基于改进的BP神经网络构建区域精密对流层延迟模型

利用神经网络算法挖掘海量数据的规律已成为科技发展的一种趋势,本文针对卫星信号的天顶对流层延迟进行建模.对流层延迟是影响卫星定位精度的重要因素之一,建立精密区域对流层模型对高精度定位有着重要的意义.对区域测站对流层延迟数据的分析,考虑到实时建模中传统BP（Back Propagation）神经网络计算量大,易出现"过拟合"现象、不稳定等因素,通过改进的BP神经网络建立了区域精密对流层模型.详细介绍了新模型的建立过程,并与常用的对流层区域实时模型进行了对比.还讨论了建模测站数目对预报精度的影响.相比现有的其他对流层延迟模型,基于改进的BP神经网络构建的区域精密对流层延迟模型无论在拟合和预报方面都有较好的精度,且随着测站数目的增加模型精度趋于平稳.改进的模型参数较少,可以进行实时的区域精密对流层延迟改正;需要播发的信息量小,适用于连续运行参考站系统（Continuously Operating Reference Stations,CORS）的应用.研究表明：改进的BP神经网络模型能够更好的充分利用大规模历史数据描述卫星信号对流层延迟的空间分布情况,适用于实时大区域精密对流层建模.基于日本地区2005年近1000多个测站的NCAR（National Center Atmospheric Research）对流层数据进行区域对流层延迟建模,结果表明改进的BP神经网络模型在拟合和预报精度上都有较大提升,RMSE（Root Mean Square Error）分别为：7.83 mm和8.52 mm,而四参数模型拟合、预报RMSE分别18.03 mm和16.60 mm.     

4D tropospheric tomography using GPS slant wet delays

Tomographic techniques are successfully applied to obtain 4D images of the tropospheric refractivity in a local dense network of global positioning system (GPS) receivers. We show here how GPS data are processed to obtain the tropospheric slant wet delays and discuss the validity of the processing. These slant wet delays are the observables in the tomographic processing. We then discuss the inverse problem in 4D tropospheric tomography making extensive use of simulations to test the system and define the resolution and the impact of noise. Finally, we use data from the Kilauea network in Hawaii for February 1, 1997, and a local 4 × 4 × 40 voxel grid on a region of 400 km² and 15 km in height to produce the corresponding 4D wet refractivity fields, which are then validated using forecast analysis from the European Center for Medium Range Weather Forecast (ECMWF). We conclude that tomographic techniques can be used to monitor the troposphere in time and space.     

基于垂直剖面函数式的全球对流层天顶延迟模型的建立

对流层延迟是无线电导航定位的主要误差源之一,其值对目标高程的变化敏感.在动态导航定位中,由于目标高程变化随机性强,延迟改正实时性需求高,已有的对流层延迟模型难以满足应用需求.本文利用2005到2006年ERA-Interim再分析气象资料积分方法计算的对流层天顶总延迟(ZTD)、天顶静力学延迟(ZHD)以及天顶非静力学延迟(ZWD)的垂直剖面研究了ZTD随高程变化的最佳拟合形式,并以此为基础建立了全球ZTD改正模型SHAO-H.该模型以大气中水汽的垂直分布规律为依据,将ZTD表示为高程的分段函数,进而再模制每段函数中各参数随时间的变化.精度评估显示:与积分ZTD相比,SHAO-H模型计算的ZTD在不同等压层上的平均bias大部分在±1 mm以内,随着高度的上升,平均RMS由39 mm减小至不足1 mm;与IGS (International GNSS Service)实测ZTD相比,SHAO-H模型的精度(bias为7.02 mm,RMS为38.50 mm)优于UNB3m模型(bias为14.67 mm, RMS为51.95 mm).SHAO-H模型具有精度稳定、计算简便等优点,适宜任意高度的用户使用.     

