基于CryoSat-2测高数据的全南极冰盖DEM的建立与精度评估

数字高程模型（DEM）是南极冰盖变化研究的基础,由于现场实测数据的稀缺,卫星测高数据是南极地区构建DEM的主要数据来源.CryoSat-2是新一代用于极地冰盖、海冰监测的测高卫星,本文利用2012-12—2015-01两个完整周期的CryoSat-2测高数据建立一个新的南极冰盖DEM.坡度是影响卫星测高精度的重要因素之一,利用改进的重定位方法对CryoSat-2数据进行坡度改正.插值方法是影响DEM精度的重要因素,通过对几种常用插值方法的比较,最后选用克里金插值方法对测高数据进行插值,建立了1 km分辨率的南极DEM.在88°S以南的CryoSat-2数据空白区,利用南极数字数据库（ADD）的等高线数据对DEM进行填补,建立了全南极冰盖DEM.利用ICESat卫星测高数据、IceBridge航空测高数据以及GPS地面实测数据对新建立的CryoSat-2 DEM进行精度验证,并与Bamber 1 km DEM、ICESat DEM、RAMPv2 DEM以及JLB97 DEM等四种国际上常用的南极DEM进行比较.结果表明：新建立的CryoSat-2 DEM的整体精度约为0.730±8.398 m;在冰穹顶部区域,DEM精度优于1 m;在冰架上,DEM精度约为4 m;在内陆冰盖大部分地区,DEM精度优于10 m;在地形复杂的山区和沿海边缘地区,DEM误差超过150 m.

     

联合卫星测高和模式资料研究海水热含量变化

本文在考虑洋底压力变化的情况下,利用2003~2008年融合多颗卫星的测高资料估计了全球和中国近海的海水热含量变化.顾及洋底压力(OBP)变化以及热膨胀系数随海水深度变化的影响,提出的改进方法提高了对中高纬度地区热含量变化的估算精度.在OBP变化较为明显的北太平洋区域I(30°N~50°N,170°E~190°E)、南印度洋区域II(40°S~60°S,100°E~120°E)和南太平洋区域III(40°S~60°S,100°W~120°W),改进方法的均方差较传统方法分别降低了16.3%、60.5%和48.4%.同时研究表明,卫星测高的精度以及盐度变化是影响中高纬度地区热含量估计精度的重要因素.在中国近海地区,东海和黄海的热含量主要表现为周年变化;南海区域的热含量除周年变化外,还存在半周年项和年际变化项,且南海的海水热含量近年有增加的趋势.     

New satellite derived sea ice motion tracks Arctic contamination

Sea ice has been reported to contain contaminants from atmospheric and nearshore sediment resuspension processes. In this study successive passive microwave images from the 85.5 GHz channels on the Special Sensor Microwave Imager (SSM/I) were merged with drifting buoy trajectories from the International Arctic Buoy Program to compute Arctic sea ice motion in the Russian Arctic between 1988 and 1994. Smooth daily motion fields were averaged to prepare monthly maps making it possible to compute the 7-year mean and mean seasonal ice motions as well as principal components of directional variability of sea ice motion for the entire Arctic and surrounding basins. These mean motion vectors are used to simulate the advection of contaminants deposited on or contained within the sea ice and subsequently transported into the Arctic Ocean in order to predict both their mean trajectories and dispersal over time. The 3-year displacement of contaminants from a number of Russian sites and one American site display various behaviours from substantial displacement and dispersal to almost no movement. This computational procedure could be applied to realtime SSM/I and ice buoy data to provide detailed, all-weather, vector motion maps of ice circulation to predict the path and dispersal of any new substance introduced to the sea ice and transported into the Arctic or Antarctic ocean surface.     

