North American Monsoon Onset In California As Indicated by GPS Precipitable Water

Precipitable water or integrated water vapor can be obtained from zenith travel-time delays from Global Positioning System (GPS) signals if the pressure and temperature at the GPS site is known. We have constructed a 3-hourly data set of precipitable water dating back to 2003 for over 500 sites in California and Nevada. Here we use this data set to study the onset of the North American Monsoon in the desert regions of California, using the normalized precipitable water index to indicate when the monsoon starts and stops. This has the advantage of having much higher spatial resolution than surface-based methods, such as dewpoint criteria and rainfall. The spatial and temporal resolution allows us to see the northwestward propagation of the monsoon onset both synoptically and seasonally and see the variation in monsoon duration.