Steric Sea-level Variations Inferred from Combined Topex/Poseidon Altimetry and GRACE Gravimetry

In this study, we propose to estimate the steric sea-level variations over a < 2-year period (April 2002 through December 2003) by combining global mean sea level (GMSL) based on Topex/Poseidon (T/P) altimetry with time-variable geoid averaged over the oceans, as observed by the GRACE (Gravity Recovery and Climate Experiment) satellite. In effect, altimetry-derived GMSL changes results from two contributions: Steric (thermal plus salinity) effects due to sea water density change and ocean mass change due to water exchange with atmosphere and continents. On the other hand, GRACE data over the oceans provide the ocean mass change component only. The paper first discusses the corrections to apply to the GRACE data. Then the steric contribution to the GMSL is estimated using GRACE and T/P data. Comparison with available thermal expansion based on in situ hydrographic data is performed. G. García: On leave from Space Geodesy Laboratory, Applied Mathematics Department, EPS, University of Alicante, Alicante, Spain.     

Characterization of Ethiopian mega hydrogeological regimes using GRACE,TRMM and GLDAS datasets

Understanding the spatio-temporal characteristics of water storage changes is crucial for Ethiopia, a country that is facing a range of challenges in water management caused by anthropogenic impacts as well as climate variability. In addition to this, the scarcity of in situ measurements of soil moisture and groundwater, combined with intrinsic “scale limitations” of traditional methods used in hydrological characterization are further limiting the ability to assess water resource distribution in the region. The primary objective of this study is therefore to apply remotely sensed and model data over Ethiopia in order to (i) test the performance of models and remotely sensed data in modeling water resources distribution in un-gauged arid regions of Ethiopia, (ii) analyze the inter-annual and seasonal variability as well as changes in total water storage (TWS) over Ethiopia, (iii) understand the relationship between TWS changes, rainfall, and soil moisture anomalies over the study region, and (iv) identify the relationship between the characteristics of aquifers and TWS changes. The data used in this study includes; monthly gravity field data from the Gravity Recovery And Climate Experiment (GRACE) mission, rainfall data from the Tropical Rainfall Measuring Mission (TRMM), and soil moisture from the Global Land Data Assimilation System (GLDAS) model. Our investigation covers a period of 8 years from 2003 to 2011. The results of the study show that the western part and the north-eastern lowlands of Ethiopia experienced decrease in TWS water between 2003–2011, whereas all the other regions gained water during the study period. The impact of rainfall seasonality was also seen in the TWS changes. Applying the statistical method of Principal Component Analysis (PCA) to TWS, soil moisture and rainfall variations indentified the dominant annual water variability in the western, north-western, northern, and central regions, and the dominant seasonal variability in the western, north-western, and the eastern regions. A correlation analysis between TWS and rainfall indicated a minimum time lag of zero to a maximum of six months, whereas no lag is noticeable between soil moisture anomalies and TWS changes. The delay response and correlation coefficient between rainfall and TWS appears to be related to recharge mechanisms, revealing that most regions of Ethiopia receive indirect recharge. Our results also show that the magnitude of TWS changes is higher in the western region and lower in the north-eastern region, and that the elevation influences soil moisture as well as TWS.     

Monitoring Sea Level in the Coastal Zone with Satellite Altimetry and Tide Gauges

We examine the issue of sustained measurements of sea level in the coastal zone, first by summarizing the long-term observations from tide gauges, then showing how those are now complemented by improved satellite altimetry products in the coastal ocean. We present some of the progresses in coastal altimetry, both from dedicated reprocessing of the radar waveforms and from the development of improved corrections for the atmospheric effects. This trend towards better altimetric data at the coast comes also from technological innovations such as Ka-band altimetry and SAR altimetry, and we discuss the advantages deriving from the AltiKa Ka-band altimeter and the SIRAL altimeter on CryoSat-2 that can be operated in SAR mode. A case study along the UK coast demonstrates the good agreement between coastal altimetry and tide gauge observations, with root mean square differences as low as 4 cm at many stations, allowing the characterization of the annual cycle of sea level along the UK coasts. Finally, we examine the evolution of the sea level trend from the open to the coastal ocean along the western coast of Africa, comparing standard and coastally improved products. Different products give different sea level trend profiles, so the recommendation is that additional efforts are needed to study sea level trends in the coastal zone from past and present satellite altimeters. Further improvements are expected from more refined processing and screening of data, but in particular from the constant improvements in the geophysical corrections.     

