US-Germany co-sponsered satellite gravimetry mission GRACE (Gravity Recovery And Climate Experiment), launched in March 2002, has been producing monthly time series of Earth gravity models up to degree and order of 120. The GRACE mission consists of two identical satellites flying on an almost polar orbit with an altitude of about 300-500 km and satelite-to-satellite ranging of about 220 km. Thanks to the payloads of space-borne GPS receivers, accelerometers and high-precision K-band satelite-to-satellite ranging mesurements, GRACE gravity models are expected to achieve more than one order of magnitude of improvement over previous models at spatial scales of a few hundred kilometers or larger. Recovery of surface mass re-distribution based on GRACE’s time-varying gravity models is applied to studies in solid Earth geophysics, oceanography, climatology and geodesy. At secular time scales, GRACE is expected to provide valuable information on global ice changes, whose variations have profound influences on global climate, and in particular, on sea level changes. At seasonal time scales, GRACE is expected to reveal surface water changes with an ac- curacy of less than 1 cm, or ocean bottom pressure changes with an accuracy of less than 1 mbar (1 mbar =102 Pa). These surface mass redistribution measurements would impove our understanding of the global and regional mass and energy cycles that are critical to human life. Using 15 GRACE monthly gravity models covering the period from April 2002 to December 2003, this study compares seasonal water storage changes recovered from GRACE data and hydrology models at global and regional scales, with particular focus on the Yangtze River basin of China. Annual amplitude of 3.4 cm of equivalent water height change is found for the Yangtze River basin with maximum in Spring and Autumn, agreeing with two state-of-the-art hydrology models. The differences between GRACE re- sults and model predictions are less than 1-2 cm. We conclude that satellite gravimetry has huge potentials
HU Xiaogong1, CHEN Jianli1,2, ZHOU Yonghong1, HUANG Cheng1 & LIAO Xinhao1 1. Center for Astro-geodynamics Research, Shanghai Astronomical Observatory, National Astronomical Observatories, Shanghai 200030, China
Using the length-of-day (LOD) data series de-rived from astronomical observations made by various mod-ern space techniques, southern oscillation index (SOI) and anomalies of sea surface temperature (SST) in tropical Pa-cific Ocean, the inter-annual components of these data series are obtained by the technique of band filtering from January 1962 to October 2002. It is found that a new ENSO event has already occurred in 2002 and been detected by the inter- annual LOD. It is shown by the wavelet analysis that the component with a period of about 46 years and the quasi-biennial component are out of phase during 2001 2002, which consequently suggests that the new ENSO event cannot be a strong one.
2002年3月成功发射的美德合作重力卫星计划GRACE（Gravity Recovery And Climate Experiment）已经开始提供阶次数达到120、时间分辨率为约1个月的地球重力场模型时变序列．GRACE的星座由两颗相距约220km，高度保持300-500km，而倾角保持约90°的近极轨卫星组成．由于采用星载GPS和非保守力加速度计等高精度定轨技术以及高精度的星．星跟踪数据反演地球重力场，在几百公里和更大空间尺度上，GRACE重力场的精度大大超过此前的卫星重力观测．根据GRACE时变重力场反演的地球系统质量重新分布对固体地球物理、海洋物理、气候学以及大地测量等应用有重要的意义．在长期时间尺度上，GRACE的结果可用于研究北极冰的变化，并进而研究极冰融化对全球气候变化，特别是对海平面长期变化的影响．在季节性时间尺度上，利用GRACE重力场的精度足以揭示平均小于1cm的地表水变化或小于1mbar的海底压强变化．除了巨大的社会和经济效益外，这些变化对了解地球系统的物质循环（主要是水循环）和能量循环有非常重要的意义．利用2002年4月至2003年12月之间共15个月的GRACE时变重力场揭示了全球水储量的明显季节性变化，并重点分析了中国长江流域水储量的变化．结果表明长江流域水储量周年变化幅度可达到3．4cm等效水高，其最大值出现在春季和初秋．根据GRACE时变重力场反演的水储量变化与两个目前最好的全球水文模型的符合相当好，其差别小于1cm等效水高．研究表明现代空间重力测量技术在监测一些大流域的水储量变化（如长江流域）、全球水循环和气候变化上有巨大的应用潜力．
