Deviation from a hydrostatic atmosphere in radio occultation data

A 1D Var sensitivity study of simulated radio occultation measurements is presented. Temperature and water vapor profiles 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 with a ray tracing tool to calculate the exact positions of the tangent point. The atmospheric profiles following the calculated tangent points trajectory in the 3D ECMWF fields are used to simulate bending angle measurements with a 1D forward model. Assimilation of these bending angles in a 1D Var tool employing the same ID forward model is performed. We analyze the sensitivity of the retrieval to the assumption of hydrostatic equilibrium for a non-vertical atmospheric scan. Deviations of more than 1% from a hydrostatic pressure profile can occur for certain mid-latitude and polar conditions. The effect upon the retrieval capabilities is usually negligible except for the most extreme cases.