dc.description.abstract | This dissertation presents efforts to assimilate FORMOSAT-3/COSMIC (F3/C) GPS Occultation Experiment (GOX) observations into the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE-GCM) by means of ensemble Kalman filtering (EnKF). The F3/C electron density profiles (EDPs) globally distributed which provide an excellent opportunity to study the three-dimensional (3D) ionospheric electron density structure. NCAR TIE-GCM simulates the Earth’s thermosphere and ionosphere by self-consistently solving for the coupled nonlinear equations of hydrodynamics, neutral and ion chemistry, and electrodynamics. F3/C EDPs are combined with the TIE-GCM simulations by an EnKF algorithms implemented in the NCAR Data Assimilation Research Testbed (DART) to obtain ’the best’ estimate of the current ionospheric state.
Since the thermosphere and ionosphere are dominated by external forces, it is difficult to generate ensemble members simply by perturbing the model initial conditions. First of all, model sensitivity experiments are designed to examine the model responses associated with forcing parameters such as the solar 10.7cm flux, the hemispheric power, the cross-tail potential, and the phase and amplitude of migrating tides. Furthermore, multiplying factors of the vertical E × B drift are introduced to diversify peak height altitude of electron density distribution over height. According to the model sensitivity experiments, the ensemble generation strategy is decided for following comprehensive EnKF study.
Observing system simulation experiments are conducted in the primary stage to investigate the optimal ensemble size, suitable horizontal and vertical localization lengths, and performance of assimilation system for F3/C EDPs as well as approaches to prevent unphysical values of state vectors. The results suggest that this EnKF assimilation system for coupled thermosphere-ionosphere model can be effectively applied to assimilate the F3/C EDPs by using 90 ensemble members, setting the vertical localization length equal to 1,000 km with additional simple bounds for particular state vectors.
Assimilation analyses obtained with real F3/C EDPs during geomagnetic quiet time and disturbed conditions are further compared with independent ground-based observations and global ionospheric maps as well as the F3/C profiles themselves. The comparison shows the improvement of the primary ionospheric parameters such as NmF2 and hmF2 and furthermore suggests that some small scale physically meaningful structures that are absent in TIE-GCM simulations are resolved. Nevertheless, some unrealistic signatures appear in the results, and high rejection rates of observations due to the outlier threshold and quality control are also found in the assimilation experiments. These issues are analyzed to identify the model biases, for example, the nighttime O+ flux at the upper boundary in TIE-GCM is decreased to reduce the nighttime rejection rates.
In the near future, the FORMOSAT-7/COSMIC-2 (F7/C2) constellation will be deployed in 2016 and provide more than 12,000 occultation soundings per day. An additionally OSSE is conducted in this dissertation to determine the impact of F7/C2 on ionospheric weather monitoring and forecasting. Results show that the denser F7/C2 observations can reconstruct 3D ionospheric structure with a dramatically shorter data accumulation period, and significantly reduce the assimilation analysis error. The results also represent a major advance in ionospheric weather monitoring of the F7/C2 mission.
Finally, this dissertation presents comprehensive investigation of the DART/TIE-GCM assimilation system for the F3/C ionospheric observations. There are a number of promising results as reported herein. However, it still needs more investigations and analysis to improve the system to be used as an operational system for monitoring and forecasting the ionospheric space weather in the future. | en_US |