Witold Rohm

Real-time retrieval of precipitable water vapor from GPS precise point positioning

Sensing of precipitable water vapor (PWV) using the Global Positioning System (GPS) has been intensively investigated in the past 2 decades. However, it still remains a challenging task at a high temporal resolution and in the real-time mode. In this study the accuracy of real-time zenith total delay (ZTD) and PWV using the GPS precise point positioning (PPP) technique is investigated. GPS observations in a 1 month period from 20 globally distributed stations are selected for testing. The derived real-time ZTDs at most stations agree well with the tropospheric products from the International Global Navigation Satellite Systems Service, and the root-mean-square errors (RMSEs) are <13 mm, which meet the threshold value of 15 mm if ZTDs are input to numerical weather prediction models. The RMSE of the retrieved PWVs in comparison with the radiosonde-derived values are <= 3 mm, which is the threshold RMSE of PWVs as inputs to weather nowcasting. The theoretical accuracy of PWVs is also discussed, and 3 mm quality of PWVs is proved achievable in different temperature and humidity conditions. This implies that the real-time GPS PPP technique can be complementary to current atmospheric sounding systems, especially for nowcasting of extreme weather due to its real-time, all-day, and all-weather capabilities and high temporal resolutions.

Capturing the Signature of Severe Weather Events in Australia Using GPS Measurements

Rapid developments in satellite positioning, navigation, and timing have revolutionized surveying and mapping practice and significantly influenced the way people live and society operates. The advent of new generation global navigation satellite systems (GNSS) has heralded an exciting future for not only the GNSS community, but also many other areas that are critical to our society at large. With the rapid advances in space-based technologies and new dedicated space missions, the availability of large scale and dense contemporary GNSS networks such as regional continuously operating reference station (CORS) networks and the developments of new algorithms and methodologies, the ability of using space geodetic techniques to remotely sense the atmosphere (i.e., the troposphere and ionosphere) has dramatically improved. Real time GNSS-derived atmospheric variables with a high spatio-temporal resolution have become an important new source of measurements for meteorology, particularly for extreme weather events since water vapour (WV), as the most abundant element of greenhouse gas and accounting for ~70% of global warming, is under-sampled in current meteorological and climate observing systems. This study investigates the emerging area of GNSS technology for near real-time monitoring and forecasting of severe weather and climate change research. This includes both ground-based global positioning system (GPS)-derived precipitable water vapour (PWV) estimation and four-dimensional (4-D) tomographic modeling for wet refractivity fields. Two severe weather case studies were used to investigate the signature of GPS-derived PWV and wet refractivity derived from the 4-D GPS tomographic model under the influence of severe mesoscale convective systems (MCSs). GPS observations from the Victorian state-wide CORS network, i.e., GPSnet, in Australia were used. Results showed strong spatial and temporal correlations between the variations in the ground-based GPS-derived PWV and the passage of the severe MCS. This indicates that the GPS method can complement conventional meteorological observations for the studying, monitoring, and potentially predicting of severe weather events. The advantage of using the ground-based GPS technique is that it can provide continuous observations for the storm passage with high temporal and spatial resolution. Results from these two case studies also suggest that GPS-derived PWV can resolve the synoptic signature of the dynamics and offer precursors to severe weather, and the tomographic technique has the potential to depict the three-dimensional (3-D) signature of wet refractivity for the convective and stratiform processes evident in MCS events. This research reveals the potential of using GNSS-derived PWV to strengthen numerical weather prediction (NWP) models and forecasts, and the potential for GNSS-derived PWV and wet refractivity fields to enhance early detection and sensing of severe weather.