Vertical Crustal Motion along the Mediterranean and Black Sea Coast Derived from Ocean Altimetry and Tide Gauge Data

Tide gauge (TG) data along the northern Mediterranean and Black Sea coasts are compared to the sea-surface height (SSH) anomaly obtained from ocean altimetry (TOPEX/Poseidon and ERS-1/2) for a period of nine years (1993–2001). The TG measures the SSH relative to the ground whereas the altimetry does so with respect to the geocentric reference frame; therefore their difference would be in principle a vertical ground motion of the TG sites, though there are different error sources for this estimate as is discussed in the paper. In this study we estimate such vertical ground motion, for each TG site, from the slope of the SSH time series of the (non-seasonal) difference between the TG record and the altimetry measurement at a point closest to the TG. Where possible, these estimates are further compared with those derived from nearby continuous Global Positioning System (GPS) data series. These results on vertical ground motion along the Mediterranean and Black Sea coasts provide useful source data for studying, contrasting, and constraining tectonic models of the region. For example, in the eastern coast of the Adriatic Sea and in the western coast of Greece, a general subsidence is observed which may be related to the Adriatic lithosphere subducting beneath the Eurasian plate along the Dinarides fault.     

Constraints on Glacial Isostatic Adjustment from GOCE and Sea Level Data

Current constraints on the glacial isostatic adjustment (GIA) process are mainly provided by relative sea-level data and GPS measurements. Due to a lack of resolving power in the shallow earth (down to about 200 km), these data sets only provide weak constraints on the shallow viscosity structure and the thickness of the lithosphere. Future high-resolution gravity data, as expected from ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) launched on March 17, 2009, are predicted to provide additional information on the shallow earth, more specifically the viscosity structure. Here we present an overview of recent developments in extracting information on rheology and stratification of the shallow earth from high-resolution quasi-steady gravity and geoid data to be obtained from GOCE.     

承压井水位固体潮M_2波海潮负荷改正

应用Schwiderski全球海潮模型,采用积分格林函数方法计算了海潮M2波的应变负荷潮。通过调和分析和残差计算,对京、津、唐地区的7个承压水位观测井的水位固体潮M2波进行了海潮负荷改正。计算结果显示,观测井水位M2波潮汐因子和相位漂移的标准偏差分别由改正前的2.263和24.57°降低到0.898和3.58°。     

Uncertainties in Steric Sea Level Change Estimation During the Satellite Altimeter Era: Concepts and Practices

This article presents a review of current practice in estimating steric sea level change, focussed on the treatment of uncertainty. Steric sea level change is the contribution to the change in sea level arising from the dependence of density on temperature and salinity. It is a significant component of sea level rise and a reflection of changing ocean heat content. However, tracking these steric changes still remains a significant challenge for the scientific community. We review the importance of understanding the uncertainty in estimates of steric sea level change. Relevant concepts of uncertainty are discussed and illustrated with the example of observational uncertainty propagation from a single profile of temperature and salinity measurements to steric height. We summarise and discuss the recent literature on methodologies and techniques used to estimate steric sea level in the context of the treatment of uncertainty. Our conclusions are that progress in quantifying steric sea level uncertainty will benefit from: greater clarity and transparency in published discussions of uncertainty, including exploitation of international standards for quantifying and expressing uncertainty in measurement; and the development of community “recipes” for quantifying the error covariances in observations and from sparse sampling and for estimating and propagating uncertainty across spatio-temporal scales.     

Hydrological variability of major French rivers over recent decades,assessed from gauging station and GRACE observations