CryoSat-2测高数据与Landsat 8光学数据融合提取南极Amery冰架接地线

本文利用CryoSat-2测高数据与Landsat 8光学数据,针对南极Amery冰架区域开展接地线提取研究.首先通过Landsat 8光学影像三次Hermite多项式插值处理,在坡度分析的基础上利用表面曲率改进方法获取接地线特征点;同时对CryoSat-2测高数据进行坡度分析和表面曲率计算,通过沿轨梯度分析方法提取接地点;最后将Landsat 8与CryoSat-2数据获取的接地点进行最小二乘融合得到融合接地线.实验结果表明,融合结果利用高空间分辨率光学数据不仅能保证接地线提取精度,同时测高数据还能够弥补光学数据受云遮挡导致的数据空缺,保持接地线的完整性.与MOA产品比较可以看出,融合数据点与MOA接地线平均距离为367 m,标准差为601 m,所有数据点中距离小于1 km的点占总数的93.19%,与MOA产品具有较好一致.本文提出的融合算法可以实现空间连续的接地线提取结果,对后续研究南极物质平衡、冰流速计算等具有重要的意义.     

CryoSat-2测高数据与Landsat 8光学数据融合提取南极Amery冰架接地线

本文利用CryoSat-2测高数据与Landsat 8光学数据,针对南极Amery冰架区域开展接地线提取研究.首先通过Landsat 8光学影像三次Hermite多项式插值处理,在坡度分析的基础上利用表面曲率改进方法获取接地线特征点;同时对CryoSat-2测高数据进行坡度分析和表面曲率计算,通过沿轨梯度分析方法提取接地点;最后将Landsat 8与CryoSat-2数据获取的接地点进行最小二乘融合得到融合接地线.实验结果表明,融合结果利用高空间分辨率光学数据不仅能保证接地线提取精度,同时测高数据还能够弥补光学数据受云遮挡导致的数据空缺,保持接地线的完整性.与MOA产品比较可以看出,融合数据点与MOA接地线平均距离为367 m,标准差为601 m,所有数据点中距离小于1 km的点占总数的93.19%,与MOA产品具有较好一致.本文提出的融合算法可以实现空间连续的接地线提取结果,对后续研究南极物质平衡、冰流速计算等具有重要的意义.

     

Improved sea ice parcel trajectories in the Arctic via data assimilation

An assimilated sea ice motion product is used to track ice parcels in several regions of the Arctic over time periods of one day to several weeks during 1992-1993. Motions simulated using a two-dimensional, dynamic-thermodynamic sea ice model are combined with motions derived from daily 85 GHz special sensor microwave/imager (SSM/I) imagery using an optimal interpolation method that minimizes error covariance. Assimilation attenuates the tracking error over the stand-alone model in comparison to buoy trajectories with the same starting location and time. The average 14-day assimilated trajectory's displacement error is as much as 34% lower than the model trajectory, while the RMS direction error is decreased by up to 10 degrees (24%). Assimilation can also yield an estimate of dispersion, which is not retrievable by point buoy observations. An assimilation approach improves estimates of ice drift and has the potential to further the understanding of ice mass flux, freshwater flux, and pollutant transport in the polar regions.     

利用多年卫星测高资料研究南海上层环流季节特征

利用10年高精度卫星测高海面高异常网格资料,联合EGM96稳态海面地形模型,构成南海海域合成海面地形的时间序列,并计算了各个时期的南海表层地转流场. 利用卫星跟踪漂流浮标观测结果与相应时期南海地转流场进行对比验证,结果显示本文结果可以很好地反映南海海域一些中小尺度的环流特征. 根据南海各季节多年平均表层环流场结构,对南海环流周年变化规律和季节特征进行了初步的探讨. 研究结果表明,南海表层环流始终处在不断演变过程之中,在时间和空间上都表现出明显的多尺度特征.     

Dipole anomaly in the Arctic atmosphere and winter Arctic sea ice motion

Arctic sea ice, an important component of the cli- mate system, has received significant attention re- cently. Arctic sea ice variation is an important indica- tor of changes in the climate system, such as global change and polar amplification, and observation and climate modeling suggests that sea ice can itself be an agent of climate change[1―4]. Previous studies[3,5] have shown that sea ice influences the surface and atmos- pheric boundary layer temperature. Potentially more important is t…     

On the potential of mapping sea level anomalies from satellite altimetry with Random Forest Regression

Ocean Dynamics - The sea level observations from satellite altimetry are characterised by a sparse spatial and temporal coverage. For this reason, along-track data are routinely interpolated into...     

