Ulrich Foelsche

Prospects of the EPS GRAS Mission For Operational Atmospheric Applications

Abstract Global Navigation Satellite System (GNSS) Receiver for Atmospheric Sounding (GRAS) is a radio occultation instrument especially designed and built for operational meteorological missions. GRAS has been developed by the European Space Agency (ESA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) in the framework of the EUMETSAT Polar System (EPS). The GRAS instrument is already flying on board the first MetOp satellite (MetOp-A) that was launched in October 2006. It will also be on board two other MetOp satellites (MetOp-B and MetOp-C) that will successively cover the total EPS mission lifetime of over 14 yr. GRAS provides daily about 600 globally distributed occultation measurements and the GRAS data products are disseminated to the users in near–real time (NRT) so that they can be assimilated into numerical weather prediction (NWP) systems. All GRAS data and products are permanently archived and made available to the users for climate applications and sc...

Atmospheric temperature change detection with GPS radio occultation 1995 to 2008

[1] Existing upper air records of radiosonde and operational satellite data recently showed a reconciliation of temperature trends but structural uncertainties remain. GPS radio occultation (RO) provides a new high-quality record, profiling the upper troposphere and lower stratosphere with stability and homogeneity. Here we show that climate trends are since recently detected by RO data, consistent with earliest detection times estimated by simulations. Based on a temperature change detection study using the RO record within 1995―2008 we found a significant cooling trend in the tropical lower stratosphere in February while in the upper troposphere an emerging warming trend is obscured by El Nino variability. The observed trends and warming/cooling contrast across the tropopause agree well with radiosonde data and basically with climate model simulations, the latter tentatively showing less contrast. The performance of the short RO record to date underpins its capability to become a climate benchmark record in the future.

GNSS Occultation Sounding for Climate Monitoring

Considerable efforts are currently invested into the setup of a Global Climate Observing System (GCOS) for monitoring climate change over the coming decades, which is of high relevance given concerns on increasing human influences. A promising potential contribution to the GCOS is a suite of spaceborne Global Navigation Satellite System (GNSS) occultation sensors for global long-term monitoring of atmospheric change in temperature and other variables with high vertical resolution and accuracy. Besides the great importance with respect to climate change, the provision of high quality data is essential for the improvement of numerical weather prediction and for reanalysis efforts. We review the significance of GNSS radio occultation sounding in the climate observations context. In order to investigate the climate change detection capability of GNSS occultation sensors, we are currently performing an end-to-end GNS

Occultations for Probing Atmosphere and Climate

occultation observing system simulation experiment over the 25-year period 2001 to 2025. We report on this integrated analysis, which involves in a realistic manner all aspects from modeling the atmosphere via generating a significant set of simulated measurements to an objective statistical analysis and assessment of 2001-2025 temporal trends. Q 200 1 Elsevier

Tropospheric water vapor imaging by combination of ground-based and spaceborne GNSS sounding data