Abstract

This study was carried out in the framework of the Surface Water and Ocean Topography (SWOT) programme of the French National Centre of Space Studies (CNES). Based on discharge measurements and Gravity Recovery and Climate Experiment (GRACE) determination of total water storage (TWS), we have investigated the hydrological variability of the main French drainage basins (Seine, Loire, Garonne and Rhône) using a wavelet approach (continuous wavelet analyses and wavelet coherence analyses). The results of this analysis have shown a coherence ranging between 82% and 90% for TWS and discharge, thus demonstrating the potential use of TWS for characterization of the hydrological variability of French rivers. Strong coherence between the four basin discharges (between 73% and 92%) and between their associated TWS data (from 82% to 98%) suggested a common external influence on hydrological variability. To determine this influence, we investigated the relationship between hydrological variability and the North Atlantic Oscillation (NAO), considered as an index of prevailing climate in Europe. Basin discharges show strong coherence with NAO, ranging between 64% and 72% over the period 1959–2010. The coherence between NAO and TWS was 62% to 67% for 2003–2009. This is similar to the coherence between NAO and basin discharges detected for the same period. According to these results, strong influence of the NAO was clearly observed on the TWS and discharges of the major French river basins.

Editor Z.W. Kundzewicz     

Seasonal Water Storage Variations as Impacted by Water Abstractions: Comparing the Output of a Global Hydrological Model with GRACE and GPS Observations

Better quantification of continental water storage variations is expected to improve our understanding of water flows, including evapotranspiration, runoff and river discharge as well as human water abstractions. For the first time, total water storage (TWS) on the land area of the globe as computed by the global water model WaterGAP (Water Global Assessment and Prognosis) was compared to both gravity recovery and climate experiment (GRACE) and global positioning system (GPS) observations. The GRACE satellites sense the effect of TWS on the dynamic gravity field of the Earth. GPS reference points are displaced due to crustal deformation caused by time-varying TWS. Unfortunately, the worldwide coverage of the GPS tracking network is irregular, while GRACE provides global coverage albeit with low spatial resolution. Detrended TWS time series were analyzed by determining scaling factors for mean annual amplitude (f _GRACE) and time series of monthly TWS (f _GPS). Both GRACE and GPS indicate that WaterGAP underestimates seasonal variations of TWS on most of the land area of the globe. In addition, seasonal maximum TWS occurs 1 month earlier according to WaterGAP than according to GRACE on most land areas. While WaterGAP TWS is sensitive to the applied climate input data, none of the two data sets result in a clearly better fit to the observations. Due to the low number of GPS sites, GPS observations are less useful for validating global hydrological models than GRACE observations, but they serve to support the validity of GRACE TWS as observational target for hydrological modeling. For unknown reasons, WaterGAP appears to fit better to GPS than to GRACE. Both GPS and GRACE data, however, are rather uncertain due to a number of reasons, in particular in dry regions. It is not possible to benefit from either GPS or GRACE observations to monitor and quantify human water abstractions if only detrended (seasonal) TWS variations are considered. Regarding GRACE, this is mainly caused by the attenuation of the TWS differences between water abstraction variants due to the filtering required for GRACE TWS. Regarding GPS, station density is too low. Only if water abstractions lead to long-term changes in TWS by depletion or restoration of water storage in groundwater or large surface water bodies, GRACE may be used to support the quantification of human water abstractions.     

Assessing and Improving Land Surface Model Outputs Over Africa Using GRACE,Field, and Remote Sensing Data

The Gravity Recovery and Climate Experiment (GRACE), along with other relevant field and remote sensing datasets, was used to assess the performance of two land surface models (LSMs: CLM4.5-SP and GLDAS-Noah) over the African continent and improve the outputs of the CLM4.5-SP model. Spatial and temporal analysis of monthly (January 2003–December 2010) Terrestrial Water Storage (TWS) estimates extracted from GRACE (TWS_GRACE), CLM4.5-SP (TWS_CLM4.5), and GLDAS-Noah (TWS_GLDAS) indicates the following: (1) compared to GRACE, LSMs overestimate TWS in winter months and underestimate them in summer months; (2) the amplitude of annual cycle (AAC) of TWS_GRACE is higher than that of TWS_LSM (AAC: TWS_GRACE > TWS_GLDAS > TWS_CLM4.5); (3) higher, and statistically significant correlations were observed between TWS_GRACE and TWS_GLDAS compared to those between TWS_GRACE and TWS_CLM4.5; (4) differences in forcing precipitation and temperature datasets for GLDAS-Noah and CLM4.5-SP models are unlikely to be the main cause for the observed discrepancies between TWS_GRACE and TWS_LSM; and (5) the CLM4.5-SP model overestimates evapotranspiration (ET) values in summer months and underestimates them in winter months compared to ET estimates extracted from field-based (FLUXNET-MTE) and satellite-based (MOD16 and GLEAM) ET measurements. A first-order correction was developed and applied to correct the CLM4.5-derived ET, soil moisture, groundwater, and TWS. The corrections improved the correspondence (i.e., higher correlation and comparable AAC) between TWS_CLM4.5 and TWS_GRACE over various climatic settings. Our findings suggest that similar straightforward correction approaches could potentially be developed and used to assess and improve the performance of a wide range of LSMs.     