Ocean tides from satellite altimetry and GRACE

Satellite altimetry and GRACE observations carry both the signature of ocean tides and have in general complementary potential to resolve tidal constituents. It is therefore straightforward to perform a combined estimation of a global ocean tide model based on these two data sources. The present paper develops and applies a three step procedure for generating such a combined ocean tide model. First, the processing of multi-mission altimetry data is described along with the harmonic analysis applied to derive initially a pure empirical ocean tide model. Then the capability of GRACE to sense particular tidal constituents is elaborated and an approach to estimate tidal constituents from GRACE is outlined. In a third step a combination strategy with optimal stochastic data treatment is developed and applied to the altimetry-only tide model EOT08a and four years of GRACE observations, leading to the combined model EOT08ag. The differential contributions of GRACE to EOT08ag remain small and are mainly concentrated to the Arctic Ocean, an area with little or poor altimetry data. In comparison with other tide models, EOT08ag is validated by K-band range residuals, the impact on gravity field modelling and on precise orbit determination and by variance reduction of crossover differences. None of these comparison exhibits a significant improvement over the altimetry-only tide model except for a few areas above 60°N. Overall the improvements of the combination remain small and appear to stay below the current GRACE baseline accuracy.     

Re-calibration of Arctic sea ice extent datasets using Arctic surface air temperature records

A new seasonal and annual dataset describing Arctic sea ice extents for 1901–2015 was constructed by individually re-calibrating sea ice data sources from the three Arctic regions (North American, Nordic and Siberian) using the corresponding surface air temperature trends for the pre-satellite era (1901–1978), so that the strong relationship between seasonal sea ice extent and surface air temperature observed for the satellite era (1979-present) also applies to the pre-satellite era. According to this new dataset, the recent period of Arctic sea ice retreat since the 1970s followed a period of sea ice growth after the mid-1940s, which in turn followed a period of sea ice retreat after the 1910s. Arctic sea ice is a key component of the Arctic hydrological cycle, through both its freshwater storage role and its influence on oceanic and atmospheric circulation. Therefore, these new insights have significance for our understanding of Arctic hydrology.

EDITOR D. Koutsoyiannis

ASSOCIATE EDITOR not assigned     

Possible differences between mean sea-levels from satellite altimetry

Summary The first attempt has been made at determining the differences between mean sea-levels, using the recent satellite altimetry data [1].     

卫星测高揭示的海面变化经纬向耦合特征及其对ENSO事件响应

本文引入3阶主张量分析方法对1993~2008年赤道太平洋地区卫星测高数据进行解析,前两个主张量可有效表征海面变化的经、纬向耦合特征,重构与对比了该时段内6次ENSO事件海面变化的经、纬向演化的空间构型与耦合作用过程.结果表明:海面的经向变化可表征ENSO强度变化,纬向变化表现为受ENSO影响的年周期波动;经、纬向张量的时间系数与MEI以及EMI指数间多尺度分析表明,两者均受El Niño Modoki影响,但在耦合尺度、能量共振关系以及相位关系上存在差异;海面变化对不同类型ENSO事件响应差异主要表现在高、低海面位置、振幅以及高、低值区分布形态与空间范围等方面.其中常规的El Niño多表现为东太平洋型ENSO,El Niño Modoki则表现为中太平洋型.不同类型的ENSO在经纬向耦合演化轨迹的周期性、规则性和方向性特征可在一定程度上作为ENSO类型区分依据.     

Determination of the geopotential scale factor from satellite altimetry

Summary The geopotential scale factor R ₀ =GM/W ₀ has been determined on the basis of satellite altimetry as R _{0=(6 363 672·5±0·3) m} and/or the geopotential value on the geoid W ₀ =(62 636 256·5±3) m ² s ^–2 . It has been stated that R ₀ and/or W ₀ is independent of the tidal distortion of surface W=W ₀ due to the zero frequency tide.

---

¶rt;a nmu amumuu u ama amnmuaa R ₀ =GM/W ₀ =(6 363 672,5±0,3) m u/uu aunmuaa a nmuu¶rt;a W ₀ =(62 636 256,5±3) m² s^–2. m, m R ₀ u/uu W ₀ auum m nm amu a a nuu ¶rt;au nmu W=W ₀ .

