Tropopause analysis over the Iranian region using GPS radio occultation data

Near-tropopause phenomena like upper level fronts and cyclones, penetrative cumulus convection and mesoscale mechanisms of exchange make important contributions to the mixing processes in the atmosphere. Spatio-temporal monitoring of the tropopause height, temperature and pressure is an appropriate tool to show the running processes in the atmosphere. In this study, GPS radio occultation data is used to investigate the tropopause height fluctuations and the relation between the stratosphere–troposphere exchange and the aforementioned phenomena over the Iranian region. The paper shows how the position of the sub-tropical jet has changed with time, using GPS radio occultation observations. The tropopause height changes latitudinally, and three different bimodal probability distribution functions are observed. The results also show that the mixing region in the south of Iran is associated with the subtropical jet in winter. However, this region shifts north of Iran due to changes in the position of the subtropical jet during the summer. Consistency of the mixing region from the radio occultation data and the total ozone of TOMS over the Iranian region is also observed.     

Variability of the upper troposphere and lower stratosphere observed with GPS radio occultation bending angles and temperatures

Recently, Lewis (2009) introduced a new method for the identification of tropopause heights (TPHs) from GPS radio occultation (RO) bending angles (α)$(\alpha )$. The method uses a covariance transform to identify transitions in a ln(α)$\mathit{ln}(\alpha )$ profile. Lewis validates the results with lapse rate tropopause (LRT) heights from one year of FORMOSAT-3/COSMIC data and radiosondes. In this study we apply the new method to the RO data sets from CHAMP/GRACE (2001–2009) and FORMOSAT-3/COSMIC (2006–2009). These results are the basis for TPH trend estimations for the time period between May 2001 and August 2009 (100 months) based on zonal monthly mean GPS RO data from CHAMP (2001–2008), GRACE (since 2006) and FORMOSAT-3/COSMIC (since 2006). Further, we compare the α$\alpha$ based TPH trends with LRT height trends and discuss the differences, which are largest in the subtropical regions (20°–40°) on both the northern and southern hemisphere. A global increase of the TPH between 5 and 9 m/yr is found for both methods and different data sets (CHAMP/GRACE alone and CHAMP/GRACE plus FORMOSAT-3/COSMIC). The results for the TPH trends are linked with bending angle and temperature trends in the upper troposphere and lower stratosphere region. Generally, an upper tropospheric warming (bending angle decrease) and a lower stratospheric cooling (bending angle increase) is noted.     

Observation of wave structures in the upper atmosphere by means of radio holographic analysis of the radio occultation data

A radio holographic approach, developed by Pavelyev (1998), Hocke (1999), Igarashi (2000), is applied to observation of wave phenomena in the upper atmosphere using Global Positioning System — “Microlab-1” satellite (GPS/MET) radio occultation data. In the current state the radio holography approach uses the radar focused synthetic aperture principle to obtain high spatial resolution, and to remove the interference part corresponding to scattering from the upper ionosphere. High spatial resolution and accuracy of the radio halographic method is validated by means of revealing the weak signal reflected from the sea in the GPS/MET radio occultation data. The radio holographic method gives a new possibility to measure directly the vertical gradient of the electron density altitude profile in the D-layer using the radio occultation signal. The results of the application of radio holographic analysis to two GPS/MET occultation events (07 February 1997, No. 0447, 0158), in the D-region of the ionosphere, are discussed. Wave structures in the electron density concentration with a vertical spatial period of 1.4–6 km, and variations in the electron density gradient from ±5·10⁹ to ±8·10⁹ [1/(m³km)], have been retrieved from the D-layer data. The features observed in the vertical electron density profiles may be connected with breaking of gravity waves in the D-layer of the ionosphere.     

