Gottfried Kirchengast

A new dynamic approach for statistical optimization of GNSS radio occultation bending angles for optimal climate monitoring utility

[1] Global Navigation Satellite System (GNSS)-based radio occultation (RO) is a satellite remote sensing technique providing accurate profiles of the Earth’s atmosphere for weather and climate applications. Above about 30km altitude, however, statistical optimization is a critical process for initializing the RO bending angles in order to optimize the climate monitoring utility of the retrieved atmospheric profiles. Here we introduce an advanced dynamic statistical optimization algorithm, which uses bending angles from multiple days of European Centre for Medium-range Weather Forecasts (ECMWF) short-range forecast and analysis fields, together with averaged-observed bending angles, to obtain background profiles and associated error covariance matrices with geographically varying background uncertainty estimates on a daily updated basis. The new algorithm is evaluated against the existing Wegener Center Occultation Processing System version 5.4 (OPSv5.4) algorithm, using several days of simulated MetOp and observed CHAMP and COSMIC data, for January and July conditions. We find the following for the new method’s performance compared to OPSv5.4: 1.) it significantly reduces random errors (standard deviations), down to about half their size, and leaves less or about equal residual systematic errors (biases) in the optimized bending angles; 2.) the dynamic (daily) estimate of the background error correlation matrix alone already improves the optimized bending angles; 3.) the subsequently retrievedrefractivityprofilesandatmospheric(temperature)profilesbenefit by improvederror characteristics,especiallyabove about 30km. Based on theseencouraging results, we work to employ similar dynamic error covariance estimation also for the observed bending angles and to apply the method to full months and subsequently to entire climate data records.

Error analysis and characterization of atmospheric profiles retrieved from GNSS occultation data

The performance of the Global Navigation Satellite System (GNSS) based radio occultation method for providing retrievals of atmospheric profiles up to the mesosphere was investigated by a rigorous Bayesian error analysis and characterization formalism. Starting with excess phase profile errors modeled as white Gaussian measurement noise, covariance matrices for the retrieved bending angle, refractivity/density, pressure, and temperature profiles were derived in order to quantify the accuracy of the method and to elucidate the propagation of statistical errors through subsequent steps of the retrieval process. We assumed unbiased phase errors (the occultation method is essentially self-calibrating), spherical symmetry in the occultation tangent point region (reasonable for most atmospheric locations), and dry air (disregarding humidity being of relevance below 10 km in the troposphere only) in this baseline analysis. Because of the low signal-to-noise ratio of occultation data at mesospheric heights, which causes instabilities in case of direct inversion from excess phase profiles to atmospheric profiles, a Bayesian approach was employed, objectively combining measured data with a priori data. For characterization of the retrievals we provide, in addition to covariance estimates for the retrieved profiles, quantification of the relationship between the measured data, the retrieved state, the a priori data, and the true state, respectively. Averaging kernel functions, indicating the sensitivity of the retrieval to the true state, contribution functions, indicating the sensitivity of the retrieval to the measurement, and the ratio of retrieval errors to a priori errors are shown. Two different sensor scenarios are discussed, respectively, an advanced receiver (AR) scenario with 2 mm and a standard receiver (SR) scenario with 5 mm unbiased RMS error on excess phase data at 10 Hz sampling rate. The corresponding bending angle, refractivity, pressure, and temperature retrieval properties are shown. Temperature, the final data product, is found to be accurate to better than 1 K below ∼40 km (AR)/∼35 km (SR) at ∼2 km height resolution and to be dominated by a priori knowledge above ∼55 km (AR)/∼47 km (SR), respectively. For all data products the use of a Bayesian framework allowed for a more complete and consistent quantification of properties of profiles retrieved from GNSS occultation data than previous work.

The gravity wave-TID relationship: insight via theoretical model-EISCAT data comparison

Abstract Atmospheric Gravity Waves (AGWs) in the thermosphere are of particular interest because of their role in the equatorward redistribution of auroral momentum and energy input. However, their direct measurement is difficult and so they are normally traced by their ionospheric signatures, the Traveling Ionospheric Disturbances (TIDs). These can be routinely observed, especially with incoherent scatter radars like the EISCAT-facility, which measure all the fundamental ionospheric parameters. In order to reliably infer AGW parameters from TID data, however, one needs to know the physics of the AGW-TID relationship as comprehensively as possible. We investigated this relationship by means of one-to-one comparison of theoretical model results with EISCAT data for several TID events. The relevant physics, the modeling procedure and the results of the comparisons are discussed. As a representative example, one typical event is presented in some detail. We found that the AGW-TID relationship can be quantitatively understood by means of careful physical modeling. A particular simulated TID shows quantitative consistency with a particular TID in EISCAT data only for a quite specific model-AGW ; thus, comprehensive AGW infonnation can be deduced by our method. We conclude that our use of TID ‘polarization information’ along a single incoherent scatter beam is basically as valuable for the unique determination of a causative AGW as is traditional TID ‘propagation/dispersion information’. The latter, however, requires several distributed stations. Finally, we address the possibility that radars like EISCAT could be used in future WAGS (Worldwide AGW Study) campaigns to provide almost real-time information on AGW activity for the benefit of mid-latitude monitoring stations.

