The importance of atmospheric water vapor in weather and climate processes is well understood (see for example the reports from the AGU Chapman Conferences on Atmospheric Water Vapor in 1994, 1999 and 2008). Less well understood is the accuracy and precision of moisture observations that are analyzed and assimilated into atmospheric models over time for the purposes of weather forecasting, climate monitoring and research. The majority of our information about moisture variability over the open ocean comes from polar orbiting satellites equipped with microwave sounders. In contrast to nadir infrared and microwave soundings made over land, the estimation of temperature and moisture from radiances measured in the microwave portion of the spectrum over the ocean is a well posed problem that is usually carried out with high reliability under most weather conditions. Quantification of the characteristics of these data and derived products usually occurs soon after launch, and checks are made sporadically thereafter with campaigns or intensive observing periods of finite duration. However, provisions to monitor the accuracy and precision of these measurements over time, along with model analyses and predictions that utilize these observations, are essentially absent in operational meteorological organizations.

In early 2010, NOAA's Earth System Research Laboratory (ESRL) installed continuously operating GPS water vapor sensing systems on two offshore oil platforms far from land in the Gulf of Mexico. The purposes were to 1) provide continuous total column water vapor and near-surface meteorological observations to the NWS Weather Service Offices in the region; 2) provide continuous geodetic observations to the Louisiana Spatial Reference Center at LSU to help them improve precise positioning along the Gulf Coast; and 3) allow ESRL to evaluate the feasibility of making independent long-term observations to monitor the characteristics of polar and geostationary satellite observations over water. Later in 2010, the NWS Pacific Region began installing continuously operating GPS water vapor sensing systems at weather forecast offices in the Western Pacific.

This paper presents the results of relatively long term comparisons of GPS total column water vapor estimates with polar orbiting satellite measurements over the Gulf of Mexico, and compares the results with GPS measurements made on islands in the Western Pacific.