GNSS掩星大气参数反演中电离层残差模拟研究
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摘要
GNSS掩星大气探测技术已广泛应用于数值天气预报和全球气候监测中。电离层是掩星大气探测的主要误差源之一，双频弯曲角线性组合法是目前应用最广泛的电离层误差改正方法。由于GNSS信号传播路径的弯曲分离和电离层高阶项的影响，经该方法改正后，反演大气参数中仍含有电离层残余误差。电离层残余误差是GNSS掩星反演中高层大气参数的主要误差。最大限度地降低电离层残余误差有利于实现GNSS掩星中高层大气高精度观测。弯曲角电离层残余误差的定性和定量研究对发展新的电离层误差改正方法意义重大。
用ECMWF大气模式和COSMIC数据，对比分析了太阳活动“宁静”期和太阳活动“活跃”期弯曲角误差特性。结果表明，太阳活动“活跃”期弯曲角标准偏差较大；平均偏差与“宁静”期相比具有明显的“负值趋向性”；电离层残余误差对平流层顶部（35~50km）和中间层底部（50~65km）弯曲角影响显著。
以MSIS90大气模式和3D NeUoG电离层模式为大气背景，模拟分析了不同电离层条件下掩星事件的弯曲角电离层残差。结果表明：弯曲角电离层残差是中间层和平流层顶部掩星大气反演参数的主要误差，其大小与太阳活动强度、地方时、掩星平面方位角密切相关。电离干扰会使弯曲角电离层残余误差增大数倍至二十多倍。
量化研究了平流层底部（15~35km）、平流层顶部（35~50km）、中间层底部（50~65km）和中间层顶部（65~80km）的区域日平均弯曲角电离层残余误差。以MSIS90大气模式和3D NeUoG电离层模式为大气背景，用GPS/MetOp-A真实轨道数据仿真模拟了2008年7月15日全天的掩星事件。弯曲角电离层残余误差分析过程中，全球被划分为GLO (global)、NHH (north hemisphere high latitude)、NHM (north hemisphere middle latitude)、EDT (equatorial day time)、SHM (southhemisphere middle latitude)和SHH (south hemisphere high latitude)六个统计区域。分析了太阳活动强度、纬度带和电离层局部球对称对弯曲角电离层残差的影响。结果表明：弯曲角电离层平均偏差是一种负的系统性偏差，且随太阳活动强度的增强而增大。六个统计区域中，EDT的弯曲角电离层偏差最大，中间层顶部、中间层底部和平流层顶部的弯曲角电离层残差平均偏差分别可达0.048μrad、0.041μrad和0.032μrad；SHH的弯曲角电离层残差平均偏差最小，其大小几乎为零。弯曲角电离层残余误差的量化研究对中高层大气弯曲角电离层残余误差的建模和修正有一定的参考价值。
用三维射线追踪法模拟分析了弯曲角电离层残差的产生机理，并对印度洋区域的电离层残差异常现象进行了初步分析。结果表明，“入射线”和“出射线”的电子密度分布不对称是弯曲角电离层误差异常的主要原因。
The global navigation satellite systems (GNSS) radio occultation (RO) techniquehas been widely used to observe the atmosphere for applications of numerical weatherprediction and global climate monitoring. The ionosphere is a major error source inGNSS RO measurements and an ionosphere-free linear combination ofdual-frequency bending angles derived from RO is commonly used to mainly removethe first-order ionospheric effect. However, the residual ionopheric error (RIE) is stillconsiderable, thus it needs to be further mitigated for high accuracy applications,especially in upper air where the RIE is more severe. To effectively mitigate the RIEeffect, characterization and quantification of the bending angle RIEs are important forobtaining benchmark-quality upper-air RO retrievals.
A comparison study of the bending angle errors in high solar activity periods andlow solar activity periods was conducted by using the European centre for mediumrange weather forecasts (ECMWF) model and constellation observing system formeteorology, ionosphere, and climate (COSMIC) data. The results show that thebending angle RIE is a main error source in the upper stratosphere (US) and lowermesosphere (LM).
