The spatial distribution of the atmospheric humidity is difficult to observe, especially for larger domains with dimensions of tens or hundreds of kilometers. The signals of satellite positioning systems like GPS or Galileo are modified by the atmospheric water vapor, and a combination of a large number of such signals provides an opportunity to observe the humidity distribution with high temporal resolution under all weather conditions. The positioning signals are collected by ground-based receiver networks and processed in a certain way in order to separate the humidity information. The information obtained in this way is a nonlocal path-integrated quantity, and tomographic techniques are required to obtain spatially and temporally resolved humidity fields. The whole processing chain from signal detection and signal processing to the final tomographic reconstruction of the atmospheric humidity distribution is presented.

Near real-time estimation of zenith total delays (ZTD) of Global Navigation Satellite Systems (GNSS) signals is operationally performed in Europe. We demonstrate that high accuracy ZTD can be provided even in real-time. Using a state-of-the-art processing strategy we estimate ZTD and horizontal gradients for 162 permanent stations in Europe over 100 days in 2019, covering the event of hurricane Lorenzo. The accuracy of real-time ZTD with respect to the final ZTD product varies from 3.3 to 10.4 mm. The accuracy of real-time ZTD with respect to the ICON numerical weather prediction model varies from 4 mm to 18 mm and we notice station-specific biases, reaching up to ±10 mm. We demonstrate that horizontal gradients show signatures under the severe weather during hurricane Lorenzo in 2019.

Global navigation satellite systems (GNSSs) have revolutionised positioning, navigation, and timing, becoming a common part of our everyday life. Aside from these well-known civilian and commercial applications, GNSS is now an established atmospheric observing system, which can accurately sense water vapour, the most abundant greenhouse gas, accounting for 60–70 % of atmospheric warming. In Europe, the application of GNSS in meteorology started roughly two decades ago, and today it is a well-established field in both research and operation. This review covers the state of the art in GNSS meteorology in Europe. The advances in GNSS processing for derivation of tropospheric products, application of GNSS tropospheric products in operational weather prediction and application of GNSS tropospheric products for climate monitoring are discussed. The GNSS processing techniques and tropospheric products are reviewed. A summary of the use of the products for validation and impact studies with operational numerical weather prediction (NWP) models as well as very short weather prediction (nowcasting) case studies is given. Climate research with GNSSs is an emerging field of research, but the studies so far have been limited to comparison with climate models and derivation of trends. More than 15 years of GNSS meteorology in Europe has already achieved outstanding cooperation between the atmospheric and geodetic communities. It is now feasible to develop next-generation GNSS tropospheric products and applications that can enhance the quality of weather forecasts and climate monitoring. This work is carried out within COST Action ES1206 advanced global navigation satellite systems tropospheric products for monitoring severe weather events and climate (GNSS4SWEC, http://gnss4swec.knmi.nl).

Slant-integrated water vapor (SIWV) data derived from GPS STDs (slant total delays), which provide the spatial information on tropospheric water vapor, have a high potential for assimilation to weather models or for nowcasting or reconstruction of the 3-D humidity field with tomographic techniques. Therefore, the accuracy of GPS STD is important, and independent observations are needed to estimate the quality of GPS STD. In 2012 the GFZ (German Research Centre for Geosciences) started to operate a microwave radiometer in the vicinity of the Potsdam GPS station. The water vapor content along the line of sight between a ground station and a GPS satellite can be derived from GPS data and directly measured by a water vapor radiometer (WVR) at the same time. In this study we present the validation results of SIWV observed by a ground-based GPS receiver and a WVR. The validation covers 184 days of data with dry and wet humidity conditions. SIWV data from GPS and WVR generally show good agreement with a mean bias of −0.4 kg m−2 and an rms (root mean square) of 3.15 kg m−2. The differences in SIWV show an elevation dependent on an rms of 7.13 kg m−2 below 15° but of 1.76 kg m−2 above 15°. Nevertheless, this elevation dependence is not observed regarding relative deviations. The relation between the differences and possible influencing factors (elevation angles, pressure, temperature and relative humidity) are analyzed in this study. Besides the elevation, dependencies between the atmospheric humidity conditions, temperature and the differences in SIWV are found.

