Retrieving temperature, water vapour and surface pressure information fromrefractive-index profiles derived by radio occultation: A simulation study

A one-dimensional variational retrieval for the assimilation of refractive- index profiles derived from radio occultation (RO) measurements has been de veloped. The method is tested by using simulated data to assess the retriev al accuracy and information content of the measurements, using various real istic estimates for the assumed error distributions. Theoretical retrieval- error estimates given by a solution covariance matrix are in good qualitati ve agreement with those derived statistically from the comparison of the so lution profiles with the precisely defined 'true' values. It is demonstrate d that the 'water vapour ambiguity' inherent in more conventional RO invers ion methods is resolved with this approach. It is found that the solution c hi(2) values, quantifying the fit to the observed refractivities and a prio ri profile estimates, are in good agreement with the theoretical distributi on, suggesting that they could be used for quality-control purposes. Furthe rmore, it is shown that the measurements contain significant surface pressu re information. This arises through the hydrostatic relationship, as a resu lt of mapping the state-vector information on pressure levels to height coo rdinates. The simulations indicate that the measurements in the tropics con tain the greatest surface pressure information, with a reduction of the bac kground error of similar to 45%. This new result has significance for the d esign of future observing systems.