Semi-automated feature-tracking of East Antarctic sea ice from Envisat ASAR imagery

While feature tracking of sea ice using cross-correlation methods on pairs of satellite Synthetic Aperture Radar (SAR) images has been extensively carried out in the Arctic, this is not the case in the Antarctic. This is due to the dynamic nature of Antarctic pack ice, its microwave signature, the tendency for SAR swath paths to be poorly aligned with the often narrow sea ice zone around the continent and inadequate satellite sampling. A semi-automated system, known as IPADS (IMCORR [IMageCORRelation] Processing, Analysis and Display System), has been developed to map fast ice and pack ice in Antarctica using multiple pairs of SAR images. The software processing pipeline uses overlapping image pairs which are geocoded and roughly registered using only data contained in the image headers. Next, fast ice maps are rapidly generated using zero motion features located within ocean regions. This also provides precise image registration. Finally, the same image pairs are re-examined for pack ice motion in a slower off-line batch process. The pack and fast ice are identified using a cluster-based search method which compares both location and motion information. Each image pair generates a NetCDF file which adds to a growing database of Antarctic sea ice motion and ice roughness. Five image-pair examples are presented to illustrate the methods used as well as their strengths and limitations. Substantial pack ice motion can often be detected in the marginal ice zone on SAR images only a few days apart.     

多时相遥感影像监测南极冰川流速

测定南极冰川的流速，对于研究南极冰雪物质平衡、南极环境变化及其对全球环境的影响和极区航行有重大意义。本论述了利用多时相卫星遥感影像监测冰川变化的原理和方法，通过对英格里特一克里斯泰森海岸不同时期的遥感影像进行几何纠正和配准及叠加处理，测量和计算出极记录冰川的平均流速，并对它们的变化特点进行了初步的分析和评价。     

Interannual variability of smoke and warm cloud relationships in the Amazon as inferred from MODIS retrievals

Aerosol and cloud data from the MODerate resolution Imaging Spectroradiometer (MODIS) onboard the Earth Observing System (EOS) Aqua are used to investigate interannual variability of smoke and warm cloud relationships during the dry-to-wet transition season (August-October) over the Amazon for two years and its association with meteorological conditions. In one year (2003), smoke aerosols are associated with an increase of cloud fraction and a decrease of cloud effective radius. These effects amplify the cooling at the surface and at the top of the atmosphere (TOA) caused by the aerosol extinction. However, in another year (2002) the cloud fraction decreases with increasing aerosol optical depth. Such a decrease of cloud fraction could offset the effect of increased reflection of solar radiation by the aerosols both at the surface and at TOA. The changes in radiative fluxes between these years would contribute to interannual changes of surface energy fluxes and radiative balance at the top of the atmosphere and influence variability of the wet season onset in the basin. In 2003, the atmosphere was more humid and less stable. These conditions may be relatively favorable for the activation of aerosol particles into cloud condensation nuclei and hence cloud droplets. In 2002, the clouds were less extensive and thinner in a relatively dry atmosphere and presumably dissipated more easily. This study suggests that the aerosol-cloud relation can be influenced by atmospheric structure and convective motions, in addition to changes in aerosols properties. An adequate characterization of aerosol-cloud relationship would require a longer time series of data that includes a variety of climate conditions. The caveat of this analysis is that differences in aerosol absorption and its vertical distribution may have contributed to the observed interannual change of smoke-cloud relationship but could not be determined due to lack of adequate measurements.     

Antarctic ice sheet and sea ice regional albedo and temperature change, 1981-2000, from AVHRR Polar Pathfinder data

Spring-summer (November, December, January) ice sheet and sea ice regional surface albedo, surface temperature, sea ice concentration and sea ice extent averages and trends from 1981 to 2000 have been calculated for the Antarctic area. In this research the AVHRR Polar Pathfinder 5-km EASE-Grid Composites and the combined SMMR and SSMI data sets from the National Snow and Ice Data Center (NSIDC), Boulder, Colorado have been employed. A regional analysis has been made for five longitudinal sectors around Antarctica: the Weddell Sea (WS), the Indian Ocean (IO), the Pacific Ocean (PO), the Ross Sea (RS) and the Bellingshausen-Amundsen Sea (BS). The IO and PO sectors show ice sheet albedos of 0.85 and temperatures of − 25 °C. The corresponding values in the RS and BS sectors are 0.80 and − 16 °C respectively. The sea ice albedo is about 0.60 in the RS, BS and WS sectors and 0.55 in the IO and PO sectors. The average sea ice temperature varies around − 12 °C. All the sectors show slight increasing spring-summer albedo trends and decreasing spring-summer temperature trends and similar interannual variability in albedo and surface temperature. The steepest ice sheet albedo trend of 0.0019 ± 0.0009/yr is found in the RS sector. The steepest sea ice albedo trend of 0.0044 ± 0.0017 /yr occurs in the PO sector. The steepest temperature trends for both the ice sheet and sea ice occur in the BS sector, having values of − 0.075 ± 0.040 °C/yr and − 0.107 ± 0.027 °C/yr respectively. The sea ice concentration shows slight increasing trends, the highest being in the PO sector (0.3 ± 0.12%/yr), whereas the sea ice extent trends are near zero with the exception of the RS sector (14,700 ±8600 km²/yr) and the BS sector (− 13,000 ± 6400 km²/yr).     

