Development Of The High-precision Global Models For The Key Parameters Of Troposphere Considering Spatiotemporal Factors

Troposphere is one of the most dominant components of near-earth space environment,and it is also the most closely connected atmosphere for human life,which plays an important role in the global water cycle,formation and evolution of severe weather,atmospheric radiation and energy balance.Temperature,pressure,water vapor,atmospheric weighted mean temperature(₈)),zenith total delay(ZTD)and zenith wet delay(ZWD)are all key parameters of troposphere.Among them,tropospheric delay(ZWD/ZTD)is the dominant errors in Global Navigation Satellite System(GNSS),Very Long Baseline Interferometry(VLBI),and other space technologies for high-precision navigation and positioning as well as the fundamental data in atmospheric science.₈)is not only a key parameter for GNSS precipitable water vapor(PWV)sounding,but also an important parameter for the calculation of ZWD by Askne's tropospheric delay model.The key parameters of troposphere,ZWD,ZTD and₈),are extremely affected by multiple factors,such as meteorological factors,terrain and latitude,which also have significant seasonal variations and spatial characteristics.In addition,the variation of key parameters of the troposphere in the vertical direction is larger than that in the horizontal direction.Recently,there is a hot topic for the establishment of real-time,high-precision tropospheric key parameters models with the increasing demands for real-time,high-precision key parameters of troposphere in GNSS navigation positioning and GNSS PWV detection.The purpose of this work is to addresses the shortages of existing global empirical models of key parameters of the troposphere,finally,the real-time,high-precision global models of key parameters of the troposphere are developed using the latest MERRA-2(the Second Modern-Era Retrospective analysis for Research and Applications)reanalysis dataset provided by NASA,the Global Geodetic Observing System(GGOS)Atmosphere gridded products and other data sources.The major research work and contributions of this study as follows:1.The data sources of this work as well as principles and methodologies of calculating the key parameters of the troposphere are described.However,there are no studies on evaluating ZWD/ZTD derived from the MERRA-2 reanalysis data on a global scale.Both ZTD products from the International GNSS Service(IGS)stations and ZWD/ZTD values calculated from radiosonde sites that distributed globally are employed to test the performance of ZWD/ZTD derived from the MERRA-2 reanalysis data over globe.Besides,a method is proposed to calculate the ZWD/ZTD values of the four MERRA-2 data grids around the global IGS stations or radiosonde sites using the elevation of the IGS stations or radiosonde sites as the integral starting height.The results are:(1)the RMS error of ZTD derived from MERRA-2 reanalysis data is better than 1.35cm when compared to the three years(2015-2017)of ZTD products from globally distributed IGS stations;moreover,the bias and RMS error of ZTD calculated from MERRA-2 data show spatiotemporal characteristics,especially at latitude that both of those show gradually decrease from the equator to the poles.(2)the precision of ZWD and ZTD calculated from MERRA-2 data are better than 1.35cm and 1.45cm,respectively,and also the spatiotemporal characteristics of bias and RMS error of ZWD and ZTD are similar to the tested results from IGS on a global scale.Thus,the ZWD and ZTD derived from MERRA-2 data have excellent performance and stability over globe,which can be suggested as a data source for the development of global ZWD/ZTD vertical stratification models.2.Some shortages are still existed in current global ZWD/ZTD vertical stratification models,such as only single gridded data as well as monthly profiles is used for modeling.To address those of drawbacks,a new approach,the sliding window algorithm,is proposed to develop the ZWD and ZTD vertical stratification models.In this work,the sliding window algorithm is introduced to divide the global into regular windows with the same size.For each window over globe,the ZWD and ZTD vertical stratification models that considering sophisticated seasonal variations of ZWD/ZTD height scale factors are established using the 6h resolution ZWD/ZTD layered profiles from all MERRA-2 grid points in each window.Finally,the high-precision global ZWD and ZTD vertical stratification models that considering spatiotemporal factors are developed,named as GZWD-H and GZTD-H models,respectively.Both the ZWD/ZTD layered profiles from 321 radiosonde site in 2017 and ZTD products from305 IGS sites in 2017 are treated as reference values,to evaluate the performance of GZWD-H and GZTD-H models in layered vertical interpolation and its application in spatial interpolation for GGOS Atmosphere gridded ZWD/ZTD.The results show that(1)both GZWD-H and GZTD-H models show the best performance in the ZWD/ZTD layered vertical interpolation against the ZWD/ZTD layered profiles from globally distributed radiosonde sites,which have the precision of 2.5mm and 27.4mm,respectively.In terms of RMS,the GZWD-H model has improved by 4%and 7%compared to the GPT2w-1 and GPT2w-5 models,respectively;while the GZTD-H model shows improvement of 10%against GPT2w model.