Sub-pixel evaluation of sea ice roughness using AMSR-E data

The rapidly changing sea ice regime in the Arctic has necessitated an evaluation of sea ice roughness at smaller scales than those provided by satellites. In this article, we evaluate sub-pixel (<5.4 km) sea ice roughness using AMSR-E brightness temperature (T_b) 89 GHz data and in situ physical roughness data acquired using a helicopter-based laser system in the southern Beaufort Sea during April–June of 2008. The analysis shows a statistically significant correlation (r² = 0.61, P-value < 0.05, regression line slope = –79.93) of T_b at horizontal polarization (H-pol) decreasing with increasing root mean square (RMS) heights. These results suggest that 89 H-pol is more sensitive (than vertical polarization (V-pol)) to the changes in physical roughness. The temporal evolution in AMSR-E T_b values at 89 H-pol and 89 V-pol shows a decrease from April to June. We conclude that solely the AMSR-E T_b at 5.4 km is insufficient to fully account for the changes occurring in the dielectric properties and surface roughness of sea ice at sub-pixel level of 1–4 km during April–June.     

Sea ice small-scale surface roughness estimation based on AMSR-E observations

In this article, the retrieval of a sea ice small-scale surface roughness parameter using a proposed model is investigated at several Advanced Microwave Scanning Radiometer Earth Observing System (AMSR-E) channels (6.9, 10.7, and 89 GHz) over the Arctic oceans. The AMSR-E 89 GHz observations with a spatial resolution of approximately 6 km?× 4 km, nearly three times the resolution of the currently operational radiometer SSM/I 85 GHz (15 km?× 13 km), are fully exploited to retrieve the total and multiyear (MY) ice concentrations through the utilization of the ARTIST sea ice (ASI) and polarization corrected temperature (PCT) algorithms, respectively. To improve the accuracy of the retrieved ice concentration, a tie-point adaption scheme was used to obtain daily adaptable tie-points for the two ice concentration algorithms. A sea ice small-scale roughness parameter was then calculated with the model proposed by Hong for the above-mentioned three frequencies. At lower frequencies, such as 6.9 and 10.7 GHz, roughness estimates are available for all ice types. However, estimates at 89 GHz are physically illegitimate over the wintertime MY ice cover. The model estimates at the two low frequencies were further studied over a protracted period (2003–2010). The annual time series of the averaged estimate over the Arctic sea ice were found to exhibit a slightly decreasing trend (?2.1 × 10^?3 and??1.9 × 10^?3 cm year^?1 for 6.9 and 10.7 GHz, respectively). Meanwhile, the winter time series showed an increasing trend whereas the summer time series showed a remarkably decreasing trend, which indicates more serious melting activity occurring over the Arctic ice.     

Analysis of sea ice motion and deformation using AMSR-E data from 2005 to 2007

Dynamics of Arctic sea ice, including motion and deformation, are studied utilizing data from the Advanced Microwave Scanning Radiometer for the Earth Observing System (EOS) (AMSR-E) during 2005 and 2007. We first derive sea ice motion maps from the satellite data in a grid of 100 km?×?100 km using a two-dimensional wavelet method. These sea ice motion results are compared with those derived from buoy data from the International Arctic Buoy Programme. Secondly, it is well known that sea ice deformation can be characterized by a strain-rate tensor calculated from the ice velocity field. Two components of the strain-rate tensor quantify the divergence and the shearing of the ice field, respectively. Daily maps for both sea ice motion and strain-rate tensor, as well as monthly averages and spatial sums, are computed and analysed. Comparison of the monthly ice motion maps for May 2005 and May 2007 indicates that the anti-cyclonic Beaufort Gyre and Transpolar Drift Stream in the western Arctic are relatively stronger during 2007 than 2005. Different patterns in the spring months' sea ice deformation rates as characterized by the absolute values of the strain-rate tensor are observed when we compare the data of 2007 with those of 2005 and 2006. The sea ice deformation activities in the spring of 2007 happen earlier and are relatively stronger than that of 2005 and 2006. These results might help to explain why the sea ice extent in the summer of 2007 is unprecedentedly low.     