Higher order ionospheric propagation effects on GPS radio occultation signals

With the increasing number of remote sensing satellites using the GPS radio occultation technique for atmospheric sounding, the estimation of higher order ionospheric effects and their mitigation have become relevant and important. Due to long ionospheric limb paths, GPS signals are strongly affected by ionospheric refraction during radio occultation. Standard dual-frequency GPS measurements may be used to estimate the first order term of the refractive index. However, non-linear terms such as the second and third order ionospheric terms and ray path bending effects are not considered in occultation measurements so far. Analysing selected CHAMP–GPS occultation events different higher order ionospheric terms are estimated and their effects on dual-frequency range estimation and total electron content (TEC) estimation are discussed. We have found that the separation between the GPS L1 and L2 ray paths exceeds the kilometer level during occultation for a vertical TEC level of more than 160 TEC units. Corresponding errors in the GPS dual-frequency range estimation and TEC estimation are found to exceed the meter and 10 TEC units level, respectively.     

A comparative and numerical study of effects of gravity waves in small miss-distance and miss-time GPS radio occultation temperature profiles

The Global Positioning System (GPS) Radio Occultation (RO) technique has global coverage and is capable of generating high vertical resolution temperature profiles of the upper troposphere and lower stratosphere with sub-Kelvin accuracy and long-term stability, regardless of weather conditions. In this work, we take advantage of the anomalously high density of occultation events at the eastern side of the highest Andes Mountains during the initial mission months of COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate). This region is well-known for its high wave activity. We choose to study two pairs of GPS RO, both containing two occultations that occurred close in time and space. One pair shows significant differences between both temperature profiles. Numerical simulations with a mesoscale model were performed, in order to understand this discrepancy. It is attributed to the presence of a horizontal inhomogeneous structure caused by gravity waves.     

GRAS radio occultation on-board of Metop

The GRAS radio occultation instrument is flying on Metop-A and belongs to the EPS (EUMETSAT Polar System). GRAS observes GPS satellites in occultation. Within this work, validation of GRAS closed-loop bending angle data against co-located ECMWF profiles extracted from model fields and occultations from the COSMIC constellation of radio occultation instruments is shown. Results confirm the high data quality and robustness, where GRAS shows lower bending angle noise against ECMWF than COSMIC and in terms of occultations per day, one GRAS ≈ two COSMIC satellites. This is partly due to the operational setup of EPS. For the investigation we focus on two observation periods where updates in the ECMWF (March 2009) and COSMIC processing (October 2009) have improved the statistics further. Bending angles biases agree to within 0.5% against ECMWF and to within 0.1% against COSMIC after the updates for altitudes between 8 and 40 km. In addition, we also analyze the impact of the Metop orbit processing on the derived GRAS bending angle data, where different GPS and Metop orbit solutions are analyzed. Results show that a batch based orbit processing would improve in particular the bending angle bias behavior at higher altitudes. Requirements for the operational processing of GRAS data are briefly outlined, options to ease the use of other positioning system satellites in the near future are discussed. A simplified analysis on the observation of several of these systems, e.g. GPS and Galileo, from one platform shows that about 16% of occultations are found within 300 km, ±3 h, thus providing similar information. A constellation of 2 GRAS like instruments would have only about 10% close-by.     

基于STK的GPS/LEO无线电掩星技术仿真研究

GPS/LEO无线电掩星技术反演地球大气参数剖面已经具有较高的精度. 国外开展了多个GPS/LEO掩星项目, 但中国还尚未深入进行相关的实验, 这制约了中国掩星技术的发展. 本文提出基于STK进行GPS/LEO掩星技术研究的方法; 根据GPS/LEO掩星的原理, 推导出掩星事件发生的条件和掩星切点的计算公式; 利用STK对掩星过程进行模拟, 得到掩星数据. 在大气球对称假设和大气模型已知的条件下, 反演得到中性大气折射指数. 通过比较模型和反演数据, 表明反演数据精度较高, 验证了利用STK模拟GPS/LEO掩星实验方法的可行性.      