Combination of Multisatellite Altimetry and Tide Gauge Data for Determining Vertical Crustal Movements along Northern Mediterranean Coast

Sea level variations (SLV) can be measured by tide gauges (TG) at the coast and by altimeters onboard satellites. The former measures the SLV relative to the coast, whereas altimetry provides the SLV with respect to a geocentric reference frame. The differences between SLV measurements from these two techniques can be used as an indirect assessment of vertical crustal motions at the TG sites. In this study, we exploit this idea, analyzing differences between sea level signals as measured by altimetric missions (TOPEX/Poseidon and Jason-1) and by 47 TG stations along northern Mediterranean coasts for the period 1993–2007. This allows us to estimate the vertical land motion along these coasts at the TG sites in this time window. For those sites where the TG is co-located or has a nearby global positioning system (GPS) station, these estimates are compared with the vertical rates derived from GPS measurements. Our results on vertical ground motion along the Mediterranean coast provide a useful source of data for studying, contrasting, and constraining tectonic models for the region.     

Groundwater Storage Change in the Jinsha River Basin from GRACE,Hydrologic Models,and In Situ Data

Groundwater plays a major role in the hydrological processes driven by climate change and human activities, particularly in upper mountainous basins. The Jinsha River Basin (JRB) is the uppermost region of the Yangtze River and the largest hydropower production region in China. With the construction of artificial cascade reservoirs increasing in this region, the annual and seasonal flows are changing and affecting the water cycles. Here, we first infer the groundwater storage changes (GWSC), accounting for sediment transport in JRB, by combining the Gravity Recovery and Climate Experiment mission, hydrologic models and in situ data. The results indicate: (1) the average estimation of the GWSC trend, accounting for sediment transport in JRB, is 0.76 ± 0.10 cm/year during the period 2003 to 2015, and the contribution of sediment transport accounts for 15%; (2) precipitation (P), evapotranspiration (ET), soil moisture change, GWSC, and land water storage changes (LWSC) show clear seasonal cycles; the interannual trends of LWSC and GWSC increase, but P, runoff (R), surface water storage change and SMC decrease, and ET remains basically unchanged; (3) the main contributor to the increase in LWSC in JRB is GWSC, and the increased GWSC may be dominated by human activities, such as cascade damming and climate variations (such as snow and glacier melt due to increased temperatures). This study can provide valuable information regarding JRB in China for understanding GWSC patterns and exploring their implications for regional water management.     

A Nonlinear Model of Level Variations in the Caspian Sea

A new thermophysical mechanism of the Caspian Sea level variations is proposed. The mechanism incorporates the effect exerted on the dynamics of the water budget of the Caspian Sea by the nonlinear dependence of the evaporation rate on the moistening of the basin.     

Efficiency of Ionospheric Model Correction by Vertical-Incidence Sounding Data from an Ionosonde during Low Sunspot Activity

Geomagnetism and Aeronomy - Numerical values of the criteria for the efficiency of the determination of the critical frequency of the F2 layer of ionosphere over European Russia are calculated with...     

Variations in the Hydrological Regime of Kara-Bogaz-Gol Gulf,Lake Issyk-Kul,and the Aral Sea Assessed Based on Data of Bottom Sediment Studies

The isotopic and material composition of bottom deposits, examined in the Kara-Bogaz-Gol Gulf, Lake Issyk-Kul, and the Aral Sea, is considered. The obtained data are used to characterize variations in the regime of the water bodies in the Holocene.     