End-to-end simulations and a detailed analysis of single-event RIEs have beenperformed to investigate the characteristics and magnitude of the bending angle RIEsin various ionospheric conditions. The results illustrate that the bending angle RIE issignificant in the mesosphere (MS) and US; its magnitude is dependant upon localtime, the intensity of solar activity and the direction of the RO plane; and ionosphericdisturbances can enlarge the bending angle RIEs by several to more than20times.
This research has for the first time quantified daily-zonal-mean bending angleRIEs in the impact height layers of the lower stratosphere (LS), US, LM and uppermesosphere (UM) using end-to-end simulation. A global ensemble of one-day ROevents were simulated and divided into six geographic zones named global (GLO),north hemisphere high latitude (NHH), north hemisphere middle latitude (NHM),equatorial day time (EDT), south hemisphere middle latitude (SHM) and southhemisphere high latitude (SHH). In the simulation, the MSIS-90atmospheric modeland the3D NeUoG ionospheric model were used. The simulated bending angleprofiles were compared with the reference—the ones simulated using the neutralatmosphere only, for calculating the biases, standard deviations and uncertainties of the bending angle RIEs. The variations of the bending angle RIEs with solar activity,latitudinal region, and with and without the assumption of ionospheric sphericalsymmetry were assessed.
The results show that the layer-average bending angle RIE biases in the US, LMand UM height layers and in all the six zones have an obvious negative tendency andthe magnitude of their absolute values increases with the rise of solar activity level.Comparison of the bending angle RIE biases of the six zones indicated that themaximum layer-average RIE biases, located in the EDT zone, in the UM, LM and USlayers are0.048μrad,0.041μrad and0.032μrad respectively. The minimumlayer-average RIE biases are in the SHH zone with the values close to zero. Theseresults suggest that the bending angle RIEs tend to have negative systematicbiases.This research has significance since these simulation results can be a referencefor the calibration of bending angles derived from real RO observations in future‘s ROdata processing.
In addition, the mechanism of the bending angle RIEs was also investigatedusing the3D ray tracing technique and those RO events with exceptionally large RIEswere analyzed. The results indicate that the asymmetry of ionospheric electron densityalong inbound‘and outbound‘ray paths is the major cause of the exceptionally largebending angle RIEs.
用ECMWF大气模式和COSMIC数据，对比分析了太阳活动“宁静”期和太阳活动“活跃”期弯曲角误差特性。结果表明，太阳活动“活跃”期弯曲角标准偏差较大；平均偏差与“宁静”期相比具有明显的“负值趋向性”；电离层残余误差对平流层顶部（35~50km）和中间层底部（50~65km）弯曲角影响显著。
以MSIS90大气模式和3D NeUoG电离层模式为大气背景，模拟分析了不同电离层条件下掩星事件的弯曲角电离层残差。结果表明：弯曲角电离层残差是中间层和平流层顶部掩星大气反演参数的主要误差，其大小与太阳活动强度、地方时、掩星平面方位角密切相关。电离干扰会使弯曲角电离层残余误差增大数倍至二十多倍。
量化研究了平流层底部（15~35km）、平流层顶部（35~50km）、中间层底部（50~65km）和中间层顶部（65~80km）的区域日平均弯曲角电离层残余误差。以MSIS90大气模式和3D NeUoG电离层模式为大气背景，用GPS/MetOp-A真实轨道数据仿真模拟了2008年7月15日全天的掩星事件。弯曲角电离层残余误差分析过程中，全球被划分为GLO (global)、NHH (north hemisphere high latitude)、NHM (north hemisphere middle latitude)、EDT (equatorial day time)、SHM (southhemisphere middle latitude)和SHH (south hemisphere high latitude)六个统计区域。分析了太阳活动强度、纬度带和电离层局部球对称对弯曲角电离层残差的影响。结果表明：弯曲角电离层平均偏差是一种负的系统性偏差，且随太阳活动强度的增强而增大。六个统计区域中，EDT的弯曲角电离层偏差最大，中间层顶部、中间层底部和平流层顶部的弯曲角电离层残差平均偏差分别可达0.048μrad、0.041μrad和0.032μrad；SHH的弯曲角电离层残差平均偏差最小，其大小几乎为零。弯曲角电离层残余误差的量化研究对中高层大气弯曲角电离层残余误差的建模和修正有一定的参考价值。
用三维射线追踪法模拟分析了弯曲角电离层残差的产生机理，并对印度洋区域的电离层残差异常现象进行了初步分析。结果表明，“入射线”和“出射线”的电子密度分布不对称是弯曲角电离层误差异常的主要原因。