Observations of ice surface temperature and thickness in the Baltic Sea

This paper presents the results of an airborne thermal infrared (TIR) experiment. The data were obtained during 6–9 February 1992 period in the Bay of Bothnia, the northern section of the Baltic Sea, in connection with an ERS‐1 field campaign. Two Aegema model 880 TIR cameras were used with nominal wavelengths at 5 and 10 µm, attached to the front of the helicopter with a Tyler mount. The camera's thermal resolution is 0.2 K; after corrections for atmosphere effects, the surface temperature accuracy is 0.5 K. The spatial resolution of individual images is 70 cm at the 300‐m flight altitude. The measurement programme was successful, producing high‐quality TIR data over ice for two different days, even under difficult weather conditions. Ice temperatures ranged from open water temperatures to 261 K for fast ice conditions on the day on the first flight. The standard deviation of the surface temperature, generally, increased with ice thickness with a value of ～0.4 K for maximum thickness, and autocorrelation length scales not exceeding a value of 5 m. Generally, all the higher values of standard deviation (>0.7) of surface temperatures were for scenes with mixed ice/open water. The results show that TIR has a substantial ability to classify ice type and thickness when the air temperature is less than 269 K, from open water at the freezing point to thin nilas and thicknesses up to 20–45 cm in the fast ice zone. In addition, a quasi‐steady sea‐ice model is used to provide a physical interpretation of the sea‐ice surface temperatures. The use of the model requires information on the atmospheric surface layer and snow thickness data, together with calibration points. The model worked well when the air temperature was around 260 K or less.     

Estimation of snow water equivalent over first-year sea ice using AMSR-E and surface observations

A SWE retrieval algorithm developed in-situ using passive microwave surface based radiometer data is applied to the Advanced Microwave Scanning Radiometer for Earth Observation System (AMSR-E). Snow water equivalent is predicted from two pixels located in Canadian Arctic Shelf Exchange Study (CASES) overwintering study area in Franklin Bay, N.W.T., Canada. Results show that the satellite SWE predictions are statistically valid with measured in-situ snow thickness data in both smooth and rough ice environments where predicted values range from 15 to 25 mm. Stronger correlation between measured and predicted data is found over smooth ice with R² value of 0.75 and 0.73 for both pixels respectively. Furthermore, a qualitative study of sea ice roughness using both passive and active microwave satellite data shows that the two pixels are rougher than the surrounding areas, but the SWE predictions do not seem to be affected significantly.     

Using remote sensing to estimate sea ice thickness in the Bohai Sea,China based on ice type

It is challenging to use traditional remote sensing techniques to accurately determine the extent and thickness of ice in the Bohai Sea, on account of the presence of sea impurities (i.e. mud, salt bubbles and sand) and shape irregularities. Accordingly, we performed a series of reflectance spectra experiments to empirically link remote measurements of surface reflectance with in situ sea ice thickness measurements in the Bohai Sea. Two years of Thematic Mapper (TM) band 2 and Moderate Resolution Imaging Spectroradiometer (MODIS) band 4 data were used to distinguish between the following sea ice types, using spectral reflectance thresholds of 6.4, 9.6, 10.3 and 12.1%: (a) clean nilas ice (a thin elastic crust of ice up to 10 cm thick that, under pressure, may deform by finger rafting; (b) nilas ice and pancake ice (roughly circular accumulations of frazil ice, usually less than about 3 m in diameter, with raised rims caused by collisions); (c) grey and grey–white ice; and (d) cumulative ice (<30 cm). By establishing a relationship between sea ice type and ice thickness, a novel, practical and low-cost remote sensing technique is introduced to estimate the extent and distribution of sea ice thickness over a large spatial scale. The results obtained by remote sensing are validated with in situ ice shape measurements. The MODIS and TM data are used to distinguish between three ice thickness grades (6–9, 10–20 and 20–30 cm).     

Analysis of sea ice motion and deformation in the marginal ice zone of the Bering Sea using SAR data

This paper presents a study of sea ice motion and deformation in the marginal ice zone (MIZ) of the Bering Sea in the winter season. Segmentation techniques and statistical methods are applied to high‐resolution synthetic aperture radar (SAR) images to derive ice motion and deformation maps. These techniques involve dynamic local thresholding (DLT), which allows separation of sea ice into different classes of thickness and type. Two ice motion characteristics are observed, one consisting of a translation and a rotation at a scale larger than about 10 km day^?1 and the other consisting of field deformations at a spatial scale of less than about 5 km over a 3‐day period. Sea ice deformation rates are calculated, and the divergence and shear feature of the sea ice in different regions identified. Possible causes, associated with wind, wave, current and internal ice forces, for the sea ice motion and deformation are discussed.     

