利用广义三角帽方法评估GRACE反演中国大陆地区水储量变化的不确定性

Uncertainties in GRACE-derived terrestrial water storage changes over China's Mainland based on a generalized three-cornered hat method

在无真实观测值的情况下,本文利用广义三角帽方法评估了五种GRACE时变重力场模型(CSR、GFZ、GRGS、HUST发布的球谐系数解和JPL发布的Mascon解)反演中国大陆地区2003—2013年水储量变化的不确定性.研究结果表明,CSR、GFZ、JPL、HUST和GRGS反演月水储量变化不确定性的区域平均RMS分别为14.4mm、26.3mm、25.3mm、26.6mm和56.1mm,其中GRGS的结果未恢复泄漏信号;在季和年尺度上,模型的不确定性均小于月尺度;扣除周期和趋势信号后,各模型反演结果更为一致.除长江流域外,CSR在13个流域的不确定性均小于其他模型,GRGS反演各流域水储量变化的不确定性通常较大,且可能高估了温带大陆性气候地区水储量的波动;CSR和JPL的不确定性受流域周边水文特征、气候类型、流域面积和形状的影响相对较小,不确定性变化范围分别为2.3～17.1mm和5.6～22.5mm,GFZ和HUST受影响较大,不确定性变化范围分别为5.5～35.1mm和4.0～40.6mm.本文的研究结果为GRACE产品不确定性评估提供了新的途径,为GRACE时变重力场模型的选取提供参考.

It remains a challenging problem to assess the uncertainties in GRACE-derived terrestrial water storage(TWS)changes due to insufficient observations in many areas globally.Particularly,China has a complicated terrain covering a range of climate settings,aquifers and levels of human interventions.Therefore,it is very important to evaluate the performance of GRACE observations from different processing centers for GRACE applications in China.Due to the absence of ground truth observations,this study analyzed the relative uncertainties in GRACE-derived TWS changes from five solutions over China's Mainland based on a generalized three-cornered hat(TCH)method,including Stokes coefficients from the Center of Space Research(CSR),the German Research Center for Geoscience(GFZ),the Groupe de Recherches de geodesie spatiale(GRGS)and the Huazhong University of Science and Technology(HUST)as well as the Jet Propulsion Laboratory(JPL)GRACE mascon solution.The results showed that the averaged uncertainties(in terms of root-mean-square,RMS)of derived monthly TWS changes over China's Mainland for CSR,GFZ,JPL,HUST and GRGS were 14.4 mm,26.3 mm,25.3 mm,26.6 mm and 56.1 mm,respectively;compared to the monthly scale,the uncertainties of each solution were lower at seasonal and annual scales for both the original and nonseasonal terms;the inverted TWS changes were more consistent at the nonseasonal term(after removing the trend and seasonal cycles from the original signals).At the basin scale,except for the Yangtze River basin,CSR showed the lowest uncertainties for the 13 river basins over China's Mainland,while GRGS showed relative large uncertainties.In addition,GRGS-based TWS showed larger variability than other GRACE solutions and two hydrological models(Global Land Data Assimilation System,GLDAS and WaterGAP Global Hydrology Model,WGHM)in the temperate continental climate region;CSR and JPL were less affected by the surrounding hydrological conditions,climate settings,size and geometry of the basins,with the uncertainties varying from 2.3~17.1 mm and 5.6~22.5 mm,respectively.Whereas GFZ and HUST were influenced more by these factors,with the ranges of the uncertainties were 5.5~35.1 mm and 4.0~40.6 mm,respectively.It should be noted that GRGS-derived TWS changes were not restored because of the difficulty in quantifying signal loss resulted from regularization for end-users.High uncertainty in GRGS suggests serious signal loss in regularized GRACE solution in the study region.Therefore,more independent data or models are necessary for validation before using the GRGS solution.This study provides a new way to assess the uncertainty in GRACE products and will be helpful for choosing proper models for specific studies.