An introduction to China FY3 radio occultation mission and its measurement simulation

GNSS (Global Navigation Satellite System) radio occultation mission for remote sensing of the Earth’s atmosphere will be performed by GNOS (GNSS Occultation Sounder) instrument on China FengYun-3 (FY3) 02 series satellites, the first of which FY3-C will be launched in the year 2013. This paper describes the FY3 GNOS mission and presents some results of measurement simulation. The key designed specifications of GNOS are also shown. The main objective of simulation is to provide scientific support for GNOS occultation mission on the FY3-C satellites. We used EGOPS software to simulate occultation measurements according to GNOS designed parameters. We analyzed the accuracy of retrieval profiles based on two typical occultation events occurring in China South–East area among total simulated events. Comparisons between the retrieval atmospheric profiles and background profiles show that GNOS occultation has high accuracy in the troposphere and lower stratosphere. The sensitivities of refractivity to three types of instrumental error, i.e. Doppler biases, clock stability and local multipath, were analyzed. The results indicated that the Doppler biases introduced by along-ray velocity error and GNOS clock error were the primary error sources for FY3-C occultation mission.     

人为活动对高平流层,中间层大气的影响

利用一维光化、辐射耦合模式研究了人为活动所排放出的废弃物ＣＨ４、Ｎ２Ｏ和ＣＯ２含量增加对高平流层，中间层大气的影响。结果指出，对流层中这些气体增加一倍将导致上中层大气水汽、Ｏ３和温度的明显变化。ＣＯ２含量增加将使上中层温度降低约１０Ｋ；而ＣＨ４含量的加倍不仅使中层顶附近水汽含量增加５０％以上，也使其温度有更明显的下降，这将更有利于於光云的形成。     

High vertical resolution water vapour profiles in the upper troposphere and lower stratosphere retrieved from MAESTRO solar occultation spectra

An algorithm has been developed that retrieves water vapour profiles in the upper troposphere and lower stratosphere from optical depth spectra obtained by the Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) instrument onboard the SCISAT satellite as part of the Atmospheric Chemistry Experiment (ACE) mission. The retrieval relies on ro-vibrational absorption of solar radiation by water vapour in the 926–970 nm range. During the iterative inversion process, the optical depth spectra are simulated at the spectral resolution and sampling frequency of MAESTRO using the correlated-k approximation. The Chahine inversion updates the water vapour volume mixing ratio (VMR), adjusting all retrieval layers simultaneously, to match the observed differential optical depth due to absorption by water vapour and ozone at each tangent height. This approach accounts for significant line saturation effects. Profiles are typically obtained from ∼22 km down to the cloud tops or to 5 km, with relative precision as small as 3% in the troposphere. In the lower stratosphere, the precision on water vapour VMR is ∼1.3 μmol/mol in an individual retrieval layer (∼1 km thick). The spectral capability of MAESTRO allows for the clear separation of extinction due to water vapour and aerosol, and for the fitting quality to be quantified and used to determine an altitude-dependent convergence criterion for the retrieval. In the middle troposphere, interhemispheric differences in water vapour VMR are driven by oceanic evaporation whereas in the upper troposphere, deep convection dominates and a strong seasonal cycle is observed at high latitudes.     

基于GPS掩星平均弯曲角的折射率反演研究

利用Abel积分变换通过掩星弯曲角计算折射率需要对高层弯曲角进行统计优化.目前由于所使用的背景场资料和具体反演方法不同,导致所发布的掩星大气数据气候统计值存在一定差异.本文使用2008年1,4,7,10月共4个月的COSMIC大气掩星附加相位数据,从纬圈平均弯曲角廓线反演相应月平均折射率,对反演结果进行比较分析.研究表明,利用掩星折射数据进行气候研究时无需逐一对掩星探测廓线进行统计优化,在40 km以下高度基于平均弯曲角的反演方法与传统统计相比能够获得几乎一致的月平均折射率,在50 km以上高度基于平均弯曲角的反演结果更加接近ECMWF资料统计.      

Comparative analysis of TEC disturbances over tropical cyclone zones in the North–West Pacific Ocean

In this paper, we study ionospheric total electron content (TEC) disturbances associated with tropical cyclones (TCs). The study relies on the statistical analysis of six cyclones of different intensity which occurred in the North–West Pacific Ocean in September–November 2005. We have used TEC data from the international network of two-frequency ground-based GPS receivers and NCEP/NCAR meteorological archive. TEC variations of different period ranges (02–20 and 20–60 min) are shown to be more intense during TC peaks under quiet geomagnetic conditions. The highest TEC variation amplitudes are registered when the wind speed in the cyclone and the TC area are maximum. The intensification of TEC disturbances is more pronounced when several cyclones occur simultaneously. We have revealed that the ionospheric response to TC can be observed only after the cyclone has reached typhoon intensity. The ionospheric response is more pronounced at low satellite elevation angles.     