一种新的高精度全球对流层天顶延迟模型

对流层延迟是影响高精度卫星导航定位的关键因素,也是大气科学研究的重要数据.针对已有全球对流层延迟模型的模型方程未同时顾及高程、纬度和季节变化以及模型构建时仅使用单一格网点数据等问题,本文提出了一种对流层天顶延迟(ZTD)全球模型构建的新方法,即引入滑动窗口算法将全球剖分为大小一致的规则窗口,利用2008—2015年全球大地观测系统(GGOS)大气格网产品构建每个窗口同时顾及高程、纬度和季节因子的全球ZTD新模型(GGZTD模型).联合未参与建模的2016年全球GGOS格网产品和2016年全球316个IGS站精密ZTD产品,检验了GGZTD模型的精度和适用性.结果表明:以GGOS大气格网ZTD产品和IGS站ZTD产品为参考值,GGZTD模型在全球的精度分别为3.58 cm和3.62 cm,相对于UNB3m模型和目前标称精度最优的GPT2w模型计算的ZTD信息,GGZTD模型在全球表现出了最优的精度和稳定性,其精度相对于UNB3m模型具有显著的提升(精度提高了30%以上),相对于GPT2w模型仍具有一定的改善;在ZTD计算时GGZTD模型相对于GPT2w模型显著地减少了模型参数,尤其相对于GPT2w-1(减少了99%).GGZTD模型只需输入位置与时间和依赖相对较少的模型参数则能在全球获得高精度和稳定的ZTD信息,极大地提升了模型的计算效率.     

Lower atmospheric anomalies following the 2008 Wenchuan Earthquake observed by GPS measurements

The Mw=8.0 Wenchuan Earthquake occurred on May 12, 2008 at the Longmen Shan fault, the western Sichuan Basin, China, killing more than ten thousand people in several cities and causing large economic losses. Global Positioning System (GPS) observations have provided unique insights on this event, including co-seismic ionospheric disturbances, co-/post-seismic crustal deformations and fault slip distributions. However, the processes and the driving mechanisms are still not clear, particularly possible seismo-lower atmospheric–ionospheric coupling behaviors. In this paper, the lower atmospheric (tropospheric) variations are investigated using the total zenith tropospheric delay (ZTD) from GPS measurements around this event. It has the first found co-seismic tropospheric anomalies during the mainshock with an increase and then a decrease, mainly in the zenith hydrostatic delay component (ZHD), while it is also supported by the same pattern of surface-observed atmospheric pressure changes at co-located GPS site that are driven by the ground-coupled air waves from ground vertical motion of seismic waves propagation. Therefore, the co-seismic tropospheric disturbances (CTD) indicate again the acoustic coupling effect of the atmosphere and the solid-Earth with air wave propagation from the ground to the top atmosphere.     

亚洲地区ECMWF/NCEP资料计算 ZTD的精度分析

对流层延迟是卫星导航定位的主要误差源,气象观测的数值预报资料可用来计算对流层延迟改正量.本文通过分布于亚洲地区的49个GPS台站一年的实测ZTD资料,对利用欧洲中尺度天气预报中心(ECMWF)分析资料、美国国家环境预报中心(NCEP)再分析资料和NCEP预报资料,计算对流层天顶延迟(ZTD)改正的有效性和可能达到的精度进行了评估,分析了ECMWF和NCEP在亚洲地区的适用程度和其分辨率对计算ZTD精度的影响.研究结果表明:(1)相对于 GPS实测ZTD,用ECMWF资料计算ZTD的bias和rms分别为-1.0 cm 和2.7 cm,优于NCEP再分析资料,可用于高精度ZTD研究和应用;NCEP预报数据计算ZTD的bias和rms分别为2.4 cm 和 6.8 cm,足以满足广大GNSS实时导航定位用户对流层延迟改正的需要.(2)bias和rms呈现明显的季节性变化,总体上夏季大,冬季小;在空间分布上随着纬度的变化不明显,但随高度的增加rms总体上有递减趋势.另外还发现,亚洲东部地区夏季日平均bias和rms和南部热带地区冬季的日平均bias和rms变化相对较大.(3)ECMWF2.5°和0.5°的资料进行了对比分析,发现0.5°分辨率资料的rms比2.5°减小1~5 mm.这些结果,为在亚洲地区的空间大地测量、导航定位和INSAR等工作中,应用ECMWF/NCEP的资料进行对流层大气延迟改正的有效性和可能达到的精度提供了重要参考.     