The global navigation satellite systems (GNSS) radio occultation (RO) techniquehas been widely used to observe the atmosphere for applications of numerical weatherprediction and global climate monitoring. The ionosphere is a major error source inGNSS RO measurements and an ionosphere-free linear combination ofdual-frequency bending angles derived from RO is commonly used to mainly removethe first-order ionospheric effect. However, the residual ionopheric error (RIE) is stillconsiderable, thus it needs to be further mitigated for high accuracy applications,especially in upper air where the RIE is more severe. To effectively mitigate the RIEeffect, characterization and quantification of the bending angle RIEs are important forobtaining benchmark-quality upper-air RO retrievals.
A comparison study of the bending angle errors in high solar activity periods andlow solar activity periods was conducted by using the European centre for mediumrange weather forecasts (ECMWF) model and constellation observing system formeteorology, ionosphere, and climate (COSMIC) data. The results show that thebending angle RIE is a main error source in the upper stratosphere (US) and lowermesosphere (LM).
End-to-end simulations and a detailed analysis of single-event RIEs have beenperformed to investigate the characteristics and magnitude of the bending angle RIEsin various ionospheric conditions. The results illustrate that the bending angle RIE issignificant in the mesosphere (MS) and US; its magnitude is dependant upon localtime, the intensity of solar activity and the direction of the RO plane; and ionosphericdisturbances can enlarge the bending angle RIEs by several to more than20times.
This research has for the first time quantified daily-zonal-mean bending angleRIEs in the impact height layers of the lower stratosphere (LS), US, LM and uppermesosphere (UM) using end-to-end simulation. A global ensemble of one-day ROevents were simulated and divided into six geographic zones named global (GLO),north hemisphere high latitude (NHH), north hemisphere middle latitude (NHM),equatorial day time (EDT), south hemisphere middle latitude (SHM) and southhemisphere high latitude (SHH). In the simulation, the MSIS-90atmospheric modeland the3D NeUoG ionospheric model were used. The simulated bending angleprofiles were compared with the reference—the ones simulated using the neutralatmosphere only, for calculating the biases, standard deviations and uncertainties of the bending angle RIEs. The variations of the bending angle RIEs with solar activity,latitudinal region, and with and without the assumption of ionospheric sphericalsymmetry were assessed.
The results show that the layer-average bending angle RIE biases in the US, LMand UM height layers and in all the six zones have an obvious negative tendency andthe magnitude of their absolute values increases with the rise of solar activity level.Comparison of the bending angle RIE biases of the six zones indicated that themaximum layer-average RIE biases, located in the EDT zone, in the UM, LM and USlayers are0.048μrad,0.041μrad and0.032μrad respectively. The minimumlayer-average RIE biases are in the SHH zone with the values close to zero. Theseresults suggest that the bending angle RIEs tend to have negative systematicbiases.This research has significance since these simulation results can be a referencefor the calibration of bending angles derived from real RO observations in future‘s ROdata processing.
In addition, the mechanism of the bending angle RIEs was also investigatedusing the3D ray tracing technique and those RO events with exceptionally large RIEswere analyzed. The results indicate that the asymmetry of ionospheric electron densityalong inbound‘and outbound‘ray paths is the major cause of the exceptionally largebending angle RIEs.
引文
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