     

A finite element sea ice model of the Canadian Arctic Archipelago

The Canadian Arctic Archipelago (CAA) is a complex area formed by narrow straits and islands in the Arctic. It is an important pathway for freshwater and sea-ice transport from the Arctic Ocean to the Labrador Sea and ultimately to the Atlantic Ocean. The narrow straits are often crudely represented in coupled sea-ice–ocean models, leading to a misrepresentation of transports through these straits. Unstructured meshes are an alternative in modelling this complex region, since they are able to capture the complex geometry of the CAA. This provides higher resolution in the flow field and allows for more accurate transports (but not necessarily better modelling). In this paper, a finite element sea-ice model of the Arctic region is described and used to estimate the sea-ice fluxes through the CAA. The model is a dynamic–thermodynamic sea-ice model with elastic–viscous–plastic rheology and is coupled to a slab ocean, where the temperature and salinity are restored to climatology, with no velocities and surface elevation. The model is spun-up from 1973 to 1978 with NCEP/NARR reanalysis data. From 1979 to 2007, the model is forced by NCEP/DoE reanalysis data. The large scale sea-ice characteristics show good agreement with observations. The total sea-ice area agrees very well with observations and shows a sensitivity to the Arctic oscillation (AO). For 1998–2002, we find estimates for the sea-ice volume and area fluxes through Admunsen Gulf, McClure Strait and the Queen Elizabeth Islands that compare well with observation and are slightly better than estimates from other models. For Nares Strait, we find that the fluxes are much lower than observed, due to the missing effect of topographic steering on the atmospheric forcing fields. The 1979–2007 fluxes show large seasonal and interannual variability driven primarily by variability in the ice velocity field and a sensitivity to the AO and other large-scale atmospheric variability, which suggests that accurate atmospheric forcing might be crucial to modelling the CAA.     

北极海冰与夏季欧亚遥相关型年际变化的联系及对我国夏季降水的影响

本文利用NCEP/NCAR等再分析资料和CAM3.1数值模式研究了夏季欧亚中高纬遥相关型年际变率与前期春季北极海冰变化的联系及其对我国夏季降水影响的可能机制.结果表明,夏季北大西洋-欧亚中高纬地区500 hPa位势高度场自然正交分解第二模态表现为"-+-+"遥相关波列,其中格陵兰岛-北大西洋和乌拉尔山地区为异常高空槽区所控制,而欧洲和贝加尔湖附近地区则为异常高压脊区,这种波列分布与欧亚中高纬EU型遥相关型十分类似.当遥相关波列为"-+-+"（"+-+-"）型分布时,前期春季巴伦支海北部和巴芬湾一带海冰偏少（多）,同期夏季巴伦支海北部一带海冰亦持续偏少（多）,同时在我国东北北部地区、长江和黄河之间地区降水明显偏少（多）.深入分析发现,巴伦支海北部和巴芬湾一带海冰偏少后,由于该地区湍流热通量明显偏强,在动力过程影响方面会形成异常Rossby波源,准定常Rossby波活动通量将向东亚地区传播,使得夏季欧亚中高纬"-+-+"遥相关波列出现.另外,海冰异常偏少后,在热动力过程影响方面,4-5月欧亚中高纬乌拉尔山-贝加尔湖以北地区积雪会出现"西少东多"偶极子型异常分布,其通过影响后期土壤湿度及下垫面热通量异常,也有利于夏季欧亚中高纬遥相关波列的维持.伴随着欧亚中高纬"-+-+"遥相关波列的出现,乌山阻塞高压偏弱,东亚槽偏浅,且亚洲副热带急流随之加强,贝加尔湖以北的副极地地区出现西风异常,东亚副热带急流北侧出现东风异常,贝加尔湖以南地区为异常反气旋控制,南下冷空气活动减弱.受到上述环流形势影响,我国东北北部地区、黄河和长江之间地区降水明显偏少.当巴伦支海北部和巴芬湾区域海冰偏多时,结论则反之.最后,基于春季海冰指数和晚春偶极子型积雪指数,我们建立了江淮流域夏季降水的预测模型,回报结果表明其对江淮流域夏季降水的年际变率具有较高的预测技巧.     