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

Elucidation of the physics of the gravity wave-TID relationship with the aid of theoretical simulations

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

Ionosphere tomography with data from satellite reception of Global Navigation Satellite System signals and ground reception of Navy Navigation Satellite System signals

The physics of the relationship between atmospheric gravity waves (AGWs) and traveling ionospheric disturbances (TIDs) is thoroughly investigated with emphasis on large-scale AGW/TIDs at F region heights at middle and high latitudes (provided field-perpendicular drifts are small). In support, simulations using a realistic AGW model (“Clark-based AGW model”) in combination with a realistic ionospheric model (“Graz Ionospheric Flux Tube Simulation (GIFTS) model”) were performed. All fundamental AGW/TID quantities are treated consistently, i.e., perturbations in neutral densities, wind, and temperature as well as disturbances in electron density, ion drift, and ion and electron temperature. The AGW-induced ionospheric response is inspected for all TlD quantities, based on their governing conservation equations. The results are discussed by means of detailed and approximative formulae as well as instructive figures which provide a firm quantitative understanding significantly beyond the current state of knowledge. Especially the physics of the electron temperature disturbance (Te-TID), up to now not yet quantitatively inspected, is thoroughly explored. A major finding is that the disturbance in specific terms of the electron energy equation is an order of magnitude more pronounced than the net disturbance determining the strength of the Te-TID. Furthermore, simulation results illustrating the natural variability of the AGW/TID quantities (1) due to varying AGW properties and (2) due to changing thermosphere/ionosphere background conditions are discussed. Features observed include the following: AGW period and magnetic field line-induced south-north asymmetries are major causes of variability. Change from high/moderate to low solar activity enhances amplitudes of most AGW/TID quantities, electron density and temperature being likely exceptions; nighttime conditions tend to lower amplitudes versus daytime. The insight gained is valuable from a basic research point of view and also for suitable AGW/TID descriptions for thermosphere/ionosphere weather modeling.

Gravity Wave Spectra from GPS/MET Occultation Observations

GPS/MET, a multichannel Global Positioning System (GPS) receiver onboard the small research satellite MicroLab 1, is the first example of a research tool of great importance for ionospheric research. In the near future, other satellites with GPS/GLONASS (Global Navigation Satellite System (also GNSS)) receivers will be launched. Their main purpose is lower atmosphere research, but because of the necessity to correct for plasma influences, “ionospheric” data will be available as a side product. The occultation of GNSS signals offers the possibility to gain very good quality height profiles of electron density by means of classical inversion techniques. The profiles are averaged horizontally. This paper concentrates on the possibility to complement inversion results with electron content data gained on the ground using beacon signals of low orbiting satellites (e.g., the U.S. Navy Navigation Satellite System (NNSS)). The data combination offers several possibilities for ionospheric tomography. Several GNSS scanning satellite scenarios are assessed, and their ionospheric imaging/tomography merits are discussed. An example result for the inversion of GPS/MET data is shown. The results of simulations with model ionosphere data are used to demonstrate tomographic reconstruction techniques based on the combination of “space” and “ground” electron content. The simulation results have direct applicability to observed data.

Occultations for Probing Atmosphere and Climate

Abstract The potential utility of radio occultation data in general, and of data from the Global Positioning System/Meteorology (GPS/MET) experiment in particular, for studying atmospheric gravity waves is discussed. Based on a validated set of ∼270 GPS/MET-derived temperature profiles, the authors produced and analyzed mean vertical wavenumber power spectra of normalized temperature fluctuations in three latitude bands (low, middle, high) within the lower stratosphere (∼15–30 km), where data accuracy was proven highest. The Fresnel diffraction limited vertical resolution and the limited height range of the dataset restricted this initial investigation to medium- to large-scale waves with vertical wavelengths of about 2–5 km. The deduced vertical wavenumber power spectra were compared with a saturation spectrum predicted by gravity wave saturation theory and generally found consistent with the theoretical saturation limit. The low-latitude power spectra exhibited almost saturation, with spectral power abo...