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GPS precipitable water measurements used in the analysis of California and Nevada climate

  • Author(s): Means, James Douglas;
  • et al.

Precipitable water (integrated water vapor) can be obtained from zenith travel-time delays of Global Positioning System (GPS) signals, if the atmospheric pressure and temperature at the site are known. There have been large numbers of GPS receivers deployed for geophysics research programs, but unfortunately most of these receivers do not have co-located barometers and thermometers. In this paper archived zenith delays are combined with estimates of GPS site station pressure and temperature from the North American Regional Reanalysis, in order to generate a seven year record of precipitable water at more than 500 sites. The precipitable water values calculated using this method have been found to be in good agreement with GPS precipitable water values from stations with barometers, as well as with radiosonde measurements of precipitable water. Precipitable water has a wide variation across the region, from just a few millimeters in the driest conditions to over 50 mm during strong episodes of the North American Monsoon. The spatial and temporal variations of precipitable water are examined, including the annual and diurnal cycles. Strong annual cycles are seen at almost all sites, and diurnal cycles are also present, increasing away from bodies of water and toward the south, where they reach more than 10% of the daily mean. Precipitable water is found to follow a lognormal distribution at all sites in the region, with some stations showing a small bimodal characteristic due to the influence of the North American Monsoon. An index is proposed that measures the bimodality and hence the "monsooniness" of a site. The elevation dependence of the precipitable water is examined and found to have an exponential decrease which is quite tightly followed in the fall, winter and spring, but more loosely in the summer. It has a greater scale height than has previously been measured elsewhere, ranging from 2.4 km in the winter to 3.1 km in the summer. The landward penetration of water vapor associated with atmospheric river events is examined using several elevation independent parameters and the influence of the atmospheric rivers is seen to be greatest in the northern coastal areas

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