卫星遥感海面风场的进展

空间技术的发展使得卫星遥感成为获取海面风速信息的重要手段。对卫星微波遥感海面风场的各种传感器的性能、特点等作了较全面地评述,并指出了卫星微波遥感信息的复合分析和数值同化将是获取海面风场的两个主要方法。     

Studies of the Antarctic sea ice edge and ice extent from satellite and ship observations

Passive microwave-derived ice edge locations in the Antarctic are assessed against in situ observations from ships between 1989 and 2000. During the growth season (March-October), the ship data agrees with satellite data very well, with r² values of 0.99 and 0.97 for the Bootstrap and Team algorithms, respectively. During the melt season (November-February), the agreement is not so good with the passive microwave ice edge typically 1-2° of latitude south of the observations. This is due to the low concentration and saturated nature of the ice, and the r² values for this period are 0.92 and 0.80 for the Bootstrap and Team algorithms, respectively. Sensitivity studies show that such an offset in the summer ice edge location can cause significant errors in trend studies of the extent of sea ice cover in the Southern Ocean. The passive microwave ice concentration at the ice edge observed by ship varies greatly, averaging 14% for the Team algorithm and 19% for the Bootstrap. Comparisons between passive microwave data and SAR, Landsat and OLS data during the ice growth season show that while small-scale details in ice edge location are lost, the passive microwave data generally provide good and consistent representation of the higher resolution imagery.     

A method for sub-pixel scale feature-tracking using Radarsat images applied to the Mertz Glacier Tongue, East Antarctica

Tracking features between pairs of satellite synthetic aperture radar (SAR) images using image cross-correlation methods is a powerful tool for defining regions of image movement such as ice flow of Antarctic glaciers. Unlike optical images, processed SAR images have various instrumental effects such as view angle distortions and lateral displacement errors caused by slant range mapping. Co-registration issues are problematic and need to be addressed if there are few/no suitable stationary tie points at or near sea level within the images. Methods are presented that use Radarsat ScanSAR images to reliably measure sub-pixel displacements with errors of ? ± 0.25 pixels but this estimate is target dependent. The techniques and issues are illustrated using examples from a long-term study of the Mertz Glacier Tongue in East Antarctica. For image pairs only a few days apart, this extensive structure forms an excellent grid-like test pattern which provides up to 5000 valid correlation matches. Images separated by 100's of days can then be processed in the same way to reveal many subtle and some large changes over time. The technique described is robust and may be widely applicable.     

Development and validation of a technique for merging satellite derived aerosol optical depth from SeaWiFS and MODIS

Merging time series of satellite derived aerosol products from independent missions can support aerosol science by combining in a consistent way temporally overlapping data sets and by increasing data coverage. A merging technique applied to satellite aerosol optical depth τ_a is presented and tested with SeaWiFS and MODIS-Aqua data. The technique relies on least squares fitting of the available τ_a spectra onto a linear or second-order polynomial relation between log-transformed τ_a and wavelengths. First, the sensor specific products are compared with field observations collected by a sun-photometer installed on the Acqua Alta Oceanographic Tower in the northern Adriatic Sea. Mean absolute percentage differences are approximately 21% at 412 and 443 nm, and increase with wavelength, with large overestimates in the red and near-infrared bands. The mean absolute differences are typically 0.04. When inter-compared, the 2 satellite products agree well, with mean absolute percentage differences lower than 20% at all wavelengths and little bias. The results of the comparison of the merger outputs with the field data are well in line with the validation results of the sensor specific products, and are comparable for the various merging procedures. The benefits of merging in terms of data coverage are briefly illustrated for the Mediterranean basin.     

