Estimation of Local-Scale Precipitable Water Vapor Distribution Around Each GNSS Station Using Slant Path Delay: Evaluation of a Severe Tornado Case Using High-Resolution NHM

Precipitable water vapor (PWV) derived from a ground-based global navigation satellite system (GNSS) can be regarded as a representative value of the PWV above each GNSS station. It is inherently difficult to capture local-scale water vapor distribution using GNSS-derived PWV. Shoji et al. (2014) proposed a new method that utilizes GNSS slant path delays (SPDs) to estimate the PWV distribution around each GNSS station. To evaluate this new method, we simulated GNSS SPDs using a high-resolution numerical weather prediction model result, emulated GNSS analysis, retrieved PWVs, and compared their accuracy with the conventional method for a severe tornado that occurred in Japan on 6 May, 2012. Comparison results demonstrate the validity of the new method for this case. The conventional procedure introduces a 0.3 to 0.7 mm root mean square error (RMSE) at the GNSS site. Errors made by simple extrapolation increased with distance and reached 1.5 mm at about 1 to 3 km. The distance dependency of PWV errors in the new procedure varied with SPD elevation angle. Using SPD with an elevation angle exceeding 150, we were able to estimate PWV with 1.5mm or better RMSE within 6km from a GNSS station.