Estimation of volume change rates of Greenland's ice sheet from ICESat data using overlapping footprints

Data from the ICESat/GLAS laser altimetry mission is used to obtain an estimate of the volume change of Greenland's ice sheet over the time span of February 2003 to April 2007. A novel processing strategy is developed and applied. It uses approximately 1 million ICESat elevation differences at geometrically overlapping footprints of both crossing and repeated tracks. The data are edited using quality flags defined by the ICESat/GLAS science team, as well as other additional criteria. In order to reduce the influence of surface slope, we propose a correction based on the ICESat/GLAS laser altimetry digital elevation model. Three slightly different processing strategies to convert the observed temporal elevation differences to elevation/volume changes are compared for 6 different drainage systems, further divided into regions above and below 2000 m in elevation. The final chosen strategy includes the correction for surface slopes, but does not include the removal of outlying elevation changes. For the region above 2000 m, a positive elevation change rate of 2 cm/year is obtained, which corresponds to a volume change rate of 21 km³/year. For the region below 2000 m the estimated elevation change rate is ? 24 cm/year, which corresponds to a volume loss of 168 km³/year. In general, the obtained results are in agreement with trends discovered by other authors that were also derived from laser altimetry. Nevertheless, the estimation obtained in this study suggests a more negative trend than those obtained previously. The differences can be explained by differences in the sampling of the region below 2000 m and, to a certain extent, by different time spans of the datasets used. A representative sampling of coastal areas is identified as the most critical issue for an accurate estimation of volume change rates in Greenland.     

Estimation of body fat from skinfold thickness

The fat content of the human body has physiological and medical importance. It may influence morbidity and mortality, it may affect the ability to withstand exposure to cold and starvation, and it may alter the effectiveness of drugs and anaesthetics. It may also be the cause of reduced insulin sensitivity in some population groups. Thus, the measurement of the total body fat provides useful information. A computer program, written in quick basic language, was developed, to compute the equivalent fat content, as a percentage of body-weight, for a range of values for the sum of the four skinfolds, of Egyptian males and females of different ages. This is based on a table developed by Durnin and Womersley. The computer program was run using skinfolds of Egyptian males and females of different ages. Satisfactory results were obtained. This program can be used to calculate the total body fat with relative ease and reasonable accuracy. This should make it of common use in many fields of medicine, physiology, nutrition and anthropology, particularly in population studies comprising a large number of subjects.     

利用MODIS数据估计中国黄东海区域测深参数

目的 现有关于漫衰减系数的研究大多是在490 nm波段建立反演模型,且未将相关研究与机载激光雷达测深能力建立联系,本文尝试获取测深参数532 nm漫衰减系数K_d(532)和透明度SD(Secchi disk depth),为机载双色激光雷达测深能力的评估和飞行方案的制定提供了重要技术参数。方法 首先分析了测深参数532 nm漫衰减系数K_d(532)和透明度SD对于评估机载双色激光系统测深能力的重要性。利用2003年春季中国黄东海区域的实测光学数据,对现有的漫衰减系数反演模式进行改进,建立了K_d(532)和K_d(490)=的线性关系以及SD和K_d(532)的幂函数关系。结果 根据2003年春季MODIS的490 nm漫衰减系数产品和上述函数关系获取了K_d(532)和SD参数。结论 本文获取测深参数K_d(532)和SD的方法能够用来有效评估该区域机载激光雷达的测深能力,准确性和精度依赖于实测光学数据的精度、分布和数量以及MODIS的K_d(490)产品的准确性。     

Estimation from aggregate data

A statistical methodology to handle aggregate data is proposed. Aggregate data arise in many fields such as medical science, ecology, social science, reliability, etc. They can be described as follows: individuals are moving progressively along a finite set of states and observations are made in a time window split into several intervals. At each observation time, the only available information is the number of individuals in each state and the history of each item viewed as a stochastic process is thus lost. The time spent in a given state is unknown. Using a data completion technique, an estimation of the hazard rate in each state based on sojourn times is obtained and an estimation of the survival function is deduced. These methods are studied through simulations and applied to a data set. The simulation study shows that the algorithms involved in the methods converge and are robust.     

Development of a water clear of sea ice detection algorithm from enhanced SeaWinds/QuikSCAT and AMSR-E measurements

