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 wa ter 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 tropos pheric delay gradients that were estimated with GIPSY OASYS 2 package (GIPS Y gradients). The results indicate that GIPSY gradients had good, linear co rrelation with WVR gradients, especially for a large gradient range. Both g radients had spike-shaped, short time-scale (similar to3 hours) peaks which were mostly associated with synoptic fronts, The GIPSY gradients were also compared with mesoscale PWC gradients calculated from zenith wet delay dat a of GPS network (NET gradients). The results show that GIPSY gradients did not have very good correlation with NET gradients, and that significant me so-scale discrepancy existed between the two gradients for a cold frontal c ase on 19 June 1998. One possible reason for this discrepancy is vertical d ifferences in RH gradients, because GIPSY gradients are sensitive to RH gra dients around the scale height of humidity (similar to 2500 m) while RH gra dients in lowermost level Rave 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 r esults indicate that large PWC gradient zones with horizontal scale of abou t several tens kilometers in cross-frontal directions were collocated with the surface wind shear zones of the fronts. This suggests that the large PW C gradients were due to humidity discontinuity around the fronts,