Estimating mean weighted temperature of the atmosphere for Global Positioning System applications

A water-vapor-weighted vertically averaged mean temperature of the atmosphere, T-m, is a key parameter in the retrieval of total water content from the measurements of Global Positioning System signal delays. The accuracy of precipitable water estimates is proportional to the accuracy of T-m. The geographic and seasonal variability of T-m based on 23 years of radiosonde soundings at 53 globally distributed stations is presented. Several methods for estimating T-m were evaluated by comparing the estimates against the actual T-m values. Site-specific climatology or site-specific linear regression was superior to the geographically and seasonally invariant regression relationship typically used to estimate T-m. Relative errors at most stations were less than 2%, which corresponds to absolute errors of precipitable water of 0.1-0.5 mm. The station-specific linear regression was superior to climatological means as an estimator except in the tropics, where correlations between T-m and T-sfc were not high, Also, a physical model was developed to indicate the relationship between T-m and other commonly used atmospheric variables.