Raman lidar (RL) and a compact diode-laser-based differential absorption lidar (DIAL) that measure continuously the vertical distribution of water vapor in the lower troposphere with a vertical resolution of 75−150 m and a temporal resolution less than 20 minutes. We have deployed these lidars to the Tokyo metropolitan area and measured continuously the water vapor distribution upwind of cumulus convection when the localized heavy rainfall frequently occurs.

We show the results of the measurements with these lidars in the summer of 2017. We also show the preliminary result of the assimilation of lidar data into the Japan Meteorological Agency non-hydrostatic mesoscale model (JMA-NHM) using an ensemble Kalman filter (EnKF) technique to study the impact of the observed data on the heavy rain forecast.