Abstract
Precipitable water derived from archived global positioning system (GPS) zenith travel-time delays is used to describe the seasonal and interannual variation of the North American monsoon in California and Nevada. A 3-hourly dataset of precipitable water from 2003 to 2009, for over 500 sites in California and Nevada using temperature and pressure interpolated from the North American Regional Reanalysis (NARR), is constructed to study the temporal and spatial extent of the North American monsoon in the desert regions of California and Nevada. The statistical distribution of precipitable water values is shown to delineate the region that is most often affected by the monsoonal influence. A normalized precipitable water index is employed to indicate when the monsoon starts and stops and to investigate spatial variability. The GPS network provides much higher spatial resolution than other meteorological networks using surface-based methods, such as dewpoint criteria and rainfall, and is seen to contain comparable ability in capturing temporal variations. This dataset reveals the northwestward propagation of the monsoon onset both synoptically and seasonally. The GPS observations indicate that in the mean the decay of the monsoon is less well defined than the onset. Seven-year reanalysis 700-mb geopotential height composites for the monsoon onset and 3 days prior indicate that the onset of the monsoon is associated with a shift in the synoptic pattern characterized by upper-level high pressure building from the east and offshore troughing retreating to the northwest.
