Remote sensing of refractivity from space for global observations of atmospheric parameters

Occultation methods based on effects connected with refraction of electromagnetic waves of both optical and radio frequency domains were numerously used by astronomers for investigations of planetary atmospheres during last decades. Application of these methods for remote sensing of terrestrial atmosphere was delayed essentially due to insufficient possibilities of space based technique to provide necessary accuracy and both temporal and spatial frequency of observations. The successful development of Global Positioning Satellite Systems during the last years on one hand, and plans for launching a great number of Low-Earth Orbit satellites, capable to carry GPS receivers, during the next several years on the other hand, provide an opportunity to create a space radio-occultation system for global observation of atmospheric parameters (pressure, temperature, humidity) and their real-time assimilation together with other observations in Atmospheric General Circulation Models. Among the set of problems connected with the development of such a system is the creation of inversion algorithms that enable to achieve maximum possible accuracy of retrieval of meteorological parameters from refraction data. This problem can be investigated by means of computational simulation prior to the realization of the space system. This report presents the first results of computational simulations on the retrieval of meteorological parameters from space refractometric data on the basis of the ECHAM 3 model developed at the Max Planck Institute for Meteorology (Roeckner et a1. 1992). For this purpose the grid fields of temperature, geopotential and humidity available from the model were interpolated and a continuous spatial field of refractivity (together with its first derivative) was generated. This field was used for calculating the trajectories of electromagnetic rays for the given orbits of transmitting and receiving satellites and for the determination of the quantities (incident angles or Doppler frequency shifts) being measured at receiving satellite during occultation. These quantities were then used for solving the inverse problem — retrieving the distribution of refractivity in the vicinity of the ray perigees. The retrieved refractivity was used to calculate pressure and temperature (using the hydrostatic equation and the equation of state). The results were compared with initial data, and the retrieval errors were evaluated. The study shows that the refractivity can be retrieved with very high accuracy in particular if a tomographic reconstruction is applied. Effects of humidity and temperature are not separable. Stratospheric temperatures globally and upper tropospheric temperatures at middle and high latitudes can be accurately retrieved, other areas require humidity data. Alternatively humidity data can be retrieved if the temperature fields are known