(2)Compared to GPT2w-1and GPT2w-5 models,GZWD-H model has improved by 17%and 35%in spatial interpolation for GGOS Atmosphere gridded ZWD against surface ZWD calculated from radiosonde profiles over globe,respectively;while for GZTD-H model,those of improvements are 8%and 28%against surface ZTD derived from radiosonde profiles for the spatial interpolation of GGOS Atmosphere gridded ZTD,respectively.Furthermore,GZTD-H model also has improved by 11%and 12%against precise ZTD products from globally distributed IGS sites compared to GPT2w-1 and GPT2w-5models,respectively.(3)In terms of model parameters,both GZWD-H and GZTD-H models have been significantly reduced and optimized against GPT2w-1 model,thus,the applicability of two models could be enhanced in GNSS atmospheric sounding and GNSS precise position.3.A new approach for the establishment of a global zenith tropospheric delay model is proposed,and a high-precision global zenith tropospheric delay grid model considering spatial-temporal factors is developed,named as GGZTD model.The drawbacks of the current global tropospheric delay models are still existed,such as the model equations do not take elevation,latitude and seasonal variations into account simultaneously,as well as only single gridded data is used for modeling.To address those of shortages,the sliding window algorithm is proposed to divide the global into regular windows with the same size.For each window over globe,the ZTD model that considering elevation,latitude and seasonal variations simultaneously is developed using GGOS Atmosphere gridded products.Finally,a high-precision global ZTD model that considering spatiotemporal factors is constructed.Both GGOS Atmosphere gridded ZTD products that were not involved in modeling in 2016 and precise ZTD products from 316 globally distributed IGS sites in 2016 are treated as reference values to assess the performance of GGZTD model.The results are:(1)GGZTD model has the best performance relative to GPT2w and UNB3m models,which shows precision of 3.58cm and 3.62cm against GGOS Atmosphere gridded ZTD products and IGS ZTD data over globe,respectively.In terms of RMS error,GGZTD model has significantly improved by 30%against UNB3m model,which still shows improvement of 2%?4%compared to GPT2w model.(2)The model parameters of GGZTD model has been significantly reduced against GPT2w model,especially compared to GPT2w-1(80times reduction).GGZTD model,fed only by the day of year and the station coordinates as well as relatively few model parameters are needed,could provide a high-precision and stable ZTD value on a global scale,and thus the calculation efficiency of the model could be greatly improved.4.In this work,the issue of a regional₈)model for global application is addressed,and a global₈)grid model that considering spatial-temporal factors is constructed,namely,GGTm model.The model equations of existing global empirical₈)models do not consider latitude,altitude and seasonal factors simultaneously;besides,another motivation of this work is to realize a regional₈)model for global application.Thus,a new approach,the sliding window algorithm,is proposed to address those problems and shortages.On a global scale,it can be divided into numerous regular areas(i.e.each area presents one regular window).For each window over globe,the₈)model that takes elevation,latitude and seasonal factors simultaneously is established using GGOS gridded₈)products from 2007 to 2017.Finally,a high-precision global₈)model that takes spatiotemporal factors into account is developed.Both GGOS gridded₈)data in 2015 and₈)profiles from globally distributed 412 radiosonde sites in 2015 are regarded as reference values to test the performance of GGTm model.The results are:(1)GGTm model has the precision of 2.89K and 3.54K against GGOS gridded₈)data and radiosonde profiles,respectively,which shows the best performance in₈)estimation over globe compared to GPT2w model and Bevis formula,especially in high altitude areas where the significant advantages were achieved for GGTm.(2)GGTm model has improved by 8%and 12%over globe against GPT2w-1 and GPT2w-5,respectively.Additionally,GGTm shows the global average RMS_PWV and RMS_PWV/PWV values of 0.26mm and 1.28%in terms of theoretic,respectively,which also realize a regional₈)model for global application.Thus,GGTm model,fed only by the day of year and target position,could provide a high-precision and reliable₈)information over globe,which shows significant potential application in global real-time high-precision GNSS PWV sounding.