The recent development of the International Global Navigation Satellite Systems Service Real‐Time Pilot Project and the enormous progress in precise point positioning (PPP) techniques provide a promising opportunity for real‐time determination of Integrated Water Vapor (IWV) using GPS ground networks for various geodetic and meteorological applications. In this study, we develop a new real‐time GPS water vapor processing system based on the PPP ambiguity fixing technique with real‐time satellite orbit, clock, and phase delay corrections. We demonstrate the performance of the new real‐time water vapor estimates using the currently operationally used near‐real‐time GPS atmospheric data and collocated microwave radiometer measurements as an independent reference. The results show that an accuracy of 1.0 ~ 2.0 mm is achievable for the new real‐time GPS based IWV value. Data of such accuracy might be highly valuable for time‐critical geodetic (positioning) and meteorological applications.

The MSTIDs are wave-like perturbations of the ionospheric plasma, which cause the most common ionospheric disturbances in mid-latitude regions. Generally the MSTIDs have velocities of several hundred meters per second and wavelengths of several hundred kilometers. The wave-like effect of the MSTID is one of the main obstacles for accurate interpolation of ionospheric corrections in a medium-scale reference GPS network. In this paper we show a new method of detecting and modeling MSTIDs using dense German GPS network. The between-epoch single difference ionospheric delays from a medium scale dense GPS network are used to estimate the parameter of the MSTID e.g. amplitude, wavelength and velocity. The efficiency of the approach is tested with data from about 320GPS stations in and near Germany. A MSTID wave moving from east to west across Germany was observed at September 27 in 2009. Its wavelength is about 302km, with a period of ∼7min and velocity of about 700m/s.

In this article, we focus on one case study from the Convective and Orographically-induced Precipitation Study (COPS), which took place in north-eastern France and south-western Germany during the summer of 2007, in a low mountain area. We investigate lee side precipitation due to shallow and deep convection during one Intensive Observation Period (IOP) of COPS which have been well documented by all the instruments. For that aim, we use a set of observations from radars, radiosoundings, satellite, and a network of Global Positioning System (GPS) receivers, as well as meteorological analyses and dedicated model simulation results. The combination of these measurements with GPS tomography results suggests the role of low level water vapour accumulation and convergence as a precursor to the convective initiation. The origin of this moistening and wind convergence seems to be linked to a slight change in the wind direction in the north-west part of the COPS domain. Using a high resolution X band radar, we also describe how small scale orography affects the precipitation locations, and we show the role of hills near the mouths of the valleys in convective enhancement. This observation is confirmed by model simulation showing that convection is no longer enhanced when the hills are suppressed. The further intensification of one convective cell over the Rhine Valley, which is climatologically frequent, is also discussed.

Water vapor plays an important role in meteorological applications; GeoForschungsZentrum (GFZ) therefore developed a tomographic system to derive 3-D distributions of the tropospheric water vapor above Germany using GPS data from about 300 ground stations. Input data for the tomographic reconstructions are generated by the Earth Parameter and Orbit determination System (EPOS) software of the GFZ, which provides zenith total delay (ZTD), integrated water vapor (IWV) and slant total delay (STD) data operationally with a temporal resolution of 2.5 min (STD) and 15 min (ZTD, IWV). The water vapor distribution in the atmosphere is derived by tomographic reconstruction techniques. The quality of the solution is dependent on many factors such as the spatial coverage of the atmosphere with slant paths, the spatial distribution of their intersections and the accuracy of the input observations. Independent observations are required to validate the tomographic reconstructions and to get precise information on the accuracy of the derived 3-D water vapor fields. To determine the quality of the GPS tomography, more than 8000 vertical water vapor profiles at 13 German radiosonde stations were used for the comparison. The radiosondes were launched twice a day (at 00:00 UTC and 12:00 UTC) in 2007. In this paper, parameters of the entire profiles such as the wet refractivity, and the zenith wet delay have been compared. Before the validation the temporal and spatial distribution of the slant paths, serving as a basis for tomographic reconstruction, as well as their angular distribution were studied. The mean wet refractivity differences between tomography and radiosonde data for all points vary from −1.3 to 0.3, and the root mean square is within the range of 6.5–9. About 32% of 6803 profiles match well, 23% match badly and 45% are difficult to classify as they match only in parts.