We develop and evaluate water clear of sea ice (open water following ice cover) detection algorithms that make use of Scatterometer Image Reconstruction (SIR) SeaWinds/QuikSCAT (QuikSCAT) backscatter (σ°) and Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) brightness temperature (T_B) measurements. Algorithm validation was performed within Canadian Arctic waters using the Canadian Ice Service Digital Archive (CISDA) ice charts, NASATeam ice concentration estimates, extended AVHRR Polar Pathfinder (APP-x) albedo data, RADARSAT-1 imagery, and MODIS imagery. Results indicate that the temporal evolution of QuikSCAT σ°, AMSR-E polarization ratio (PR18), and AMSR-E vertical spectral gradient ratio (GR3618) can detect water clear of sea ice events, however mean differences due to frequency dependent characteristics of the data (spatial resolution; sensitivity to open water) were apparent. All water clear of sea ice algorithms are in good agreement with the timing and clearing patterns given by the CISDA. The QuikSCAT algorithm provided a more representative ice edge and more details on the ice clearing process due to higher spatial resolution, however, transient clearing events were better represented by the AMSR-E PR(18) or (GR3618) algorithm. By exploiting the strengths of each sensor, we found that a QuikSCAT and AMSR-E fused algorithm provide improved open water area estimates by as much as 11%. The fusion of QuikSCAT and AMSR-E PR(18) yielded in the most spatially representative open water detection. The residual surface of the water clear of sea ice algorithms was found to provide another measure of the average September minimum pan-Arctic sea ice extent within 6% of the NASATeam algorithm estimates.     

Estimation of the surface heat budget and the thickness of thin sea-ice in Arctic polynyas with Landsat 5 TM

A method was developed to relate the thickness of sea-ice to its surface temperature and other meteorological parameters controlling the surface heat exchange between sea-ice and the atmosphere. The method was based on a one-dimensional thermodynamic sea-ice model, allowing an optional snow cover. The surface temperature and the albedoof sea-ice were derived from radiance measurements of the Thematic Mapper. The method was applied to the NorthEast Water polynya in winter (East of Greenland) to estimate the polynya's ice thickness and its surface energy exchange. The results were compared with the pack-ice zone surrounding the polynya. The polynya is covered by thin ice surrounded by thicker multi-year ice. The surface heat exchange in the polynya (-150Wm-2) was significantly larger than above Arctic pack-ice (-25Wm-2 to -50Wm-2).     

Estimation of burnt coal seam thickness in central Borneo using JERS-1 SAR data

We analysed the interaction of microwaves with a burnt coal seam. This analysis approximated a burnt coal seam as a microwave absorber. The impedance of an incident wave with horizontal polarization (transverse magnetic mode) was derived in order to calculate the relationship between burnt coal seam thickness and backscattering coefficient. The result was confirmed by simulating scattering on a burnt coal seam using the Finite Difference Time Domain (FDTD) method. Both were similar. This relationship was used to estimate burnt coal seam thickness in central Borneo using Japanese Earth Resources Satellite (JERS-1) SAR data. Estimation results and ground data were similar.     

On the classification of melt season first-year and multi-year sea ice in the Beaufort Sea using Radarsat-2 data

Interpreting satellite microwave sea ice data during the melt season is difficult. Warm temperatures allow for a greater presence of water in the liquid phase at the surface and within the ice, resulting in similar backscattering responses for first-year ice (FYI) and multi-year ice (MYI). Differentiating these ice types is important, especially during summer, in view of the higher presence of seasonal marine traffic, functioning of the ecosystem, and the Inuit use of the marine icescape in summer. In this article, we investigate the similarities between geophysical, thermodynamic, and dielectric characteristics of the late-season MYI and FYI, and discuss how this can lead to a false detection of MYI. The study uses Radarsat-2 data for ice detection during summer. This involves an analysis of co-polarization versus cross-polarization (HH vs. HV), various incident angles (20°, 35°, and 45°), and ice types (FYI vs. MYI). Statistical analyses of the measurements obtained in 2009 identify the difficulty in differentiating ice types during summer. The results show that the physical and electromagnetic properties of the ice surfaces are virtually identical with few differences in the scattering of microwave energy. We conclude with suggestions on how a more effective differentiation of MYI and FYI types in the summer season can be accomplished.     

Validation of AMSR-E soil moisture using L-band airborne radiometer data from National Airborne Field Experiment 2006

AMSR-E has been extensively evaluated under a wide range of ground and climate conditions using in situ and aircraft data, where the latter were primarily used for assessing the T_B calibration accuracy. However, none of the previous work evaluates AMSR-E performance under the conditions of flood irrigation or other forms of standing water. Also, it should be mentioned that global soil moisture retrievals from AMSR-E typically utilize X-band data. Here, C-band based AMSR-E soil moisture estimates are evaluated using 1 km resolution retrievals derived from L-band aircraft data collected during the National Airborne Field Experiment (NAFE'06) field campaign in November 2006. NAFE'06 was conducted in the Murrumbidgee catchment area in southeastern Australia, which offers diverse ground conditions, including extensive areas with dryland, irrigation, and rice fields. The data allowed us to examine the impact of irrigation and standing water on the accuracy of satellite-derived soil moisture estimates from AMSR-E using passive microwave remote sensing. It was expected that in fields with standing water, the satellite estimates would have a lower accuracy as compared to soil moisture values over the rest of the domain. Results showed sensitivity of the AMSR-E to changes in soil moisture caused by both precipitation and irrigation, as well as good spatial (average R = 0.92 and RMSD = 0.049 m³/m³) and temporal (R = 0.94 and RMSD = 0.04 m³/m³) agreement between the satellite and aircraft soil moisture retrievals; however, under the NAFE'06 ground conditions, the satellite retrievals consistently overestimated the soil moisture conditions compared to the aircraft.     

