The Diurnal Cycle of GPS Precipitable Water in California

California and Nevada have large numbers of GPS receivers designed to study earthquakes and plate motions. We have used archived travel time delay data from these sites, combined with North American Regional Reanalysis (NARR) data to calculate the precipitable water (PW) at each site from the delays. This allows us to make regional high-resolution maps of PW. Currently we have done this for the time period 2002--present, with values every three hours. We have analyzed this data set to determine if a diurnal signal is present in PW. We have found a measurable diurnal signal in PW across much of the study region, with the strongest signal seen during the summer months in the south, and the weakest signals seen in the cooler months in the north. This is as might be expected, considering that wintertime diurnal signals can be masked by the passage of mid-latitude baroclinic disturbances.