Project 4469

This study will first simulate, then verify and ultimately use data from the next generation Global Navigation Satellite System (GNSS) Radio Occultation (RO) technique for meteorological forecasting and climate analysis over the Australian and Antarctic regions. The space-based GNSS RO signals received by the new generation Constellation Observing System for Meteorology Ionosphere & Climate (COSMIC-2) low Earth orbit satellites are refracted by the Earth's atmosphere. Sophisticated atmospheric retrieval processes use this refraction data to determine high precision and high vertical resolution profiles of important atmospheric parameters such as temperature, pressure and water vapour pressure. The COSMIC-2 GNSS RO technique has the potential to become an important new atmospheric monitoring type for operational meteorology, providing significant information on the thermodynamic state of the atmosphere and improving weather forecasting as well as atmospheric and climate analyses.

The project team has made significant progress over the last year. We developed software that reads in GPS Radio Occultation (RO) data received from COSMIC and other Low Earth orbit satellites. The software calculates height profiles of temperature, pressure, water vapour pressure and refractivity at the GPS RO tangent point location (closest approach to the Earth along the GPS-LEO path). The software also reads in radiosonde data from weather balloons, from which we can make direct comparisons with the GPS RO parameter measurements. Weather balloons which are typically released at 0000 UT and 1200 UT daily by weather stations across the world. Our focus is on the Australian and Antarctic regions.

Secondly an investigation by EUMETSAT and the United Kingdom Meteorological Office (UKMO) revealed regions in the Earth's atmosphere where a large percentage of GPS RO events exhibit anomalous bending angle results. The anomalous GPS RO results are characterized as those with L1 bending angles that are greater than the corresponding L2 bending angles. The exact source of these anomalous RO has been unclear to the RO data user community; i.e., data processing artefact or atmospheric phenomenon. We identified the regions of increased occurrence of anomalous RO and explained the mechanisms that produce them and proved our reasoning using 3-D numerical ray tracing simulations of the anomalous GPS RO paths.

The project team has again made significant progress over the last year.

We have extended the functionality of software for numerical simulations by incorporating wave optics technique. The simulation tool allows for computation of phase and amplitude of GNSS signals and their further inversion to geophysical parameters such as bending angle and refractivity. The software utilizes a radioholographic method in the lower troposphere to determine occultation profiles as single-valued functions with respect to altitude (i.e. multipath effect mitigation).

The wave optics propagator was used for the assessment of refractivity mismodeling. When radio occultation measurements are incorporated to weather models, it is commonly assumed that the atmosphere is composed of gaseous components. Hence, the contribution of clouds is neglected. We found that clouds in the lowermost layer of the troposphere, namely planetary boundary layer (PBL), can be frequently observed and should be considered as a valuable source of information about the vertical distribution of refractivity. The altitude of maximum cloud contribution can be used as a indicator of super-refractions that cause inversion errors in radio occultation refractivity.

In the upper troposphere and lower stratosphere, we used monthly means of RO data to compute geostrophic winds. The RO technique demonstrated capability to derive direction and magnitude of jet streams which is comparable to reanalysis data from global atmospheric models.

Another achievement is the software we developed to determine the zenith total delay data from the Victorian GNSS continually operating receiver network. The results are now automatically sent the Australian government Bureau of Meteorology. The test results so far have shown improvement in rainfall forecasts over south-eastern Australia. This technique has the potential to be added to the BoM's forecasting system and to improve weather forecasts which will benefit all Australians

This project set out to investigate some key science questions relating to the monitoring of the atmosphere over the Australian and Antarctic regions using Global Navigation Satellite Systems (GNSS) radio occultation (RO). In recent years, GNSS RO has been adopted by many weather forecasting agencies around the world due to its novelty of un-biases temperature soundings all over the world at all times of the day. This project solved some key problems in the use of GNSS RO. In particular, new knowledge was uncovered as to the cause of anomalous RO events (i.e., events that appeared to have "unphysical" characteristics) that appeared over the polar regions during winter/nighttime hours. It was found that these anomalous RO events were caused by ionospheric gradients, and were therefore found to be "physical" and thus suitable for atmospheric monitoring/weather forecasting purposes. Further, some key issues were identified in the comparison of GNSS RO and radiosonde observations that are important for ongoing efforts in RO measurement quality assessment. Also, some new knowledge was uncovered about the impacts of clouds of GNSS RO observations. Finally, an unexpected outcome of this project was the incorporation of ground-based GPS observations into operational weather forecasting, made possible by the continuous monitoring of water vapour in the atmosphere above Australia using GPS receiver stations. This is a new operational capability that can be expanded to beyond the Australian mainland, including the Antarctic region, in the future.