Climatological signals from long term behaviors of atmospheric zenith delays and their gradients from the Japanese dense GPS array

Atmospheric delays estimated by GPS together with station coordinates are known to provide valuable information for meteorological studies. Last year, Heki and Yoshida (2009 AGU Fall Meeting) reported climatological signals in the long term behaviors of the atmospheric water vapor over the last 12 years using zenith wet delays (ZWD) derived by combining zenith total delays (ZTD) at ~1000 receivers of GEONET GPS stations in Japan and surface meteorological data from ~200 observatories. ZWD can be converted to precipitable water vapor (PWV), and long-term changes of PWV presented climatological signals including interannual changes due to El Nino/Southern Oscillation (ENSO), longer-term changes such as North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), and secular increase due to global warming. At that time, we used the so-called F2 solution by GSI (Geospatial Information Authority), a government institute operating the GEONET. Recently, a new generation solution (F3 solution) has been publicized from GSI. In addition to various kinds of updating in GPS data analyses, an important improvement is the inclusion of the tropospheric delay gradient as additional parameters to be estimated. We studied time series of the tropospheric delay gradients 2004-2010, and found that (1) almost all stations had time-averaged gradient of ~1 mm toward south, and (2) southward gradients are stronger in winter. The point (1) simply indicates higher temperature (i.e. more water vapor) in the south, and the point (2) would reflect stronger north-south temperature gradient in Japan in winter. On the other hand, the gradient vectors showed insignificant time-averaged east-west components. Miyazaki et al. [2003] studied the relationship between the positioning errors and the atmospheric delay gradients, i.e. negligence of gradient may result in position deviation in the opposite direction to the gradient vectors, especially around the typhoon and the weather front. Vertical components of the position are also affected by tropospheric gradients, i.e. they are anti-correlated with the north components of the gradients. We are going to justify these tendencies over a long term by comparing station coordinates between F2 and F3 solutions. Iwabuchi et al. [2003] investigated the influence of atmospheric delay gradients on the estimated ZTD values, i.e. tropospheric delay gradient of 1 mm results in ~1 mm error of the ZTD if neglected. We also try to confirm these results comparing estimated ZTD values between F2 and F3 solutions. Tropospheric delays are the sum of those by dry atmosphere and water vapor. Here we try to separate their contributions to the observed tropospheric delay gradient by using available meteorological data, e.g. sea level pressure and water vapor, and spatial gradients of the estimated ZTD and ZWD.