Signature of atmospheric oscillations in GPS-measured tropospheric delay

Ground-based GPS finds potential applications in many atmospheric studies such as spatial distribution of columnar water vapor as well as the tidal oscillations in the atmosphere. The zenith tropospheric delay (ZTD) derived from GPS data at two Indian IGS stations are used to establish its potential for studying the atmospheric tidal, intra-seasonal and planetary oscillations. The major tidal oscillations observed in ZTD data are diurnal, semi-diurnal and their harmonics. Prominent intra-seasonal oscillations observed in ZTD are reported for the first time in this context. These intra-seasonal oscillations are Madden–Julian Oscillation (30–70 days, 60–90 days, 100–120 days) and planetary waves (like 27, 16 and 5–10 days periodicities). Quantification of these periodicities will provide a useful handle to improve the empirical models employed in the estimation of tropospheric delay.     

两种精化的对流层延迟改正模型

对流层延迟是全球导航卫星系统(Global Navigation Satellite System, GNSS)导航定位中的重要误差源,其量值主要受气象条件影响.采用传统对流层建模思路,利用GPT2模型来提供相对准确的气温、气压和相对湿度,然后利用Saastamoinen模型来计算天顶对流层延迟,由此构建了GPT2+Saas模型;采用新的对流层建模思路,直接针对天顶对流层延迟的时空特性建模,构建了与GPT2+Saas模型相匹配的GZTDS格网模型.以GGOS Atmosphere格网数据为参考,GPT2+Saas模型(Bias: 0.2 cm; RMS: 4.2 cm)和GZTDS模型(Bias: 0.2 cm; RMS:3.7 cm)较UNB3m模型精度分别提升34%和43%.以IGS (International GNSS Service)数据为参考, GPT2+Saas (Bias: 0.5 cm; RMS: 4.7 cm)和GZTDS (Bias: -0.3 cm; RMS:3.8 cm)相对UNB3m模型精度分别提升10%和27%.针对GPT2+Saas模型在少数测站出现精度异常的情况进行了研究,探讨了可能的原因.在两种不同思路构建的精化对流层模型中,GZTDS模型不仅表现出更高的精度,而且在时间稳定性和地理稳定性上也表现出优越性.     

Application of predictive regional ionosphere model to medium range RTK positioning

Real Time Kinematic (RTK) GPS positioning over longer distances requires a support of atmospheric (ionospheric and tropospheric) corrections, since the atmospheric errors decorrelate with the growing distances and cannot be completely eliminated by double differencing of the satellite observations. Currently, the most commonly used approach is to derive the atmospheric corrections at the reference station network and provide them in real time to the roving receiver. Another solution, proposed here, is to use predictive atmospheric models in order to derive the atmospheric corrections. This paper presents the test results of the performance assessment of the predictive ionosphere model (UWM-IPM) application to medium-range RTK positioning. The rover data collected within 25 to 67 km from the closest reference station were processed in the kinematic mode with the support of the ionospheric corrections derived from the UWM-IPM model. The RTK solution was derived in both single-and multi-baseline modes, and compared to the two reference solutions obtained without the ionospheric corrections. All numerical tests were carried out using the MPGPS software developed in cooperation with The Ohio State University; a recent extension to the software, developed at the University of Warmia and Mazury in Olsztyn, introduces the predictive ionosphere model to the RTK solution. The test results are very promising, and indicate that predicted ionosphere corrections can effectively support medium-range RTK positioning, and allow for fast ambiguity resolution over distances of several tens of kilometers under moderate ionospheric conditions.     

Impact of the surface temperature and vertical shear of zonal wind on the dynamics of a simple two-layer model of the atmosphere

