Gravity potential spherical harmonic series@en

Time variable gravity fields, reflecting variations of mass distribution in the system Earth is one of the key parameters to understand the changing Earth. Mass variations are caused either by redistribution of mass in, on or above the Earth's surface or by geophysical processes in the Earth's interior. The first set of observations of monthly variations of the Earth gravity field was provided by the US/German GRACE satellite mission beginning in 2002. This mission is still providing valuable information to the science community. However, as GRACE has outlived its expected lifetime, the geoscience community is currently seeking successor missions in order to maintain the long time series of climate change that was begun by GRACE. Several studies on science requirements and technical feasibility have been conducted in the recent years. These studies required a realistic model of the time variable gravity field in order to perform simulation studies on sensitivity of satellites and their instrumentation. This was the primary reason for the European Space Agency (ESA) to initiate a study on ''Monitoring and Modelling individual Sources of Mass Distribution and Transport in the Earth System by Means of Satellites''. The goal of this interdisciplinary study was to create as realistic as possible simulated time variable gravity fields based on coupled geophysical models, which could be used in the simulation processes in a controlled environment. For this purpose global atmosphere, ocean, continental hydrology and ice models were used. The coupling was performed by using consistent forcing throughout the models and by including water flow between the different domains of the Earth system. In addition gravity field changes due to solid Earth processes like continuous glacial isostatic adjustment (GIA) and a sudden earthquake with co-seismic and post-seismic signals were modelled. All individual model results were combined and converted to gravity field spherical harmonic series, which is the quantity commonly used to describe the Earth's global gravity field. The result of this study is a twelve-year time-series of 6-hourly time variable gravity field spherical harmonics up to degree and order 180 corresponding to a global spatial resolution of 1 degree in latitude and longitude. In this paper, we outline the input data sets and the process of combining these data sets into a coherent model of temporal gravity field changes. […]时变重力场（Time variable gravity fields）可反映地球系统内的质量分布变化，是解析动态地球的关键参数之一。质量变化既可能源于地球表面、地表之上或内部的质量再分布，也可能由地球内部的地球物理过程引发。2002年起，美德联合的GRACE卫星任务首次获取了地球重力场月度变化的观测数据，至今仍为科学界提供宝贵的研究资料。然而，GRACE已远超设计服役寿命，因此地球科学领域目前正寻求继任卫星任务，以延续GRACE开启的气候变化长时序观测序列。近年来，学界已围绕科学需求与技术可行性开展了多项研究，而这些研究需要构建贴合实际的时变重力场模型，以开展卫星及其载荷灵敏度的仿真分析。这也是欧洲空间局（European Space Agency, ESA）启动题为"利用卫星监测并模拟地球系统中质量分布与输运的各类源项"研究的核心动因。此项跨学科研究的目标是基于耦合地球物理模型，构建尽可能贴合实际的仿真时变重力场，用于受控环境下的仿真实验。为此，研究采用了全球大气、海洋、陆地水文与冰盖模型，并通过在各模型间统一强迫场，以及引入地球系统不同圈层间的水体交换，实现模型耦合。此外，研究还模拟了固体地球过程引发的重力场变化，包括持续的冰川均衡调整（glacial isostatic adjustment, GIA），以及伴随同震与震后信号的突发性地震事件。将所有单一模型的模拟结果进行整合，并转换为重力场球谐级数（spherical harmonic series）——这是描述全球地球重力场的通用表达方式。本研究的最终成果为一套时长12年、时间分辨率为6小时的时变重力场球谐级数数据，其阶数与次数最高可达180，对应全球经纬度空间分辨率为1°。本文将概述本研究用到的输入数据集，以及将这些数据集整合为统一的时变重力场变化模型的完整流程。……