Occultations for Probing Atmosphere and Climate: Setting the Scene.- 1. Occultation Methodology in General.- Wave Optics Algorithms for Processing Radio Occultation Data in the Lower Troposphere: A Review and Synthesis.- The Radio-Holography Approach for GNSS Occultation Data Analysis: Review and Application to Resolving Fine Structures in the Atmosphere and Mesosphere.- Open Loop Tracking and Inverting GPS Radio Occultation Signals: Simulation Study.- Fourier Analysis of GNSS-LEO Radio Occultation Signals, Resolution and Limitations.- Canonical Transform Methods for Radio Occultation Data.- Unfolding of Radio Occultation Multipath Behavior Using Phase Models.- Abel Integral Inversion in Occultation Measurements.- Does a Priori Information Improve Occultation Measurements?.- Retrieval of Atmospheric Refractivity Profiles from Ground-Based GPS Measurements.- 2. GNSS-LEO Occultation.- GRAS-SAF Radio Occultation Data from EPS/Metop.- Deviations from a Hydrostatic Atmosphere in Radio Occultation Data.- Sensitivity of GNSS Occultation Profiles to Horizontal Variability in the Troposphere: A Simulation Study.- Advancement of GNSS Radio Occultation Retrieval in the Upper Stratosphere.- Ensemble-Based Analysis of Errors in Atmospheric Profiles Retrieved from GNSS Occultation Data.- Refractivity Profiles Obtained by Abel Inversion from a Down Looking GPS Radio Occultation Experiment at Mt. Fuji: Preliminary Results and Future Plan.- 3. LEO-LEO Occultation.- An Active Microwave Limb Sounder for Profiling Water Vapor, Ozone, Temperature, Geopotential, Clouds, Isotopes and Stratospheric Winds.- An Overview of the University of Arizona ATOMS Project.- The ACE+ Mission: An Atmosphere and Climate Explorer Based on GPS, GALILEO, and LEO-LEO Radio Occultation.- Simulating the Influence of Horizontal Gradients on Retrieved Profiles from ATOMS Occultation Measurements - A Promising Approach for Data Assimilation.- Water Vapor Profiling Using Absorptive Occultation Measurements: A Comparison Between SAGE III and ATOMS.- The Genesis of the ACE+ Anti-Rotating Satellites Concept.- 4. Stellar and Solar Occultation.- The Stellar Occultation Technique: Past Achievements, Recent Developments, and Future Challenges.- Envisat/GOMOS Stellar Occultation: Inversion Schemes and First Analyses of Real Data.- Atmospheric Density, Pressure and Temperature Profile Reconstruction from Refractive Angle Measurements in Stellar Occultation.- Stratospheric Temperature and Ozone Sounding with ENVISAT/GOMOS Stellar Occultation.- Information Approach to Channel Selection for Stellar Occultation Measurements.- The Solar Occultation Mission ACE: An Overview.- Mesospheric Temperature and Ozone Sounding by the SMAS Solar Occultation Sensor.- 5. Use of Occultation Data.- Utility of Occultations for Atmospheric Wave Activity Studies: Results of GPS/MET Data Analyses and Future Plan.- Stratospheric Gravity Wave Fluctuations and Sporadic E at Mid-Latitudes with Focus on Possible Effects of the Andes.- The Detection of Upper Level Turbulence via GPS Occultation Methods.- Evaluation of Refractivity Profiles from CHAMP and SAC-C GPS Radio Occultation.- Ionospheric Radio Occultation Measurements and Space Weather.- The Mars Atmospheric Constellation Observatory (MACO) Concept.- Author Index.

Pre-Operational Retrieval of Radio Occultation Based Climatologies

The Global Navigation Satellite System (GNSS) comprises the U.S. system GPS (Global Positioning System), its Russian pendant GLONASS, and presumably, in the future, the European system Galileo. The potential of GNSS-based phase delay measurements for accurately estimating vertically and slant-path-integrated water vapor has been demonstrated recently for radio links between GPS satellites and ground-based GPS receivers. GNSS-based radio occultation, on the other hand, has been demonstrated via the GPS/Meteorology experiment to deliver accurate near-vertical profiles of atmospheric variables such as temperature and humidity with high vertical resolution. Height-resolving imaging of atmospheric water vapor becomes feasible when occultation profiles from spaceborne receivers in Low Earth Orbits (LEO) are combined with ground-based GNSS data from a colocated receiver network. We developed a two-dimensional, height-resolving tomographic imaging technique following the Bayesian approach for optimal combination of information from different sources. Using simulated GNSS-based water vapor measurements from LEO and ground, we show representative results derived from simple synthetic refractivity fields as well as from a realistic refractivity field based on a European Centre for Medium-Range Weather Forecasts (ECMWF) analysis. For cases located poleward of ∼40° we found a new simple mapping function to perform best within our forward model scheme, where the only free parameter is the climatological scale height in the troposphere, the exact value of which is not critical. The mapping function exploits the ratio between the straight-line ray path length within the first two scale heights above surface and the “effective height” defined by these first two scale heights. We found our technique capable of reconstructing atmospheric features like water vapor maxima near the top of the trade wind inversion. Adjustment of the integral over the water vapor profile measurements to the horizontally averaged ground-based vertical integrated water vapor data efficiently mitigates potential biases in the former data. Accuracies are best in areas with high absolute humidities but also over drier areas such as Finland, useful two-dimensional information can still be obtained. Thus it is attractive to apply the developed technique in a next step to real data.