The paper presents and analyzes, from the point of view of smooth dynamic systems theory, a two-layer baroclinic model of the troposphere in geostrophic approximation. The model describes airflow in β-channel within the tropospheric part of the main Hadley circulation cell. It enables to obtain, after application of the Galerkin method, a fairly simple low-parametric dynamic system describing the phenomena of non-linear interactions, bifurcations and blocking in the atmosphere. This enables to take into consideration such basic factors influencing the atmospheric dynamics like the heat exchange within the surface, orography, vertical variability of zonal wind and hydrostatic stability. Impact of zonal thermal variability of the surface and vertical shear of zonal wind in the troposphere on the orographic bifurcation was investigated and the oscillation character in the dynamic system after Hopf bifurcation of the second kind was analyzed. Additionally, the model dynamics was investigated in conditions including momentum forcing in the upper and lower parts of the troposphere and excluding orographic interaction, as well as in the conditions of thermal interaction between the troposphere and the surface for the vertical shear of zonal wind in both tropospheric layers. Impact of the mean zonal wind in the troposphere on the properties of model dynamics was assessed. It was proved that zonally varied surface temperature and layered mean zonal wind in the atmosphere are the parameters that have basic influence on the model dynamics. They cause numerous bifurcations and strongly influence the periods of oscillations of the model variables. They are often Hopf bifurcations of the second kind during which tropospheric states fairly distant from the ones before the bifurcations are generated. This significantly influences the model predictability.     

基于GNSS的再分析资料对流层延迟精度评估

对流层延迟对导航定位精度有着重要的影响,而再分析资料提供的高精度气象参数计算的对流层延迟可应用于定位过程中以提升定位精度.本文针对三种再分析资料计算的对流层延迟进行精度评估,并将其应用在精密单点定位中,分析其对定位精度的影响.首先,利用2020年全球范围内125个IGS (International GNSS Service)站的对流层天顶总延迟(Zenith Total Delay,ZTD)作为真值对三种再分析资料(ERA5、MERRA2、CRA40)计算的ZTD进行了精度评估,并分析其时空分布特性.研究结果表明：ERA5-ZTD的均方根误差(RMS)最小(12.1 mm),其次为CRA40-ZTD (15.8 mm)和MERRA2-ZTD (16.9 mm),整体上ERA5-ZTD的精度最高;据所选的IGS站点的比较结果发现赤道平均偏差(BIAS)呈现负值,在中高纬度地区CRA40的精度优于MERRA2,在低纬度地区则相反,而ERA5在各纬度平均精度均为最优;当考虑季节因素时,三者计算的ZTD-RMS在夏秋季较大,其中ERA5的RMS季节变化最稳定.之后还利用180个探空站点对三者计算的ZTD、天顶干延迟(Zenith Hydrostatic Delay,ZHD)、天顶湿延迟(Zenith Wet Delay,ZWD)进行了进一步精度评估,ERA5、MERRA2、CRA40的年均ZTD-RMS分别为13.2,18.7,14.9 mm.而年均ZHD-RMS分别为8.2,8.5,8.2 mm,差异较小,年均ZWD-RMS分别为9.3,14.6,11.6 mm,变化较大.最后,使用ERA5、MERRA2、CRA40计算的ZTD进行精密单点定位实验,结果显示,相对于Saastamoinen模型,其在N方向分别提升了59%、56%、47%、E方向提升了51%、47%、41%、U方向提升了81%、78%、75%.总体而言,ERA5对流层延迟精度在各方面表现均为最优.

     

Improving GPS positioning estimates during extreme weather situations by the help of fine-mesh numerical weather models

Space geodetic applications require to model troposphere delays as good as possible in order to achieve highly accurate positioning estimates. However, these models are not capable to consider complex refractivity fields which are likely to occur during extreme weather situations like typhoons, storms, heavy rain-fall, etc. Thus it has been investigated how positioning results can be improved if information from numerical weather models is taken into account. It will be demonstrated that positioning errors can be significantly reduced by the usage of ray-traced slant delays. Therefore, meso-scale and fine-mesh numerical weather models are utilized and their impact on the positioning results will be measured. The approach has been evaluated during a typhoon passage using global positioning service (GPS) observations of 72 receivers located around Tokyo, proving the usefulness of ray-traced slant delays for positioning applications. Thereby, it is possible reduce virtual station movements as well as improve station height repeatabilities by up to 30% w.r.t. standard processing techniques. Additionally the advantages and caveats of numerical weather models will be discussed and it will be shown how fine-mesh numerical weather models, which are restricted in their spatial extent, have to be handled in order to provide useful corrections.     