基于船载太阳光度计的中国海域MODIS卫星反演气溶胶光学参数验证

利用2006~2007年中国海洋大学东方红2号科考船在中国黄海和东海海域的POM-01MK2太阳光度计气溶胶光学参数的观测资料,与MODIS反演结果进行比较,给出了光学厚度、| ngstrm指数和粒子有效半径的对比结果。结果表明：两种观测方法得到的气溶胶光学厚度的一致性较好,相关系数（标准差）达0.97（0.08）,73%的对比结果在期望误差（Δτ=±0.03±0.05τ）之内;季节分类对比结果,秋季和春季相关系数（标准差）均为0.97（0.08）;海域分类对比结果,黄海北部海域和黄海南部海域相关系数（标准差）分别为0.98（0.08）和0.76（0.10）。MODIS反演得到的| ngstrm指数偏低,相关系数（标准差）为0.67（0.23）;按季节分类得到,秋季和春季相关系数（标准差）分别为0.71（0.27）和0.62（0.19）;按海域分类得到,黄海北部海域和黄海南部海域相关系数（标准差）分别为0.87（0.07）和0.70（0.30）。粒子有效半径的对比结果偏差和离散度较大,相关系数（标准差）仅为0.31（0.10）。造成这种现象的原因可能是该海域沙尘气溶胶和人类源二次气溶胶浓度较高导致海上气溶胶光学性质同MODIS反演中使用的光学参量有较大差别。     

面向对象的高分辨率影像采煤塌陷地提取

煤炭开采引起了塌陷等一系列的地质环境问题,与常规监测方法相比,遥感技术可以实现大范围、高效率、周期性的动态监测。在遥感影像分类方法中,面向对象的遥感影像分类方法能更好地利用高分辨率遥感影像中丰富的纹理和几何结构信息。针对煤炭开采导致的地表塌陷地的特点,在归纳整理遥感影像中塌陷地判识准则的基础上,重点探讨了面向对象的遥感影像分类方法中塌陷地的自动提取规则。综合利用ERDAS IMAGINE9.2、ENVI4.7和ENVI4.4 ZOOM进行数据处理,以安徽省淮南矿务集团潘三矿区为实验区,用该方法利用SPOT５影像进行了塌陷地信息提取实验,结果证明,面向对象分类方法能有效地从高分辨率遥感影像中自动提取塌陷地相关信息。     

Improving soil wetness variations monitoring from passive microwave satellite data: The case of April 2000 Hungary flood

Precipitation-runoff processes are correlated with catchment's hydrological conditions before the precipitation. Thus, an estimation of these conditions, particularly regarding soil wetness variations, is of considerable importance to improve the reliability of flood warning. In this paper, a new methodology is presented which, on the basis of microwave satellite observations, could permit us to monitor soil wetness variations at a global scale. The proposed method seems able to overcome the problems connected to surface roughness and vegetation cover that mainly limit the soil moisture estimations from satellite in the microwave region.Preliminary results achieved for the flooding event which occurred in the Carpathian basin (Hungary) in April 2000 will be described in detail. They seem to confirm the reliability of the proposed technique in the identification of different amounts of soil wetness, not only during and after the considered event, but, in order to possibly use it for warning system purposes, in the phase preceding the event as well.Such an approach is automatic and, for construction, globally exportable. Moreover, because of the complete independence from the specific satellite platform, such a technique could be easily exported to the new generation of satellite sensors with improved performances like AMSR-E aboard EOS-Aqua and MIRAS aboard SMOS.     

Detection of small-scale roughness and refractive index of sea ice in passive satellite microwave remote sensing

Polar ice masses and sheets are sensitive indicators of climate change. Small-scale surface roughness significantly impacts the microwave emission of the sea ice/snow surface; however, published results of surface roughness measurements of sea ice are rare. Knowing the refractive index is important to discriminate between objects. In this study, the small-scale roughness and refractive index over sea ice are estimated with AMSR-E observations and a unique method. Consequently, the small-scale surface roughness of 0.25 cm to 0.5 cm at AMSR-E 6.9 GHz shows reasonable agreement with the results of known observations, ranging from 0.2 cm to 0.6 cm for the sea ice in the Antarctic and Arctic regions. The refractive indexes are retrieved from 1.6 to 1.8 for winter, from 1.2 to 1.4 for summer in the Arctic and the Antarctic, which are similar to those of the sea ice and results from previous studies. This research shows the physical characteristics of the sea ice edges and melting process. Accordingly, this investigation provides an effective procedure for retrieving the small-scale roughness and refractive index of sea ice and snow. Another advantage of this study is the ability to distinguish sea ice from the sea surface by their relative small-scale roughness.     

