Wanqiang Yao

Combination of Ground- and Space-Based Data to Establish a Global Ionospheric Grid Model

The global ionospheric maps (GIMs) that are established using ground-based Global Navigation Satellite System (GNSS) data are important means to study the variations of the ionosphere. However, the uneven distribution of ground GNSS stations, which particularly exist at large gaps in the vast ocean areas, results to low accuracy and reliability of GIMs in the marine region and other areas that lack GNSS sites. The ocean altimetry satellites orbit can cover most of the marine areas, and dual-frequency signals can obtained vertical total electronic content (VTEC) at the nadir track. Low-Earth-orbit occultation observations also obtain much global ionospheric uniform distribution information. The combination of the space-based ionospheric data and ground-based GNSS observation data can effectively improve the accuracy and reliability of GIM in marine areas. However, the systematic bias that exists between ionospheric data obtained by different systems must also be considered during the data combination. This paper used both ground-based GNSS data and space-based data to establish a global ionospheric model, whereas the systematic bias between the space-based ionospheric data and ground-based GNSS data can be seen as parameters to estimate. The results show that, by adding space-based data, the accuracy of GIM on the ocean areas has been improved to make up the deficiencies of the existing GIMs.

Real-time precise point positioning-based zenith tropospheric delay for precipitation forecasting

GPS-based Zenith Tropospheric Delay (ZTD) estimation should be easily obtained in a cost-effective way, however, the most previous studies focus on post-processed ZTD estimates using satellite orbit and clock products with at least 3–9 hours latency provided by International GNSS Service (IGS), which limits the GNSS meteorological application for nowcasting. With the development of IGS’s real-time pilot project (RTPP), this limitation was removed by April, 2013 as real-time satellite orbit and clock products can be obtained on-line. In this paper, on the one hand, the GPS-derived ZTD estimation was evaluated using the IGS final and real-time satellite products based on independently developed PPP software. On the other hand, the analysis of the time series of GPS-derived ZTD by least-square fitting of a broken line tendency for a full year of observations, and a forecasting method for precipitation is proposed based on the ZTD slope in the ascending period. The agreement between ZTD slope and the ground rainfall records suggested that the proposed method is useful for the assisted forecasting, especially for short-term alarms.

Troposphere Water Vapour Tomography: A Horizontal Parameterised Approach

Global Navigation Satellite System (GNSS) troposphere tomography has become one of the most cost-effective means to obtain three-dimensional (3-d) image of the tropospheric water vapour field. Traditional methods divide the tomography area into a number of 3-d voxels and assume that the water vapour density at any voxel is a constant during the given period. However, such behaviour breaks the spatial continuity of water vapour density in a horizontal direction and the number of unknown parameters needing to be estimated is very large. This is the focus of the paper, which tries to reconstruct the water vapor field using the tomographic technique without imposing empirical horizontal and vertical constraints. The proposed approach introduces the layered functional model in each layer vertically and only an a priori constraint is imposed for the water vapor information at the location of the radiosonde station. The elevation angle mask of 30◦ is determined according to the distribution of intersections between the satellite rays and different layers, which avoids the impact of ray bending and the error in slant water vapor (SWV) at low elevation angles on the tomographic result. Additionally, an optimal weighting strategy is applied to the established tomographic model to obtain a reasonable result. The tomographic experiment is performed using Global Positioning System (GPS) data of 12 receivers derived from the Satellite Positioning Reference Station Network (SatRef) in Hong Kong. The quality of the established tomographic model is validated under different weather conditions and compared with the conventional tomography method using 31-day data, respectively. The numerical result shows that the proposed method is applicable and superior to the traditional one. Comparisons of integrated water vapour (IWV) of the proposed method with that derived from radiosonde and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim data show that the root mean square (RMS)/Bias of their differences are 3.2/−0.8 mm and 3.3/−1.7 mm, respectively, while the values of traditional method are 5.1/−3.9 mm and 6.3/−5.9 mm, respectively. Furthermore, the water vapour density profiles are also compared with radiosonde and ECMWF data, and the values of RMS/Bias error for the proposed method are 0.88/0.06 g/m3 and 0.92/−0.08 g/m3, respectively, while the values of the traditional method are 1.33/0.38 g/m3 and 1.59/0.40 g/m3, respectively.

Studies of precipitable water vapour characteristics on a global scale

ABSTRACT Atmospheric water vapour plays an important role in hydrological, global climate change, atmospheric, and meteorological processes. In this study, precipitable water vapour (PWV) data set for 2004–2017 was first estimated with an average accuracy of about 1.28 mm globally using the products provided by the International Global Navigation Satellite System Service and Global Geodetic Observation System Atmosphere and then the spatio-temporal trends of PWV variation were characterized. Periodic signals of the annual, semi-annual, and seasonal variations of PWV time series were detected based on the Lomb–Scargle periodogram and analysed by dividing the whole world into five geographical zones. From a global perspective, the average PWV has an increasing trend, which may be caused by global warming effects and anthropogenic activities. Analysis of different PWV amplitudes also shows that the main component of the PWV is annual amplitude except in low latitude zones. In addition, the PWV differences between weekends and weekdays for four seasons are also analysed globally, and the result indicates that the weekend effects caused by anthropogenic activity depend on season and region

Modeling the plasmasphere based on LEO satellites onboard GPS measurements: MODELING PLASMASPHERE BASED ON LEO GPS

The plasmasphere, which is located above the ionosphere, is a significant component of Earth’s atmosphere. A global plasmaspheric model was constructed using the total electron content (TEC) along the signal propagation path calculated using onboard Global Positioning System observations from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) and MetOp-A, provided by the COSMIC Data Analysis and Archive Center (CDAAC). First, the global plasmaspheric model was established using only COSMIC TEC, and a set of MetOp-A TEC provided by CDAAC served for external evaluation. Results indicated that the established model using only COSMIC data is highly accurate. Then, COSMIC and MetOp-A TEC were combined to produce a new global plasmaspheric model. Finally, the variational characteristics of global plasmaspheric electron content with latitude, local time, and season were investigated using the global plasmaspheric model established in this paper.