Retrieval of Water Vapor Inhomogeneity Using the Japanese Nationwide GPS Array and its Potential for Prediction of Convective Precipitation

Procedures for retrieving two indices indicating the degree of inhomogeneity of water vapor using the carrier phase of a global positioning system (GPS) have been introduced. One index describes the spatial concentration of water vapor, while the other indicates higher-order water vapor inhomogeneity. Horizontal scales of the two indices are approximately considered to be 60 km and 2-3 km, respectively. The characteristics of the water vapor field over Japan in August 2011 were studied using the spatiotemporal variation in these two indices along with GPS-derived precipitable water vapor (PWV). The monthly averaged indices indicate distinct diurnal variation in the mountainous region of central Honshu and coincidence with the diurnal variation in precipitation frequencies in the area. The relations between these indices and precipitation were examined statistically. The results indicate that the inhomogeneity indices are more strongly correlated with strong rainfall than PWV. PWV seemed to relate to precipitation of less than 10 mm h−1, but did not exhibit much of a relation with precipitation greater than 10 mm h−1. These relations hold true for both present and imminent precipitation. The spatiotemporal variations in the indices of a thunderstorm on August 11, 2011, were also examined. Both water vapor concentration (WVC) and higher-order inhomogeneity indicated an increase ahead of the initiation of convective precipitation. The results suggest that the two GPS-derived indices of water vapor inhomogeneity reflect local variations in the water vapor associated with the convection phenomena and can potentially be used for monitoring thunderstorms.