Cut-off low monitoring by the French VHF-ST-radar network during the ESTIME campaign

In order to investigate mesoscale strato–tropospheric exchanges, the field campaign ‘Echanges Stratosphère–Troposphère: Investigations à Moyenne Echelle’ was conducted in France from late 1993 to mid 1995 and focused on cut-off low events. It involved the French research network of five VHF (Very High Frequency) ST (Strato–Tropospheric) radars deployed in southern France. Observations corresponding to three Intensive Observing Periods are reported here. The radar data analyzed and discussed are time–height diagrams of the aspect ratio (AR), that is, the vertical to oblique beam-returned power ratio, used for monitoring three cut-off low events. In order to discuss the validity of the method, 506 h of radar AR data were compared with time–height diagrams of the static stability and the humidity obtained from synoptic European Center for Medium-range Weather Forecasts model analyses. A dataset corresponding to 297 h of observations is analyzed and discussed here. It is concluded that the AR is a good tracer to document cut-off low events, including tropopause folding identification and the detection of tropospheric air masses of enhanced stability, in dry or weakly humid cases. On the other hand, although the effects of the specific humidity and its gradients on VHF radar echo power could not be extensively investigated, our results suggest that the same parameter cannot be used at mid- and lower-tropospheric levels when the effects of specific humidity significantly reinforce the moist static stability. It is important to take into account these insights in the context of future observing campaigns in which a network of VHF-ST-radars will be involved, and where their role will be to observe and to document the evolution of upper-level features or potential vorticity streamers, or more generally stratospheric–tropospheric exchanges.     

青藏高原对流层臭氧含量变化的太阳周期活动信号分析

利用1979~1992年卫星TOR对流层臭氧数据库资料,以及同期太阳辐照度数据序列,考察青藏高原对流层臭氧含量变化与太阳辐射周期变化之间的关系.分析表明,青藏高原对流层臭氧分布表现出与太阳辐照度相同的变化趋势,存在着明显的太阳周期变化特征.逐月线性回归分析表明,太阳辐照度增加导致青藏高原对流层臭氧增加的正效应.在太阳周期内,太阳辐射增加可使青藏高原对流层臭氧、平流层臭氧和臭氧总量分别增加1.31、4.97、6.628DU,或4.07%、2.04%、2.28%.该特征与赤道太平洋地区完全相反,分析产生差异的原因,至少应包括两方面因素：一是背景大气NO_X和水汽含量的差异;二是青藏高原频繁发生的平流层-对流层大气物质交换和输送.     

Simulation of slow atmospheric turbulent flows induced by lithospheric sources

The paper presents the results of numerical experiments with a simplified nonstationary model of turbulent flow of viscous compressible gas for two classes of flows comparable in scale with the height of the troposphere. In the first experiment, the possibility for light tropospheric gases to reach the tropopause during their intense release from the lithosphere is explored on the example of methane, and the gas volumes required for this are estimated. In the second experiment, interaction of the wind flow with orographic heterogeneities is studied, and the development of the intense eddy process is demonstrated.     

基于GNSS网络的实时精密单点定位及精度分析

基于局域GNSS网络,以历元间、星间差分技术实时估计了GPS卫星相对钟差的历元间差值;针对所估计实时精密钟差的特征,推导了实时精密单点定位的估计模型.对所估计得到的相对钟差的历元间差值、实时定位结果分别进行了比较、分析.结果表明,相对钟差的历元间差值与IGS的最终星历相比其精度可以达到0.08 ns;每小时观测的实时静态定位结果在N、E、U三个方向的精度分别为1.47、3.62、4.09 cm.动态模式,实时结果在N、E、U三个方向的精度分别为2.63、3.82、5.20 cm.与采用IGS最终轨道和钟差解算的结果相比较,实时计算结果优于采用精密轨道和精密钟差计算结果.     

Impact of a simple parameterization of convective gravity-wave drag in a stratosphere-troposphere general circulation model and its sensitivity to vertical resolution

