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An Evaluation of Slant Water Vapor From Using a High Resolution Numerical Weather Model

Braun, J., Ha, S.Y., Rocken, C., and Kuo, Y.H., UCAR/COSMIC
Fourteenth Atmospheric Radiation Measurement (ARM) Science Team Meeting

The integral amount of water vapor along a path through the atmosphere is called a measurement of slant water vapor (SW). A ground based GPS station is able to simultaneously measure SW in the direction of all GPS satellites that are visible. A GPS station is typically able to track between six and twelve satellites at any time, with each satellite having a different elevation and azimuth direction with respect to the station. A previous comparison (Braun et al., 2003) of SW measured both by GPS and a pointing microwave radiometer showed a root mean square agreement between the two techniques of 1.3 mm in integrated water vapor. In this study, we have investigated the errors associated with GPS derived SW by evaluating how well the standard processing technique was able to retrieve SW from simulated observations which were created using a high-resolution (3 kilometer horizontal grid resolution) numerical weather prediction model. This simulation depicted the development and movement of a squall line that passed over a large portion of the United States Southern Great Plains region and generated large variations in water vapor over multiple scales. This storm represents a time period when SW observations from a network of GPS stations should be more valuable for characterizing the distribution of water vapor in the atmosphere than zenith-integrated quantities of water vapor. Using this simulation we have determined that SW provides precise measurements of the spatial variation in water vapor, while sometimes containing systematic errors that affect the accuracy of GPS derived SW. Nevertheless, this simulation illustrates the value of SW measurements in multiple directions when compared to a single zenith integrated quantity.

Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).