A composite DOP approach to excluding bottom reflectance in mapping water parameters of shallow coastal zones from TM imagery

In this paper we proposed a composite depth of penetration (DOP) approach to excluding bottom reflectance in mapping water quality parameters from Landsat thematic mapper (TM) data in the shallow coastal zone of Moreton Bay, Queensland, Australia. Three DOPs were calculated from TM1, TM2 and TM3, in conjunction with bathymetric data, at an accuracy ranging from ±5% to ±23%. These depths were used to segment the image into four DOP zones. Sixteen in situ water samples were collected concurrently with the recording of the satellite image. These samples were used to establish regression models for total suspended sediment (TSS) concentration and Secchi depth with respect to a particular DOP zone. Containing identical bands and their transformations for both parameters, the models are linear for TSS concentration, logarithmic for Secchi depth. Based on these models, TSS concentration and Secchi depth were mapped from the satellite image in respective DOP zones. Their mapped patterns are consistent with the in situ observed ones. Spatially, overestimation and underestimation of the parameters are restricted to localised areas but related to the absolute value of the parameters. The mapping was accomplished more accurately using multiple DOP zones than using a single zone in shallower areas. The composite DOP approach enables the mapping to be extended to areas as shallow as <3 m.     

Algorithm for retrieval of the effective snow grain size and pollution amount from satellite measurements

We present an algorithm for retrieval of the effective Snow Grain Size and Pollution amount (SGSP) from satellite measurements. As well as our previous version (Zege et al., 2008, 1998), the new algorithm is based on the analytical solution for snow reflectance within the asymptotic radiative transfer theory. The SGSP algorithm does not use any assumptions on snow grain shape and allows for the snow pack bidirectional reflectance distribution function (BRDF). The algorithm includes a new atmospheric correction procedure that allows for snow BRDF. This SGSP algorithm has been thoroughly validated with computer simulations. Its sensitivity to the atmosphere model has been investigated. It is shown that the inaccuracy of the snow characteristic retrieval due to the uncertainty in the aerosol and molecular atmosphere model is negligible, as compared to that due to the measurement errors at least for aerosol loads typical for polar regions. The significant advantage of the SGSP over conventional algorithms, which use a priori assumptions about particle shape and (or) not allow for the BRDF of the individual snow pack, is that the developed retrieval still works at low sun elevations, which are typical for polar regions.     

Characterising groundwater use by vegetation using a surface energy balance model and satellite observations of land surface temperature

This study presents a novel ‘model-data’ approach to detect groundwater-dependent vegetation (GDV), through differences in modelled and observed land surface temperatures (LST) in space and time. Vegetation groundwater use is inferred where modelled LST exceeds observed LST by more than a threshold determined from consideration of systematic and random errors in model and observations. Modelled LST was derived from a surface energy balance model and LST observations were obtained from Terra-MODIS thermal imagery. The model-data approach, applied in the Condamine River Catchment, Queensland, Australia, identified GDV coincident to existing mapping. GDV were found to use groundwater up to 48% of the time and for as many as 56 consecutive days. Under driest of conditions, groundwater was estimated to contribute up to 0.2 mm h⁻¹ to total ET for GDV. The ability to both detect the location and water-use dynamics of GDV is a significant advancement on previous remote-sensing GDV methods.     

基于改进型统计区域增长的遥感图像分割

随着遥感技术的发展,高分辨率遥感图像内容变得越来越复杂,信息表现更加精细,将某些中、低分辨率图像的图像分割、 分类算法直接应用到这类图像上时,其效果往往不理想。在传统区域增长算法的基础上,进行了四重优化,采用四邻域策略组合成像素对,严格化合并准则,在不同尺度下将像素对进行合并,得到较好的分割影像。实验证明,该算法较大程度上实现了目标的提取,减少了传统区域合并算法的计算量,与原算法相比较更多地考虑了像素的空间特性,具有很好的抗噪能力,能够适应于大数据量高分辨率遥感影像的多尺度分割。     