A numerical algorithm based on Fermat's Principle was developed to simulate the propagation of Global Positioning System (GPS) radio signals in the refractivity field of a numerical weather model. The unique in the proposed algorithm is that the ray‐trajectory automatically involves the location of the ground‐based receiver and the satellite, i.e. the posed two‐point boundary value problem is solved by an implicit finite difference scheme. This feature of the algorithm allows the fast and accurate computation of the signal travel‐time delay, referred to as Slant Total Delay (STD), between a satellite and a ground‐based receiver. We provide a technical description of the algorithm and estimate the uncertainty of STDs due to simplifying assumptions in the algorithm and due to the uncertainty of the refractivity field. In a first application, we compare STDs retrieved from GPS phase‐observations at the German Research Centre for Geosciences Potsdam (GFZ STDs) with STDs derived from the European Center for Medium‐Range Weather Forecasts analyses (ECMWF STDs). The statistical comparison for one month (August 2007) for a large and continuously operating network of ground‐based receivers in Germany indicates good agreement between GFZ STDs and ECMWF STDs; the standard deviation is 0.5% and the mean deviation is 0.1%.

Atmospheric water vapor plays a significant role in atmospheric convection and in the development of clouds and precipitation. As one of the key parameters for modern weather prediction, the atmospheric water vapor has high temporal and spatial variability. The lack of observations of the atmospheric water vapors in space and time limits the accuracy of short-term weather forecasts. Therefore, the spatial and temporal resolution of the atmospheric water vapor observations needs to be improved. Using the Global Positioning System (GPS) in meteorology provides a unique opportunity for this need. Radio signals emitted by the GPS satellites are bent and delayed depending on the temperature, pressure and water vapor. Based on the tropospheric delay, the water vapor distribution within the troposphere can be determined. However, to detect the water vapor distribution with a resolution of kilometer scale in horizontal and sub-kilometer scale in vertical direction, the existing GPS networks must be densified. Due to economic reasons, this densification is recommended with single frequency (SF) receivers. For normal dual-frequency (DF) GPS receivers the observations of the second frequency L2 can be used to eliminate the ionospheric delay by forming a linear combination with the observations of the first frequency L1. In the SF data processing a different ionospheric delay handling is required. In this thesis it is shown that the epoch-differenced ionospheric delay correction is sufficient for estimating the tropospheric delay, e.g., the Zenith Total Delay (ZTD), from SF GPS data. Based on this result, the Satellite-specific Epoch-differenced Ionospheric Delay model (SEID) was developed. In the SEID model the ionospheric corrections for SF data are generated from the observations of surrounding reference stations equipped with DF receivers. With the derived ionospheric corrections and the SF data, pseudo L2 data are generated, which can be processed using existing GPS processing software packages without any changes. In order to evaluate the performance of the SEID model, 24 simulated densification scenarios with different reference station densities and varying numbers of reference stations were defined and investigated. The validations showed very promising results: for densification scenarios with mean distances of the SF station to reference stations below 80 km, the ZTD accuracy of the SF receivers is comparable with those of the DF receivers. The study shows that the ZTD reliability of the SF data is improved with decreased reference station distance and increased number of reference stations. The approach is validated with data from a very dense GPS network with mixed SF and DF receivers in Germany. The ZTDs derived from the SF and DF data were compared. Their differences in Root Mean Square (RMS) are about 3 mm which is negligible compared to the differences due to processing with various state-of-the-art software packages of about 7 mm. To assess the possibility of densifying an existing GPS network with low-cost SF GPS receivers, an evaluation study was carried out. Observations from 258 German DF GPS stations are treated as observations from SF GPS stations, i.e., only L1 GPS observations are used. ZTD, Slant Total Delay (STD) and Slant Water Vapor (SWV) products, derived from the SF data using the SEID model, are validated using tropospheric products derived from DF data, a Water Vapor Radiometer (WVR) and European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. The three Scientific Technical Report STR 12/09 DOI: 10.2312/GFZ.b12099 Deutsches GeoForschungsZentrum GFZ validation studies show that the ZTD, STD and SWV products obtained from SF data are almost of the same high-quality as those from the DF data. Compared to the tropospheric products from the DF data the ZTD from the SF data have an accuracy of 3 mm in RMS, and the relative accuracy of SF STDs is almost constant for all elevation angles and equals ~ 0.18%, which is not degrading with decreasing elevation angles. The SWV between GPS and WVR agree equally well; the standard deviation increases almost linearly from 1.3 kg∙m near the zenith to about 2 kg∙m at 20° elevation. The quality of the tropospheric products derived from SF data is fully adequate for atmosphere sounding. The easy implementation and the accuracy of the SEID model can speed up the densification of existing networks with SF receivers.

