Signature of the atmospheric compressibility factor in COSMIC, CHAMP, and GRACE radio occultation data

It is shown that the deviation of air from an ideal gas has nonnegligible effects when assimilating GPS radio occultation (GPSRO) data in a Numerical Weather Prediction (NWP) system. Therefore an assimilation system that aims to be unbiased to within the threshold of detection should account for this effect. GPSRO data are vertically referenced in terms of mean sea level altitude. Many other data types are vertically referenced in pressure units. The assimilation system may use yet another vertical coordinate. The required transformations between vertical coordinate systems should not induce significant biases. In the context of NWP the threshold of detection for a systematic height bias is on the order of 1-2 m. This study demonstrates that this level of accuracy cannot be obtained unless the deviation of air from an ideal gas, known as compressibility factor, is properly taken into account. With the current volume of GPSRO data an inconsistency between pressure and altitude scales larger than the mentioned threshold can lead to the development of nonnegligible biases in NWP assimilation cycles. Consideration of the compressibility factor realigns the altitude- and pressure-based scales to better than 1 m in the entire troposphere. Impacts are appreciated not only from global averages but from zonal averages as well.