基于冰和水导电特性的新型冰层厚度传感器

包含有离子的水溶液在外施电场作用下是导电的。随着温度的变化,水和冰的导电率也发生变化。基于水和冰的这一导电特性,提出了一种新的冰层厚度传感器结构及其检测方法。这一新型冰层厚度传感器及其检测方法对于在恶劣的检测环境下进行水文检测具有积极的作用。     

A continuous global record of near-surface soil freeze/thaw status from AMSR-E and AMSR2 data

Long term and consistent records of near-surface soil freeze/thaw (F/T) status are required for understanding hydrological, ecological, and biogeochemical responses of land surface to global warming. To create such a record, we compiled and inter-calibrated satellite observations from the Advanced Microwave Scanning Radiometer – Earth Observing System (AMSR-E) and its successor, AMSR2, using linear regression models, and then applied a discriminant algorithm to the calibrated observations to map global F/T status from 2002 to 2018. The new global F/T dataset was rigorously assessed using in situ air and surface temperatures, and modelled soil temperature. Results show that agreement between remotely sensed F/T status and that determined by in situ or modelled temperature exceeds 85% and 79% for ascending and descending orbits, respectively. Moreover, consistency between the F/T datasets derived from two sensors is around 0.8 after calibration, in nonoverlapping time frames. With such an accuracy and consistency, we calculated frost days and frost trends using the F/T dataset. The mean annual number of frost days of high northern latitudes (>45° N) is 279.2 ± 44.1 days. Based on Mann-Kendall’s tau-b test, 7.7% of global lands show a significant warming trend, and most of which are concentrated in the Western United States, Northern and Eastern Canada, Northern Europe and Western China. Such a spatial distribution was found to be consistent with the global land surface temperature anomalies trend from 2002 to 2018. Both the results of applications and favourable accuracy indicate the potential of this long, consistent F/T record to track global temperature change.     

Retrieval of aerosol spectral optical thickness from AVIRIS data

The ER-2, with the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) onboard, overflew Rapid City, South Dakota, and the United States Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Site, Oklahoma, on 9 August and 15 August 1993, respectively. High contrast natural and artificial surfaces present in the imagery were used as a basis for retrieving aerosol spectral optical depth (SOD) over these two sites. Coincident measurements of spectral optical depth from a surface-based sunphotometer also were obtained and used as a validation of the AVIRIS derived retrievals. The accuracy of the retrievals is discussed as a function of measurement uncertainty and surface contrast. The results indicate that, given sufficiently small sensor errors and spectrally uniform surfaces with a reflectance difference of at least 0.5, aerosol spectral optical depth over clear continental atmospheres can be retrieved from high spatial resolution space-based imagery to an accuracy of approximately 0.1. Although only two cases are reported here and additional tests are required, these preliminary results suggest that background aerosol spectral optical depth (i.e., τ_aerosol, <01) cannot be retrieved with adequate accuracy from space; however, the aerosol spectral optical depth of more polluted atmospheres (i.e., τ_aerosol < 0.2) can be retrieved with adequate accuracy.     

Sea ice altimeter processing scheme at the EODC

Sea ice presents a serious impediment to both shipping and off-shore operations in the polar regions. Since sea ice conditions can change within a matter of hours, near real time monitoring is required. Airborne data are available in some areas, but collection is expensive and coverage limited. Satellite images can provide wider coverage, but cloud cover, darkness and the need for rapid processing and dissemination can limit their use. Information on sea ice cover over longer periods is needed for global climate monitoring. Microwave sensors provide the most practical means of monitoring global sea ice cover since they can operate both at night and day and observe through clouds. Previous studies have concentrated on the use of passive microwave data.

Here we discuss the routine monitoring of sea ice using the ERS-1 radar altimeter. The low data rate and somewhat simple nature of the data, lend themselves to the mapping of global sea ice cover and to operational applications.

We review the processing adopted at the U.K. EODC.