Evaluation of uncertainty in gravity wave potential energy calculations through GPS radio occultation measurements

The application of the Global Positioning System (GPS) radio occultation (RO) method to the atmosphere enables the determination of height profiles of temperature, among other variables. From these measurements, gravity wave activity is usually quantified by calculating the potential energy through the integration of the ratio of perturbation and background temperatures between two given altitudes in each profile. The uncertainty in the estimation of wave activity depends on the systematic biases and random errors of the measured temperature, but also on additional factors like the selected vertical integration layer and the separation method between background and perturbation temperatures. In this study, the contributions of different parameters and variables to the uncertainty in the calculation of gravity wave potential energy in the lower stratosphere are investigated and quantified. In particular, a Monte Carlo method is used to evaluate the uncertainty that results from different GPS RO temperature error distributions. In addition, our analysis shows that RO data above 30 km height becomes dubious for gravity waves potential energy calculations.     

General aspect of GPS data use for atmospheric science

The radio link between a GPS satellite and a GPS receiver is appropriate for bistatic radar sounding of the Earth's atmosphere, ionosphere, and ocean surface (latter in case of GPS reflection). Measurements of GPS phases and amplitudes are currently performed by spaceborne, airborne, mountain- and ground-based GPS receivers. In the present paper, an uniform approach based on geometrical optics and spherical symmetry of the atmosphere is applied to various GPS observation configurations. Atmospheric mapping function, influence and retrieval of ionospheric layers/disturbances, tropospheric water vapor, and possible measurement of vertical winds and wave velocities are investigated by use of simulation data of GPS phase path excess and bending angle.     

ADVANCES ON STUDY OF MIDDLE AND UPPER ATMOSPHERE AND THEIR COUPLING WITH LOWER ATMOSPHERE

In this paper advances on study of middle and upper atmosphere and their cou pling with lower atmosphere in China in recent two years are briefly reviewed.This review emphasized three aspects, ie. (1) analysis and observation of mid and upper atmosphere over China; (2) theoretical and modelling study of grav ity wave activities in middle atmosphere and their relation to lower atmospheric processes; (3) coupling between the stratosphere and troposphere.     

Characterisation of residual ionospheric errors in bending angles using GNSS RO end-to-end simulations

Global Navigation Satellite System (GNSS) radio occultation (RO) is an innovative meteorological remote sensing technique for measuring atmospheric parameters such as refractivity, temperature, water vapour and pressure for the improvement of numerical weather prediction (NWP) and global climate monitoring (GCM). GNSS RO has many unique characteristics including global coverage, long-term stability of observations, as well as high accuracy and high vertical resolution of the derived atmospheric profiles. One of the main error sources in GNSS RO observations that significantly affect the accuracy of the derived atmospheric parameters in the stratosphere is the ionospheric error. In order to mitigate the effect of this error, the linear ionospheric correction approach for dual-frequency GNSS RO observations is commonly used. However, the residual ionospheric errors (RIEs) can be still significant, especially when large ionospheric disturbances occur and prevail such as during the periods of active space weather. In this study, the RIEs were investigated under different local time, propagation direction and solar activity conditions and their effects on RO bending angles are characterised using end-to-end simulations. A three-step simulation study was designed to investigate the characteristics of the RIEs through comparing the bending angles with and without the effects of the RIEs. This research forms an important step forward in improving the accuracy of the atmospheric profiles derived from the GNSS RO technique.     

