GPS analysis requires statistical information on temporal variations of precipitable water vapor content (PWC), since temporal variability of PWC is assumed in the analyses. Spatial scales of PWC variations also are necessary for incorporation of the analyzed GPS data into numerical weather prediction systems. The objective of the present study is to investigate temporal and spatial characteristics of the PWC variations in Japan. For this purpose, slant-path PWC was observed with ground-based microwave radiometers (MWRs) for the directions of GPS satellites in the eastern part of the Kanto Plain during several observation periods in 2000 and 2001. Three components, vertical PWC, horizontal gradient and higher-order inhomogeneity, were retrieved from the slant-path observation data. Deviations of vertical PWC from ten-day averages (hereafter referred to as vertical deviations) were then calculated in order to remove seasonal changes of this component. Simultaneous observation data at three MWR sites also were used for the rough estimation of horizontal scales of the three PWC components.
The results show: 1) The vertical deviations marked were about 20 times as large amplitudes as the other two components, while the variations due to the gradient had even smaller amplitudes than the inhomogeneity; 2) The vertical deviations had large spectral power against periods around 5-6 days and 8-9 days, while the gradient was dominated by diurnal variations; and, 3) It was roughly estimated that the vertical deviations (gradient) had the horizontal scales of several hundred (several ten) kilometers. The horizontal scale of the PWC inhomogeneity was considered to be less than 10 km. The vertical deviations (the gradient) were considered to be closely related with large-scale meteorological disturbances (local-scale circulations) on the basis of the above temporal and spatial characteristics.