Estimation of PWC gradients over the Kanto Plain using GPS data: Validation and possible meteorological implications

Simultaneous GPS and water vapor radiometer (WVR) observations were carried out in Tsukuba during May–June 1998, for the validation of precipitable water content (PWC) gradients estimated from single-site GPS data. Slant path PWC observed by WVR were fitted into hourly PWC gradients (WVR gradients) using the least-square method. GPS PWC gradients were retrieved from tropospheric delay gradients that were estimated with GIPSY OASYS 2 package (GIPSY gradients). The results indicate that GIPSY gradients had good, linear correlation with WVR gradients, especially for a large gradient range. Both gradients had spike-shaped, short time-scale (∼ hours) peaks which were mostly associated with synoptic fronts. The GIPSY gradients were also compared with meso-scale PWC gradients calculated from zenith wet delay data of GPS network (NET gradients). The results show that GIPSY gradients did not have very good correlation with NET gradients, and that significant meso-scale discrepancy existed between the two gradients for a cold frontal case on 19 June 1998. One possible reason for this discrepancy is vertical differences in RH gradients, because GIPSY gradients are sensitive to RH gradients around the scale height of humidity (∼2500 m) while RH gradients in lowermost level have largest weights for NET gradients. To study PWC gradients associated with the fronts, GPS gradients were compared with other meteorological data over the Kanto Plain for two frontal cases. The results indicate that large PWC gradient zones with horizontal scale of about several tens kilometers in cross-frontal directions were collocated with the surface wind shear zones of the fronts. This suggests that the large PWC gradients were due to humidity discontinuity around the fronts.