Recent results on the Venus atmosphere from pioneer Venus radio occultations

Radio occultation measurements of the temperature structure of the Venus atmosphere have been obtained during seven occultation “seasons” extending from December 1978 to December 1983. Approximately 123 vertical profiles of temperature from about 40 km to about 85 km altitudes have been derived. Since these measurements cover latitudes from both poles to the equator, they have shown the latitudinal dependence of thermal structure. There is a smooth transition from the troposphere to the mesosphere at latitudes below about 45°, with the tropopause at about 56 km. The troposphere then rises to about 62 km in the “collar cloud” region between about 60° and 80° latitude, where a strong temperature inversion (up to 30 K) is present. In the polar areas, 80°–90°, the mesosphere becomes isothermal and there is no inversion. This latitudinal behavior is related to the persistent circulation pattern, in which a predominantly zonal retrograde motion at latitudes below 45° gradually changes to a circumpolar vortex at the “collar cloud” latitudes. Indeed, the radio occultation data have been used in a cyclostrophic balance model to derive zonal winds in the Venus atmosphere, which showed a mid-latitude (50°–55°) jet with a speed of about 120–140 ms^?1 at about 70 km altitude /1,2/. The observations obtained in 1983 and 1984 have shown that above the tropopause there is considerable temporal variability in the detailed thermal structure, suggesting that the persistent circulation pattern is subject to weather-like variability.     

Location of layered structures in the ionosphere and atmosphere by use of GPS occultation data

A method is introduced to locate the layered structures in the atmosphere and ionosphere based on simultaneous observations of radio wave intensity and phase variations in trans-ionospheric satellite-to-satellite links. The method determines location of a tangent point on the trans-ionospheric ray trajectory where gradient of refractivity is perpendicular to the ray trajectory and influence of a layered structure on radio wave parameters is maximal. An estimate of the location of a layer can be obtained from a combination of the phase and intensity variations. This new technique was applied to measurements provided during FORMOSAT-3 and CHAMP radio occultation (RO) missions. For the considered RO events the location of the inclined plasma layer in the lower ionosphere is found and the electron density distribution is retrieved. The method is checked by measuring the location of the tangent point on the ray trajectory in the neutral gas in the atmosphere. The results showed a fairly good agreement.     

FY-3D卫星的北斗掩星分布特征与误差特性

利用2018年1－3月FY-3D卫星的掩星折射率数据,研究了北斗导航卫星系统的掩星分布特点、数据精度以及误差统计特征。北斗导航卫星系统同步静止轨道掩星沿卫星轨道呈弧状分布在南北两极地区,倾斜轨道掩星在东西半球低纬度地区分别形成一小一大两个空洞,中地球轨道掩星则全球均匀分布。北斗掩星折射率数据精度在探测核心区域,即12~32 km范围内,与ERA5再分析资料计算的折射率相比,平均偏差的标准差约为1.5%,在核心区外,标准差从1.5%逐渐增大到6%。静止轨道掩星的平均偏差在高层略大于倾斜轨道和中地球轨道掩星。下降掩星在20 km以上区域的标准差大于上升掩星,20 km以下区域小于上升掩星。高纬地区北斗掩星标准差最小,低纬地区最大,对流层中下层尤其明显。分析结果表明,北斗掩星的数据精度和误差特征与GPS掩星数据相似。      

2000年7月和2003年10月大磁暴期间东亚地区中低纬电离层的GPS TEC的响应研究

利用武汉电离层观象台研制的GPS TEC的现报方法及现报系统,对东亚地区GPS台网的观测数据进行处理分析,特别对2000年7月14-18日和2003年10月28日至11月1日两次特大磁暴期间的数据进行了对比考察,文中分析了两次磁暴间的电离层响应,得到对应不同磁暴时段电离层TEC的不同变化情况,着重揭示了TEC赤道异常峰的压缩和移动以及赤道异常随时间的压缩—反弹—恢复的过程,并结合高纬电离层的部分响应机制进行了说明,结果显示,两次磁暴期的电离层响应表现出了各自不同的特点,从而反映出因季节变化引起的高纬电离层暴时能量注入的不同而造成的全球性电离层扰动的不同形态,由此看出,磁暴期间电离层TEC的变化直接与太阳扰动发生的时间及其对高纬电离层的耦合有关,若短时期内连续发生多次磁暴,则电离层反应更加复杂,不能简单地当做单一磁暴叠加处理。