Observations of Lake Baikal ice from satellite altimetry and radiometry

We demonstrate the potential of combining satellite altimetry and radiometry for lake ice studies using the example of Lake Baikal in Siberia. We show the synergy using active and passive microwave observations available from the recent satellite altimetry missions (TOPEX/Poseidon, Jason-1, ENVISAT and Geosat Follow-On), complemented by the SSM/I passive data. We assess the applicability of altimetric and radiometric data for ice/water discrimination, and discuss the drawbacks and benefits of each type of sensor. An ice discrimination method, based on the combined use of the data from the four altimetric missions and SSM/I, is proposed and validated using available in situ observations and MODIS imagery. The method is applied to the entire satellite data set and used to define specific dates of ice events (first appearance of ice, formation of stable ice cover, first appearance of open water, complete disappearance of ice) and associated uncertainties. Using these satellite-derived estimates, we can extend the existing time series of ice events in the Southern Baikal up to 2004 and provide new information on the Middle and Northern Baikal, regions where no recent in situ ice cover observations are available. Our data show that over the last 10-15 years, trends towards earlier ice formation and later ice break-up result in a tendency for longer fast ice duration over the whole Lake Baikal. The methods proposed here have been tested for Lake Baikal, but they are applicable for other lakes and water bodies with seasonal ice cover.     

Retrieval of total suspended matter concentration in the Yellow and East China Seas from MODIS imagery

A retrieval algorithm, of total suspended matter (TSM) concentration in the Yellow Sea (YS) and East China Sea (ECS) was developed using observations made in the 2003 Spring and Autumn cruises over the YS and the ECS. Analysis of the in-situ backscattering coefficients of the suspended particles (b_bp) indicates that the accuracy becomes worse when the concentration of TSM (C_TSM) is higher than 20 mg/l. The accuracy of the b_bp is improved by using a bio-optical model in which b_bp is optimized with a non-linear least-square Levenberg-Marquardt method. The remote sensing reflectance (R_rs) is obtained by means of the optimization. The optimized R_rs for waters with C_TSM higher than 20 mg/l, together with the measured R_rs for waters with C_TSM lower than 20 mg/l, are used to establish the relationships between R_rs(748), R_rs(869) and R_rs(645), which are used in the iterative method for atmospheric correction. Two atmospheric correction algorithms are switched according to the water turbidity. The shortwave infrared wavelengths (SWIR) method is used for waters with high-turbidity, and the iterative method is used otherwise. Results of the atmospheric correction were then applied to the Tassan model modified in this paper to compute the C_TSM. Comparison between the retrieval results from MODIS imagery and the in-situ measurements indicates that the algorithms described in this paper can provide a reliable estimation of the C_TSM distributions in the YS and ECS.     

Assessment of NO2 satellite observations for en-route aircraft emissions detection

In this work we examine the possibility of using satellite remote sensors for the detection of air traffic emissions produced during the en-route segment of flight in the Upper Troposphere/Lower Stratosphere region (8000-12,000 m). NO₂ has been considered as the tracer of aircraft plumes with highest possibility of being successfully detected from space. An analysis of the technical potential of the current orbital sensors capable of measuring NO₂ in the proximity of the tropopause has been conducted. In order to estimate an upper bound for the NO₂ column related to aircraft emissions, the Canary Islands Corridor has been selected for conducting a simple emission calculation exercise based on real air traffic and operational data, assuming an ideal atmospheric scenario. The results obtained in this approximation have been compared to the actual information retrieved from space sensors. An in-depth inspection of the NO₂ column data for two particular areas (Canary Islands Corridor and North Atlantic Flight Corridor) obtained in recent years by SCIAMACHY and OMI has also been carried out.The general conclusions of this viability study are not optimistic. The estimated maximum NO₂ column value attributable to aircraft emissions at cruise altitudes were lower than the detection limits associated with SCIAMACHY and OMI for NO₂ column measurements. As a result, detecting and quantifying the actual NO₂ levels in aircraft corridors by space remote sensing is a very challenging task.     

The infrared spectral signature of volcanic ash determined from high-spectral resolution satellite measurements

High-spectral resolution infrared spectra of the earth's atmosphere and surface are routinely available from satellite sensors, such as the Atmospheric Infrared Sounder (AIRS) and the Infrared Atmospheric Sounding Interferometer (IASI). We exploit the spectral content of AIRS data to demonstrate that airborne volcanic ash has a unique signature in the infrared (8-12 μm) that can be used to infer particle size, infrared opacity and composition. The spectral signature is interpreted with the aid of a radiative transfer model utilizing the optical properties of andesite, rhyolite and quartz. Based on the infrared spectral signature, a new volcanic ash detection algorithm is proposed that can discriminate volcanic ash from other airborne substances and we show that the algorithm depends on particle size, optical depth and composition. The new algorithm has an improved sensitivity to optically thin ash clouds, and hence can detect them for longer (~ 4 days) and at greater distances from the source(~ 5000 km).