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

A global, 8-year (1997-2004), 2-hourly data set of atmospheric precipitable water (PW) has been produced from ground-based Global Positioning System (GPS) measurements of zenith path delay (ZPD). An analysis technique was developed to convert ZPD to PW and was validated by comparing radiosonde and microwave radiometer data. The PW data are available every two hours at about 350 International GPS Service (IGS) ground stations from 1997 to 2004. The advantages of GPS-derived PW include high temporal sampling resolution, availability under all weather conditions, high accuracy (~2 mm in PW) and long term stability. 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 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.