Using GPS Water Vapor Measurements to Validate, Calibrate, and Improve Microwave Satellite Water Vapor Retrievals

Abstract:

Measurements of total column water vapor can be made by ground-based GPS receivers in all weather conditions. Over the world’s oceans, total column water vapor can also be retrieved from measurements made by satellite-borne microwave imaging radiometers, such as the Special Sensor Microwave Imager (SSM/I). We have analyzed the satellite vapor retrievals in the vicinity of small, isolated islands that host one or more GPS stations that can be processed to produce water vapor measurements. The analysis shows that, in general, there is very good agreement between the two very different types of measurements, with the RMS difference between individual measurements typically less than 1.5 mm and no evidence for system-wide difference in water vapor trends. A more detailed look at the comparison data reveals small inconsistencies. In many cases, we can assign the cause of these to either the GPS station, or the satellite data, by comparing results from different satellites and/or different stations. For example, a discontinuity seen in the GPS/satellite differences for a given station that is similar for a number of different satellites can be assigned to the GPS station. In another example, a comparison of GPS data with an earlier version of the satellite data allowed us to correct a small bias in the satellite retrievals for high values of total column vapor. Because of the high quality of both the GPS and microwave satellite measurements, the results of GPS/satellite intercomparisons are extremely valuable both as a validation tool and a source of information for future improvements.