Mass-induced sea level variations in the Red Sea from GRACE,steric-corrected altimetry,in situ bottom pressure records,and hydrographic observations

An annual amplitude of ∼18 cm mass-induced sea level variations (SLV) in the Red Sea is detected from the Gravity Recovery and Climate Experiment (GRACE) satellites and steric-corrected altimetry from 2003 to 2011. The annual mass variations in the region dominate the mean SLV, and generally reach maximum in late January/early February. The annual steric component of the mean SLV is relatively small (<3 cm) and out of phase of the mass-induced SLV. In situ bottom pressure records at the eastern coast of the Red Sea validate the high mass variability observed by steric-corrected altimetry and GRACE. In addition, the horizontal water mass flux of the Red Sea estimated from GRACE and steric-corrected altimetry is validated by hydrographic observations.     

Gravity Anomaly Map over The Black Sea Using Corrected Sea Surface Heights from ERS1, ERS2 and Topex/Poseidon Satellite Altimetry Missions

Approximately nine-year data from ERS1, ERS2 and TOPEX/POSEIDON (T/P) satellite altimetry missions have been used for the recovery of gravity anomalies over the Black Sea. The Corrected Sea Surface Height product of Aviso/Altimetry has been proven to be homogeneous after a cross-over adjustment. The Least Squares Collocation method was applied in a so-called remove-restore procedure. The residual geoid heights, obtained by subtracting EGM96 geoid heights from cross-over adjusted sea surface heights, were inverted to recover residual gravity anomalies in a grid structure over the Black Sea. Finally, EGM96 free air gravity anomalies were added to the predicted residual gravity anomalies to obtain the free air gravity anomalies. In order to check the consistency with respect to an external source, these computed free air gravity anomalies were compared to ship gravity observations, and to alternative satellite altimetry derived gravity anomalies. Comparisons with the observed gravity data yielded that external consistency of the gravity anomalies computed in this study is about 3 mGal for individual ship tracks. Overall external consistency in the test area is 4.8 mGal. Comparison with other satellite altimetry derived gravity anomalies presented a good agreement.     

海洋地磁参量实时测量野值的在线辨识与改正

地磁参量实时测量野值的在线辨识与改正是影响地磁导航算法定位概率和精度的重要因素.地磁导航定位的研究目前主要集中于匹配定位算法,地磁参量实时测量数据处理的研究较少,本文将多层递阶非平稳时间序列预测模型引入海洋地磁参量实时测量野值在线辨识与改正,利用多层递阶模型对实时测量值进行一步预测,根据迪克松准则进行野值辨识,利用中值滤波和一步预测值对野值进行改正,仿真数据和实测数据的实验室仿真结果表明,本文提出的野值在线辨识与改正算法不仅可以检测全部孤立型野值而且对多数斑点型野值也有较好的辨识效果,对产生野值处信号的复原误差小于5％.     

Second Tesseral Harmonic Torque Due to the DynamicS of the Oceanic Surface Layer as Detected by TOPEX/POSEIDON Altimetry 1993-2000

Estimates of the second tesseral torque due to the variations in the radial space position of the mean ocean surface as monitored by TOPEX/POSEIDON altimeter system are derived. The magnitude of the studied torque may be compared to the tidal torque and to the tesseral torque caused by deformations due to the Earths rotation. However, such torque estimates strongly depend on the thickness of the ocean surface layer adopted in the spherical model of which the dynamics is believed to be responsible for the derived torque. The dependence on the thickness is discussed.     

The El Salvador and Philippines Tsunamis of August 2012: Insights from Sea Level Data Analysis and Numerical Modeling

We studied two tsunamis from 2012, one generated by the El Salvador earthquake of 27 August (Mw 7.3) and the other generated by the Philippines earthquake of 31 August (Mw 7.6), using sea level data analysis and numerical modeling. For the El Salvador tsunami, the largest wave height was observed in Baltra, Galapagos Islands (71.1 cm) located about 1,400 km away from the source. The tsunami governing periods were around 9 and 19 min. Numerical modeling indicated that most of the tsunami energy was directed towards the Galapagos Islands, explaining the relatively large wave height there. For the Philippines tsunami, the maximum wave height of 30.5 cm was observed at Kushimoto in Japan located about 2,700 km away from the source. The tsunami governing periods were around 8, 12 and 29 min. Numerical modeling showed that a significant part of the far-field tsunami energy was directed towards the southern coast of Japan. Fourier and wavelet analyses as well as numerical modeling suggested that the dominant period of the first wave at stations normal to the fault strike is related to the fault width, while the period of the first wave at stations in the direction of fault strike is representative of the fault length.