Recovery of marine gravity anomalies from ERS1, ERS2 and ENVISAT satellite altimetry data for geoid computations over Norway

Free-Air Anomalies (FAA) for the Norwegian marine area including some parts of the North Sea, the Norwegian Sea and the Barents Sea are computed from satellite altimetry data. A total of 84 cycles of ERS2 along-track data, 25 cycles of ENVISAT along-track data and high density ERS1 data during its geodetic mission are used. The new geopotential model from the Gravity Recovery and Climate Experiment (GRACE) mission, GGM02S (Tapely et al., 2005) is used to compute the long wavelength contributions of the geoid and the FAA. To correct data for mean dynamic topography, the available Levitus climatology model (Levitus and Boyer, 1994) is used. Corrected data are then used to compute along-track gradients in each cycle-pass to suppress the orbital and the atmospheric errors below the noise level of the altimeter. Resulted gradients are then stacked and the east-west and the north-south components of the deflection of verticals are computed where ascending and descending tracks meet each other. Finally, the inverse Vening-Meinesz formula is implemented on the gridded deflections to compute FAA. Results are then compared with available marine and airborne data. Standard deviations of ± 4.301 and ± 6.159 mGal in comparison with airborne and marine FAA were achieved. Thereafter, the derived anomalies are combined with marine and airborne FAA together with the land FAA to compute a fine resolution geoid for Norway and the surrounding marine areas. This geoid is evaluated over sea and land with the synthetic geoid (the geoid derived from the mean sea surface by subtracting the mean dynamic topography) and Global Positioning System (GPS)-levelling and the standard deviations of the differences are ± 20.9 and ± 12.8 cm respectively. ali.soltanpour@ntnu.no, hossein.nahavandchi@ntnu.no, kourosh.ghazavi@ntnu.no     

Using satellite altimetry data to augment flow estimation techniques on the Mekong River

Satellite altimetry is routinely used to provide levels for oceans or large inland water bodies from space. By utilizing retracking schemes specially designed for inland waters, meaningful river stages can also be recovered when standard techniques fail. Utilizing retracked waveforms from ERS‐2 and ENVISAT along the Mekong, comparisons against observed stage measurements show that the altimetric measurements have a root mean square error (RMSE) of 0·44–0·65 m for ENVISAT and 0·46–0·76 m for ERS‐2. For many applications, however, stage is insufficient because discharge is the primary requirement. Investigations were therefore undertaken to estimate discharges at a downstream site (Nakhon Phanom (NP)) assuming that in situ data are available at a site 400 km upstream (Vientiane). Two hypothetical, but realistic scenarios were considered. Firstly, that NP was the site of a de‐commissioned gauge and secondly, that the site has never been gauged. Using both scenarios, predictions were made for the daily discharge using methods with and without altimetric stage data. In the first scenario using a linear regression approach the altimetry data improved the Nash‐Sutcliffe r² value from 0·884 to 0·935. The second scenario used known river cross‐sections while lateral inflows were inferred from a hydrological model: this scenario gave an increase in the r² value from 0·823 to 0·893. The use of altimetric stage data is shown to improve estimated discharges and further applications are discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Low-frequency sea level variability and impact of recent sea ice decline on the sea level trend in the Arctic Ocean from a high-resolution simulation

Ocean Dynamics - The Arctic Ocean is undergoing significant changes, with rapid sea ice decline, unprecedented freshwater accumulation, and pronounced regional sea level rise. In this paper, we...     

Detection and classification of surface roughness in an Arctic marginal sea ice zone

Sea ice dynamic and thermodynamic processes are important and highly variable elements of the marginal ice zone (MIZ). This study examines the detection and classification of statistically separable sea ice classes in the MIZ through a range of temporal and spatial scales. A helicopter‐based laser system was used to obtain large‐scale and a ship‐based laser profiler to identify small‐scale roughness types, respectively. The analysis of variance of surface height data from helicopter‐ and ship‐based laser systems, active microwave (AMW) C‐band backscattering data and passive microwave (PMW) (37 and 89 GHz) brightness temperature data reveal different classes that statistically differ from one another. We found significant statistical difference in variances in AMW data with six classes that differ in VV polarization, three classes in VH polarization, and five classes in HH polarization in the MIZ (e.g. snow‐covered first‐year ice, ice rubble, pancake ice, frost flowers, melt pond, flooded ice, and ice edge) of southeastern Beaufort Sea. The PMW emission was not as effective at discrimination, yielding only one statistically separable class. The results can potentially be extended to satellite‐based investigations of the MIZ at regional scales. Copyright © 2012 John Wiley & Sons, Ltd.