Systematic westerly biases in the southern hemisphere wintertime flow and easterly equatorial biases are experienced in the Météo-France climate model. These biases are found to be much reduced when a simple parameterization is introduced to take into account the vertical momentum transfer through the gravity waves excited by deep convection. These waves are quasi-stationary in the frame of reference moving with convection and they propagate vertically to higher levels in the atmosphere, where they may exert a significant deceleration of the mean flow at levels where dissipation occurs. Sixty-day experiments have been performed from a multiyear simulation with the standard 31 levels for a summer and a winter month, and with a T42 horizontal resolution. The impact of this parameterization on the integration of the model is found to be generally positive, with a significant deceleration in the westerly stratospheric jet and with a reduction of the easterly equatorial bias. The sensitivity of the Météo-France climate model to vertical resolution is also investigated by increasing the number of vertical levels, without moving the top of the model. The vertical resolution is increased up to 41 levels, using two kinds of level distribution. For the first, the increase in vertical resolution concerns especially the troposphere (with 22 levels in the troposphere), and the second treats the whole atmosphere in a homogeneous way (with 15 levels in the troposphere); the standard version of 31 levels has 10 levels in the troposphere. A comparison is made between the dynamical aspects of the simulations. The zonal wind and precipitation are presented and compared for each resolution. A positive impact is found with the finer tropospheric resolution on the precipitation in the mid-latitudes and on the westerly stratospheric jet, but the general impact on the model climate is weak, the physical parameterizations used appear to be mostly independent to the vertical resolution.     

Variations in zonal mean and planetary wave properties of the stratosphere and links with the troposphere

Zonal mean data and amplitudes and phases of planetary zonal waves were derived from daily hemispheric maps for tropospheric and stratospheric levels, for the four winters 1975–76 to 1978–79. Important year-to-year fluctuation in zonal means and wave activity are described, most notable of which are the changes from 1975–76 to 1976–77. Comparison of the relative strengths of the stratospheric and tropospheric jet streams shows a strong negative correlation (–0.8) between monthly mean zonal stratospheric winds (at 10 mb, 65°N) and zonal tropospheric winds (at 200 mb, 32.5°N, in the jet core) and a positive correlation (+0.7) between the stratospheric 10 mb winds and the tropospheric 200 mb winds at 65°N. Parameters correlated were the departures from the climatological mean zonal winds. The structure of correlation between wave amplitudes in the same wave number (1, 2) at different altitudes and between wave numbers 1 and 2 is investigated. We find a high correlation (+0.93) between wave 1 in the stratosphere (10 mb height) and wave 2 (height) in the troposphere at 65°N; but only a weak correlation (+0.2) between wave 1 amplitudes in the stratosphere and troposphere. These results suggest the possible importance of wave-wave interactions in processes linking the stratosphere and troposphere. The wave correlations presented here are based on comparisons of monthly means of daily amplitudes; the correlation structure in individual wave developments may differ, in view of the likelihood of altitudinal lags in wave amplification.     

用传输函数构建的大气重力波传播理论模式

本文根据考虑大气热传导和黏滞的重力波复色散关系,采用传输函数的概念,基于重力波的线性理论,构建了用于研究对流层内重力波激发源与电离层响应之间的传输函数数值模式.在相空间中讨论了传输函数振幅的分布特性,并以地面单位脉冲源为例,分析了从地面到300 km高空的响应,得到了物理量的时空分布特征.结果表明：（1）对内重力波的传播而言,大气相当于一个滤波器,只有波动周期在15～30 min,水平波长在200～450 km之间的重力波扰动才最容易到达300 km电离层高度;（2）电离层的响应主要在与地面的激发源之间相隔较远的水平距离上发生;（3）黏滞和热传导系数在低层对上传重力波的影响较小,随着高度的增加它们对重力波的影响越来越大;（4）在低层计算的波动频率与Row理论的计算结果比较一致,然而到了高层却相差较大.     

Observations of acoustic-gravity waves in the ionosphere generated by severe tropospheric weather

Atmospheric waves influence the dynamics and energetic budget of the upper atmosphere. Using the continuous HF Doppler sounder, we study the wave activity in the ionosphere during tropospheric convective storms in western and central part of the Czech Republic. The study is focused on acoustic-gravity waves in the period range 2–30 minutes. We discuss possible methods of distinguishing the waves emitted by meteorological sources from waves of different origin, particularly waves of geomagnetic origin. In two cases out of twenty-five analysed, we found waves in the infrasonic period range which might be generated by exceptionally intense meteorological activity in the troposphere. The results differ considerably from those previously obtained in North America. In the central part of the United States, infrasonic waves were frequently observed during convective storms. As a possible reason, we discuss different intensity and dynamics of weather systems in both regions.