One-dimensional variational (1-D Var) retrieval of temperature, water vapor, and a reference pressure from radio occultation measurements : A sensitivity analysis

A 1-D Var retrieval study of simulated radio occultation measurements is presented. Temperature and a water vapor profile are retrieved along with a reference pressure to generate the pressure profile by applying the hydrostatic equation. High-resolution European Center for Medium-Range Weather Forecasts (ECMWF) atmospheric fields are used by a ray tracing tool to calculate the exact positions of the tangent point. The 1-D atmospheric profiles following the calculated tangent point trajectory in the 3-D ECMWF fields are used to simulate bending angle measurements with a 1-D forward model. Assimilation of these bending angles in a 1-D Var tool employing the same 1-D forward model is performed. We analyze the sensitivity of the retrieval to changes in vertical resolution, horizontal smearing of the tangent point trajectory, and the assumption of hydrostatic equilibrium for a nonvertical atmospheric scan. We find that retrievals calculated without adequate vertical resolution can have significant errors in temperature and water vapor. Errors in the retrieval by assuming hydrostatic equilibrium for a nonvertical scan generally cause only minor errors in the retrieved profiles. A study into the occurrence of rays curving down toward the Earth surface indicates that about 5–10% of the profiles could experience so-called critical refraction at altitudes between 0.5 km and 2 km in case of the applied high-resolution ECMWF data.Upprättat; 2003; 20070507 (pafi