We have developed procedures for analyzing several kilometer scale water vapor variation using slant path delays (SPD) retrieved from ground-based GNSS (Global Navigation Satellite System) stations. The information on water vapor inhomogeneity could be utilized as a supplementary monitoring tool of development of cumulus convections.
Following MacMillan (1995), the slant path delay (SPD) between a GNSS satellite and a receiver at the elevation angle ∏ and direction angle ∆ can be written in the following form:
where ZTD is zenith total delay, Gn and Ge are delay gradient, and µ is postfit residual. The postfit residuals contain information on higher-order atmospheric inhomogeneity (HI). Shoji et al. (2004) demonstrated that the horizontal scale of the ZTD can be considered as about 600 km, the gradient component (Gn and Ge) as 60 km, and the HI as 2 to 3 km. This result insists that ZTD, Gn and Ge, and HI relate to atmospheric motion of the meso- , meso- , and meso- scales, respectively.
Shoji (2013) introduced two new indices indicating the degree of anisotropy of water vapor utilizing the GNSS SPDs as:
(1) Water vapor concentration (WVC) index
Inner product of nabla operator and gradient vector (Gn and Ge)
(2) Water vapor inhomogeneity (WVI) index
Standard deviation of µ after removing non-atmospheric noises
The relationships between these indices and precipitation were examined statistically. The results indicate that the anisotropy indices are more strongly correlated with strong rainfall than PWV (Precipitable Water Vapor), whereas PWV seems to be related to weak and/or modest precipitation. These relations hold true for both present and upcoming precipitation.
Shoji et al. (2014) proposed a procedure for estimating the PWV distribution around ground-based GNSS stations on a scale of several kilometers. This procedure utilizes the difference between the ZTD above a GNSS station and the zenith mapped SPD. This difference can be used to estimate the PWV gradient in each SPD direction by assuming an exponential distribution for the horizontal water vapor gradient.
The procedure was tested using an estimation of the PWV variation associated with the parent storm of an F3 Fujita scale tornado that occurred in Ibaraki prefecture on May 6, 2012. The PWV gradient estimated using the proposed procedure revealed a several kilometer scale strong PWV gradient and its enhancement near the tornado.
The results suggest that these information on water vapor inhomogeneity and gradient derived from GNSS analysis reflect local water vapor variation associated with convection phenomena and can be potentially used for an additional monitoring tool of thunderstorms.