A GNSS water vapour tomography system developed to reconstruct spatially resolved humidity fields in the troposphere is described. The tomography system was designed to process the slant path delays of about 270 German GNSS stations in near real-time with a temporal resolution of 30min, a horizontal resolution of 40km and a vertical resolution of 500m or better. After a short introduction to the GPS slant delay processing the framework of the GNSS tomography is described in detail. Different implementations of the iterative algebraic reconstruction techniques (ART) used to invert the linear inverse problem are discussed. It was found that the multiplicative techniques (MART) provide the best results with least processing time, i.e., a tomographic reconstruction of about 26,000 slant delays on a 8280 cell grid can be obtained in less than 10min. Different iterative reconstruction techniques are compared with respect to their convergence behaviour and some numerical parameters. The inversion can be considerably stabilized by using additional non-GNSS observations and implementing various constraints. Different strategies for initialising the tomography and utilizing extra information are discussed. At last an example of a reconstructed field of the wet refractivity is presented and compared to the corresponding distribution of the integrated water vapour, an analysis of a numerical weather model (COSMO-DE) and some radiosonde profiles.

In order to increase the spatial resolution of tropospheric delays derived from GPS observations, existing GPS networks assembled from dual frequency (DF) receivers must be densified. For economic reasons, low‐cost single‐frequency (SF) receivers are considered for the densification. The Satellite‐specific Epoch‐differenced Ionospheric Delay model (SEID) was developed at the German Research Centre for Geosciences (GFZ) to derive the ionospheric corrections for SF GPS receivers. Those corrections allow synthesizing a L2 observable for SF receivers, and existing GPS processing packages can analyze the resulting observables (L1 and synthesized L2) using the same methodology as with DF receivers. The SEID model has already been successfully applied to tropospheric Zenith Total Delay (ZTD) and station coordinates estimation. To assess the possibility of densifying an existing GPS network with low‐cost SF GPS receivers, observations from 258 German DF GPS stations are treated as observations from SF GPS stations (only L1 GPS observations are used). While in a previous study ZTD products were validated, in this study Slant Total Delay (STD) and Slant Water Vapor (SWV) products, derived from SF data and the SEID model, are validated using tropospheric products derived from DF data, a Water Vapor Radiometer (WVR) and a numerical weather model.

In the afternoon of 15 July 2007, a thunderstorm was initiated within a line of cumulus clouds which formed parallel to the crest of the Black Forest mountains during the Intensive Observation Period (IOP) 8b of the Convective and Orographically‐induced Precipitation Study (COPS). This paper extends the analysis of processes that led to convection initiation (CI), i.e. the transition from shallow to deep convection, on this day with the data from several COPS instruments that have not been considered in previous studies. In particular, the boundary‐layer structure, lids and the water‐vapour field in the pre‐convective environment of the event are discussed. For this purpose, we investigated measurements of water‐vapour lidars, temperature lidars and wind lidars, profiles from radiosondes, in situ aircraft data and gridded data of weather stations as well as GPS integrated‐water‐vapour data and satellite imagery. Thermally driven circulation systems formed over both the Black Forest and the Vosges mountain ranges which resulted in local convergence zones. These superimposed with the large‐scale convergence in the Black Forest area. In the presence of sufficient moisture and updraught, clouds formed close to the mountain crests. The related latent‐heat release allowed larger thermals to be produced, which may have had a positive feedback on stabilizing these convergence zones as a whole. We believe that differences in the moisture field explain why convection remained shallow and sparse over the Vosges mountains because these differences were responsible for differences in convective inhibition (CIN). The stationary location of the convergence zone over the southern Black Forest was probably decisive for CI because it constantly transported sensible and latent heat into the area in which CI took place. Copyright © 2011 Royal Meteorological Society

