GGOS Atmosphere

In modern geodesy and in particular in space geodetic techniques various effects of the atmosphere have to be considered. The atmosphere delays (or advances) radio signals emitted by satellites, e.g. of the GNSS (Global Navigation Satellite Systems), or by distant radio sources observed by VLBI (Very Long Baseline Interferometry), atmosphere pressure loading causes deformation of the Earth`s surface up to more than one centimeter, Earth gravity observations from dedicated satellites have to be reduced for atmospheric influences, and a considerable part of the variations of Earth rotation (polar motion, length of day) is due to processes in the atmosphere. Thus, the atmosphere plays an important role for the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) with its central theme `Global deformation and mass exchange processes in the System Earth`. In recent years, models for all these effects related to the atmosphere have been developed which are based on data from numerical weather models. However, the parameters describing each of these effects have been determined at various institutions, using different weather models with different resolutions, and applying different geophysical models and hypotheses. The overall goal of project `GGOS Atmosphere` is to determine consistent and homogenous models for (1) atmosphere loading corrections, (2) atmosphere angular momentum functions, and (3) gravity field coefficients for the atmosphere based on a common data stream with predominantly the same underlying meteorological parameters like pressure, temperature, humidity, and wind velocity. Main tasks of the project are the download of high-resolution data sets from the European Centre for Medium- Range Weather Forecasts (ECMWF) extracting the best models with the highest spatial and temporal resolution available, and to consistently determine all three target quantities mentioned above. The influence of different ECMWF data classes and of the various geophysical models on these quantities will be investigated, and as soon as best-suited data classes and geophysical models are found the routines will be implemented at the supercomputing facilities of the ECMWF to save the download of the enormous data amounts. Pressure loading corrections, angular momentum functions and gravity field coefficients for the atmosphere will be consistently determined for the whole history of space geodetic observations and provided to the international science community. This will contribute to a better understanding of the System Earth that is based on a detailed knowledge of the interactions between geometry, rotation and gravity field of the Earth.

After a series of detailed tests with various data sets and model development, four geodetic parameters have been determined consistently from one common data stream of the European Centre for Medium-Range Weather Forecasts (ECMWF) and are now provided operationally to the international scientific community. These four parameters are: (1) angular momentum time series of the atmosphere which are necessary to explain and predict Earth rotation variations; (2) atmospheric pressure loading parameters describing the deformation of the solid Earth due to pressure changes; (3) gravity field coefficients of the atmosphere which are needed to reveal hydrological variations and mass changes observed by gravity space missions; and (4) delays of microwave signals in the troposphere in order to improve position estimates, e.g. from Global Positioning System (GPS) observations. For the first time, all these parameters have been determined consistently, including the consistent treatment of underlying models, like the application of the same topography and reference pressure, identical models for atmospheric (thermal) tides, or the treatment of the oceanic response to pressure variations. Since one of the major goals of the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) is the integration of geometry, rotation, and gravity field of the Earth, this project is a strong contribution to GGOS.Moreover, many other research topics have been addressed in the course of the project. The applicability of the torque approach for the atmosphere as alternative to the angular momentum approach for polar motion investigations at intra-seasonal time scales has been assessed, thereby deriving valuable information on the reliability of present-day atmospheric re-analysis systems. Furthermore, the concept of ray-tracing through numerical weather models for the correction of space geodetic observations has been successfully tested as well as the application of the gravity field coefficients for the reduction of superconducting gravimeters. Based on the tropospheric delays computed operationally, a new climatological model has been developed which will be very useful for positioning and navigation purposes.Findings of the project have been presented in a series of papers, in talks and posters at many international conferences, and in lectures at the Vienna University of Technology. The latter activities finally resulted in the edition of a textbook on atmospheric effects in space geodesy which will be useful for many students and researchers working in that field.