مقایسه‌ مجموعه داده‌های era5 و era-interim در محاسبه‌ تغییرات جرم کوتاه‌مدت جوی و اثرات آن بر ارتفاع ژئویید

میدان گرانی حاصل از مشاهدات گرانی‌سنجی ماهواره‌ای مانند grace شامل خطاهایی ناشی از نوفه‌ دستگاهی، نمونه‌برداری مکانی ناهم‌سانگرد (anisotropic) و خطای تداخل سیگنال (aliasing) و زمانی ناشی از نقص مدل‌سازی تغییرات جرم کوتاه‌مدت زمین است. کیفیت مشاهدات grace در ماموریت gracefo بهبود یافت، اما خطای تداخل سیگنال زمانی همچنان یک عامل تاثیرگذار بر گرانی محاسباتی از مشاهدات می‌باشد. به همین دلیل محاسبه تغییرات جرم کوتاه‌مدت و تصحیح اثر این تغییرات بر مشاهدات جرم ماهواره‌ای و به بیان دیگر تصحیح خطای تداخل سیگنال (dealiasing) ضروری می‌باشد. این پژوهش بر بخش جو زمین تمرکز کرده و تنها تغییرات جرم جوی فرکانس بالا را با راه‌حل انتگرال‌گیری سه‌بعدی، با در نظرگرفتن جو سه‌بعدی و پارامترهای جوی سطحی و چند سطحی حاصل از مدل عملیاتی ecmwf و مجموعه‌ داده‌های دوباره آنالیز شده erainterim و era5 با پوشش جهانی محاسبه می‌کند. همچنین ضرایب هارمونیک کروی تحت‌اثر این تغییرات محاسبه و اثر تغییرات جرم جوی بر تغییرات ارتفاع ژئویید و تغییرشکل قائم زمین بررسی شد. مقایسه‌ تغییرات جرم حاصل از دو داده بیانگر اختلاف کم و نزدیک به صفر بین دو داده می‌باشد. همچنین اثر این تغییرات جرم بر نوسان ژئویید و تغییرشکل قائم زمین بسیار ناچیز است. بیشینه‌ ضریب همبستگی 70 درصد بین تغییر شکل حاصل از سری زمانی gps و تغییر شکل حاصل از دو مجموعه داده بیانگر اعتبار نتایج به‌دست آمده می‌باشد.

Investigation of short-term atmospheric mass variations and their effects on geoid height using meteorological data

Modern missions such as CHAMP, GRACE and GOCE which derive the Earth’s static and timevariable gravity field with unprecedented accuracy with monthly or even submonthly resolution, are also sensitive to shortterm (weekly or shorter) nontidal mass variations due to mass transports and mass redistribution phenomena in the atmosphere, the oceans and the continental water storage. GRACE derived gravity solutions contain errors mostly due to instrument noise, anisotropic spatial sampling and temporal aliasing caused by incomplete reduction of shortterm mass variations in models. Improving the quality of satellite gravimetry observations, in term of using more sensitive sensors and increasing the spatial isotropy, has been discussed in the context of the designed scenarios of GRACEFollow On (GRACEFO) mission. Temporal aliasing is still a factor that affects the quality of the gravity field. For GRACE data processing only the shortterm variations are of importance, because with the monthly Grace gravity field solutions it is planned to provide data for determination of the seasonal variations. Shortterm mass variations cannot be measured adequately by GRACE. Therefore they are removed from measurements beforehand using geophysical models (dealiasing). This paper specifically focuses on the atmosphere of Earth and its mass variations using the ITG3D method. In this paper, various type of data such as the atmospheric pressure parameter, the multilevel geopotential, temperature and humidity parameters from European Center for MediumRange Weather Forecasts (ECMWF) have been used to perform threedimensional integral solution. ERAInterim and ERA5 reanalysis are considered as the datasets. In the procedure of calculations, the shape of earth is approximated as an ellipse. As a first step in calculation procedure, it is necessary to remove the effect of longterm variations. In order to eliminate this effect; the mean variations of atmospheric mass over a specific period should be subtracted from the mass variation. Atmospheric dealiasing products can be illustrated as sets of spherical harmonic coefficients, which are estimated using atmospheric mass variations. Then, the effect of atmospheric mass changes on geoid height and vertical deformation were calculated. In the computation, the ECMWF data on 1 January 2015 at 00:00h were used, while the mean atmospheric mass variations were derived from the means of the years 2015 and 2016. The results of the comparison between two datasets demonstrated that the maximum differences in parameters are located in Asia and Antarctic. The results indicate that the mean of difference between atmospheric mass variations from ERAInterim and ERA5 is 0.23 kgm2. The results show that the difference between the coefficients is about one percent of their values. In addition, the geoid height from ERA5 changes on average of 0.16 cm whereas this parameter varies on average 0.17 cm using ERAInterim data due to atmospheric mass variations. The difference of vertical deformation from two datasets is 0.002 cm on average. The atmospheric mass variations calculated by the two data sets (ERAInterim and ERA5) is not significantly different. The validation results of the vertical deformation of the two data also show a high correlation with the GPS time series.