The German Research Centre for Geosciences (GFZ) operates a GNSS water vapour tomography system using about 350 German GNSS stations. The GNSS data processing at the GFZ works in near real-time and provides zenith total delays, integrated water vapour and slant delay data operationally. This large data set of more than 50,000 slant delays per hour is used to reconstruct spatially resolved humidity fields by means of tomographic techniques. It can be expected that additional observations from the future Galileo system provide more information with improved quality. A simulation study covering 12h at 14 July 2009 was therefore started to estimate the impact of GPS, Galileo and GLONASS data on the GNSS tomography. It is shown that the spatial coverage of the atmosphere with slant paths is highly improved by combining observations from two or three satellite systems. Equally important for a reliable tomographic reconstruction is the distribution of slant path intersections as they are required to locate the integrated delay information. The number of intersection points can be increased by a factor of 4 or 8 if two or three systems are combined and their distribution will cover larger regions of the atmosphere. The combined data sets can be used to increase the spatiotemporal resolution of the reconstructed humidity fields up to 30km horizontally, 300m vertically and 15min. The reconstruction quality could not be improved considerably using the currently available techniques.

Two dryline-like humidity drops without considerable temperature change were detected by the ground-based microwave radiometer profiler (MWRP) at the Richard-Assmann-Observatory Lindenberg (52.21 degrees N, 14.12 degrees E) on April 28, 2007. The detailed analysis of these two events includes cloud radar and radar wind profiler measurements at the site as well as data from the surface synoptic network and from integrated water vapour (IWV) maps derived from GPS. The first more pronounced humidity drop is part of a roughly 200 km long line that meets the criterion of a classical dryline or dewpoint front, namely of a moisture gradient larger 3.5 g m(-3) per 100 km. This dewpoint front is ahead of an approaching cold front and is caused by strong downdraft induced by low tropospheric wind shear due to weakening of a midtropospheric high over Germany. It consisted in particular in two kernels of variable size depending on their stage. The fate of the kernels - migration, speed, unification and divorce - is described in detail. Their lifetime was a bit more than 9 hours. The second humidity drop at the site was observed after the passage of the cold front and was caused by dry advection behind the front. Both events are predicted by the numerical weather prediction model COSMO-EU of the German Weather Service to some extent.

In this work, we investigate the relationship between the structure and evolution (from initiation to decay) of precipitation systems, and the associated water vapour distributions during the COPS (Convective Orographically-induced Precipitation Study). This international field campaign took place over an area from the Vosges to the Black Forest Mountains, across the Rhine Valley, in summer 2007. In particular, we consider water vapour retrieval through GPS integrated water vapour 2D maps and 3D tomography, and compare these to precipitation systems observed with the ground-based C-band POLDIRAD weather radar.We have demonstrated the predominant role of water vapour as a precursor to convective initiation for local convective cell generation. Water vapour accumulation on the crest of the orography is associated with ridge convection, while water vapour passing over the mountain top and creating valley outflows generates lee-side convection, often triggered by a small hill positioned within or close to the valley exit, or by a local convergence with the water vapour field over the plain.We have also noted that frontal systems seem to develop preferentially where the largest amount of water vapour is available. Likewise, in the case of frontal systems, well-formed synoptic-scale storms are associated with high water vapour signatures, while weaker systems with embedded convection appear to trail high water vapour areas where the convective element is associated with local water vapour depletion. This latter aspect could be the signature of convective cloud formation, when water vapour is transferred into liquid water, before the onset of precipitation. Copyright (C) 2011 Royal Meteorological Society

Natural hazards and climate change are of major concern to the society. Huge losses are reported in recent years. It is widely believed that modern GNSS technologies are effective in hazard monitoring and climate change detection and modelling. Considering the limitations of current accuracy and reliability, sophisticated strategies and models have to be developed. We introduce and overview recent GNSS activities at GFZ in this field, including ground and satellite based atmospheric sounding, reflectometry and GNSS seismology. In addition we summarize recent hardware developments, where new robust on-site hardware systems combining GNSS receiver and other sensors (e.g. seismic sensors and weather sensors) are developed. The main focus of our contribution is recent results of GNSS analysis software developments for real-time applications, where multi-technique (e.g. SLR and GNSS) and multi-system (e.g. GPS and GLONASS) data source can be handled uniquely. The software can run in real-time as well as post-processing modes, and precision of several mm for ground surface deformation can be achieved. We overview the ground and spaced based GNSS atmosphere researches based on the estimation and assimilation of atmosphere parameters, which are among those parameters estimated from our GNSS software. Related projects, applying these new developments, are also introduced.