The CHAMPCLIM Project: An Overview

CHAMPCLIM is a joint project of WegCenter/UniGraz and GFZ Potsdam. The overall aim of the project is to exploit the CHAMP (CHAllenging Minisatellite Payload for geoscientific research) radio occultation (RO) data in the best possible manner for climate monitoring. This paper focuses on describing the pre-operational status and technical aspects of the CHAMPCLIM processing system at WegCenter/UniGraz. For creating RO based climatologies we ingest, on the one hand, the complete CHAMP RO dataset provided by GFZ at excess phase level (GFZ level 2, ∼180 profiles/day), which is processed to obtain atmospheric profiles of refractivity, geopotential height, and temperature (in future also humidity). On the other hand, we use operational atmospheric analysis fields from the European Centre for Medium-Range Weather Forecasts (ECMWF), at T42L60 resolution, as reference for quality control and evaluation. For delivering climatologies operationally, which will be prepared at monthly, seasonal, and annual time scales, our aim is to provide them with a delay of at most two weeks after the last measurement (e.g., JJA 2003 seasonal climatology available by September 14, 2003, latest). The climatologies are set up in overlapping equal-area and non-overlapping almost equal-area grids. In order to monitor the error characteristics of the climatologies, various types of error statistics (vs. ECMWF analyses) are performed. The main emphasis of this paper lies on processing the complete 2002–2004 data — starting from March 2002 when the CHAMP data stream became stable and quasi continuous — and on creation of climatologies including error estimates. The spatial set up of the climatologies, exemplary seasonal climatologies (as far as processed) as well as preliminary climatological error estimates are presented.

Global Climatologies Based on Radio Occultation Data: The CHAMPCLIM Project

The CHAMP radio occultation (RO) data provide the first opportunity to create real RO based climatologies on a longer term. CHAMPCLIM is a joint project of the Institute for Geophysics, Astrophysics, and Meteorology (IGAM) in Graz and the GeoForschungsZentrum (GFZ) in Potsdam. The overall aim of CHAMPCLIM is to ensure that the CHAMP RO data are exploited in the best possible manner, in particular for climate monitoring. The main objectives of the CHAMPCLIM project can be summarized in form of three areas of study as follows: RO data processing advancements for optimizing climate utility, RO data and algorithms validation based on CHAMP/GPS data, and global RO based climatologies for monitoring climate change. Here we show a summary of the current activities and exemplary results.

New Horizons in Occultation Research

The German/US research satellite CHAMP (CHAllenging Minisatellite Payload for geoscientific research) continuously records about 230 radio occultation (RO) profiles per day since March 2002. The mission is expected to last at least until 2007, thus CHAMP RO data provide the first opportunity to create RO based climatologies on a longer term. CHAMPCLIM is a joint project of the Wegener Center for Climate and Global Change (WegCenter) in Graz and the GeoForschungsZentrum (GFZ) in Potsdam. It aims at exploiting the CHAMP RO data in the best possible manner for climate research. For this purpose, CHAMP excess phase data provided by GFZ are processed at WegCenter with a new retrieval scheme, especially tuned for monitoring climate variability and change. The atmospheric profiles which pass all quality checks (∼150 profiles/day) are used to create climatologies on a monthly, seasonal, and annual basis. Here, we focus on dry temperature climatologies from the winter season (DJF) 2002/03 to the summer season (JJA) 2004, obtained by averaging-and-binning. The results show that useful dry temperature climatologies resolving horizontal scales >1000 km can be obtained even with data from a single RO receiver. RO based climatologies have the potential to improve modern operational climatologies, especially in regions where the data coverage and/or the vertical resolution and accuracy of RO data is superior to traditional data sources.

Sensitivity of GNSS Occultation Profiles to Horizontal Variability in the Troposphere: A Simulation Study

Several GPS Radio Occultation (RO) missions (GRACE-A (GRavity And Climate Experiment), FORMOSAT-3/COSMIC (FORMOsa SATellite mission-3/Constellation Observing System for Meteorology, Ionosphere, and Climate), and MetOp) started data provision in 2006 and 2007. Together with the measurements from CHAMP (CHAllenging Minisatellite Payload, since 2001) and the recently launched (June 15, 2007) TerraSAR-X an operational multi-satellite constellation for precise GPS based atmospheric sounding became reality. The data base is supplemented by measurements from SAC-C (Satelite de Aplicaciones Cientificas-C). Our contribution briefly reviews current GFZ activities regarding processing and application of GPS RO data from different satellites. These activities include precise satellite orbit determination and the provision of near-real time analysis results for weather forecast centers within 2 h after measurement. Available satellite data are used for climatological investigations of global gravity wave characteristics.