A global, 2-hourly atmospheric precipitable water dataset from ground-based GPS measurements for diurnal cycle and other climate studies

The main goals of this study are 1) to develop an analysis technique for deriving and continuously updating a near real-time, global, 2-hourly data set of atmospheric precipitable water (PW) using existing ground-based Global Positioning System (GPS) measurements of zenith path delay (ZPD), and 2) to apply the PW data to study the diurnal variations in PW over the globe. The dataset can also be used for other climate studies, such as estimating the diurnal sampling errors in twice-daily radiosonde humidity and quantifying errors/biases in global radiosonde PW data. The advantages of GPS-derived PW, including high temporal sampling resolution, availability under all weather conditions, long term stability, low cost, and large coverage, make the PW dataset unique for climate studies. The GPS ZPD data are currently available every two hours at about 380 GPS ground stations from 1997 to the present as part of the International GPS Service (IGS) products. Diurnal variations in PW over North America have been studied by analyzing 30-min PW data for 1996-2002 from GPS observations at 54 North America stations. Significant diurnal variations of PW are found over most of the North America. The diurnal (24 hour) cycle, S1, explains >50% of the sub-daily variance and has an amplitude of 1.0-1.8 mm in the central and eastern U. S. during summer and is weaker in other seasons. The S1 peaks around noon in winter and from mid-afternoon to midnight in summer. The semidiurnal (12 hour) cycle is weak, with an amplitude of a few tenths of 1 mm. This study will analyze the global, 2-hourly GPS-PW data to study diurnal variations in water vapor over the globe. Underlying physical processes will also be investigated by examining relationships with other atmospheric parameters, such as surface wind and divergence fields, precipitation and atmospheric pressure tides.