Determining the 4D Dynamics of Wet Refractivity Using GPS Tomography in the Australian Region

The Earth’s climate and weather is a highly dynamic and complex system. Monitoring and predicting meteorological conditions with a high accuracy and reliability is, therefore, a challenging task. Water vapour (WV) has a strong influence on the Earth’s climate and weather due to the large energy transfers in the hydrological process. However, it remains poorly understood and inadequately measured both spatially and temporally, especially in Australia and the southern hemisphere. Four dimensional (4D) WV fields may be reconstructed using a tomographic inversion method that takes advantage of the high density of ground-based GPS Continue Operating Reference Station (CORS) networks. Recent development in GNSS tomography technique based on the dense Australian national positioning infrastructure has the potential to provide near real time 4D WV solutions at a high spatial and temporal resolution for numerical weather prediction, severe weather monitoring and precise positioning. This paper presents a preliminary study using the most advanced state CORS network—GPSnet as a test bed and introduces 4D GPS tomography in Australia and evaluates different parameters for voxel and height resolution and the influence of a priori data through simulations in a controlled field. Preliminary analyses of a real data campaign using a priori information are presented. These preliminary results conclude that the most optimal setup for GNSS tomography models in Victoria is: ∼55 km horizontal resolution and 15 vertical layers with a smaller spacing in the lower troposphere and a larger spacing towards the tropopause. Further analysis will be undertaken to optimize the parameter settings for real data processing. The initial investigation into real data analysis has concluded an overall RMS error of 5.8 ppm with respect to the operational Australian Numerical Weather Prediction (NWP) model for 1 day.

Simulating the Impact of Refractive Transverse Gradients Resulting From a Severe Troposphere Weather Event on GPS Signal Propagation

In this study, the effects of transverse refractive gradients in the ionosphere and in the lower atmosphere on GPS signal paths for both ground-based receivers and receivers on board low Earth orbital satellites are examined. A three-dimensional numerical ray tracing technique, based on geometrical optics, together with the models of the ionosphere, lower atmosphere, and magnetic field, are used to simulate GPS signal propagation. The average transverse refractive gradients were determined from a tropospheric storm event over Melbourne, Australia, on 6th of March, 2010. The traditional GPS ionospheric and atmospheric retrieval methods assume spherical stratification of the refractivity in the atmosphere and typically do not take into consideration the transverse refractivity gradients acting on the GPS signals. The transverse displacements of the GPS signal paths are calculated for both ground-based stations and for low Earth orbit radio occultation paths.

Comparing GPS Radio Occultation Observations with Radiosonde Measurements in the Australian Region

GPS Radio Occultation (RO) is a robust space-based Earth observation technique, with the demonstrated potential for atmospheric profiling and meteorological applications. The GPS RO technique uses GPS receivers onboard Low Earth Orbit (LEO) satellites to measure the received radio signals from GPS satellites to obtain atmospheric profiles such as temperature, pressure, water vapour and electron concentration in the ionosphere using complicated atmospheric retrieval processes. The Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) was launched in April 2006. GPS RO data from this constellation of six LEO micro-satellites provides an observational data type for operational meteorology, providing significant information on the thermodynamic state of the atmosphere with the potential to improve atmospheric analyses and prognoses. Thus it is important to know and understand how COSMIC RO measurements compare to conventional atmospheric and meteorological sounding devices. In this study the COSMIC GPS RO temperature and pressure profiles are compared to those measured from radiosondes (Vaisala RS-92) in the Australian region.

GNSS tomography, from epoch local solution to country-wide near real time system

GNSS meteorology is the remote sensing of the atmosphere (particularly troposphere) using Global Navigation Satellite Systems (GNSS) to derive information about its state. Over past five years the studies on GNSS tomography were performed in the Wroclaw University of Environmental and Life Sciences on the GNSS tomography. The results for epoch wise researches, allows to develop stable tomography numerical model (its optimal size and shape), assess errors, define special constraints, include in the model results of the analysis of the air flow. This paper presents the issues of the Near Real Time (NRT) tomographic model construction, the building of the required IT infrastructure, developing algorithms for effective numerical processing, requirements for NRT products, means of integration with other measurement techniques, comparison with weather prediction models.