Earth Grid Research Articles

Advanced earth grid monitoring for HV network safety compliance

ABSTRACT Electrical energy is a critical component for human comfort, but mismanagement of this energy can lead to human injury and even fatalities. High voltage safety compliance hinges on the presence of an adequate earthing system within the infrastructure. Various factors influence the effectiveness of the earthing system, including soil resistivity in the surrounding area and the availability of auxiliary paths for fault currents. Weather conditions, changes in the surrounding environment, and alterations within the high-voltage network significantly impact these elements. Any modifications to the auxiliary path conditions can have severe consequences on the network’s safety compliance. The current industry practice lacks continuous monitoring of the earth grid and fault current paths, with no provision for frequent current injection tests on the entire earth grid system. To close the existing gap, this paper proposes a novel approach through the introduction of an Earthing IoT Monitoring system. This system enables network owners to monitor safety compliance across the entire network, identify fault current return paths, and pinpoint any necessary preventive maintenance measures. The paper also delves into the crucial roles played by soil resistivity and split factors in the earthing system domain. A case study is included to support the innovative nature of the proposed system.

Transmission line earthing system; Advanced design diagram for safety compliance

An electrical high-voltage power system is essential to support everyday electrical demands. Due to the increase in electrical loads, a more rigid design is required to ensure system compliance with standards and policies. High-voltage safety is the number one priority when it comes to the design, installation, commissioning, and operation of the electrical power system network. Safety compliance is getting more complex due to the joining infrastructure between residential and high-voltage projects. Nowadays, high-voltage infrastructure, including substations and transmission lines, is constructed near residential properties. This neighboring setup increased the safety concerns of high voltage systems, especially under fault or system malfunction situations. Safety concerns are addressed during the design stage and verified during the commissioning stages.The paper discusses the minimum transmission line earthing system requirements. This document provides a clear and consistent approach to assess transmission line earthing system design. The work contents comply with current IEEE standards including rigid author research outcomes. The works reduce the complexity of the assessment and explain the critical role of different sectors within the transmission line network. The work introduces the boundaries when the fault current analysis should include the earth grid resistance. A case study is also included.

A climate data record of year-round global sea-ice drift from the EUMETSAT Ocean and Sea Ice Satellite Application Facility (OSI SAF)

Abstract. Sea ice in the polar regions can move several tens of kilometres per day under the actions of winds, ocean currents, and internal stresses. Long-term observations of the rate and patterns of this motion are needed to characterize the full response of the polar environment to climate change. Here, we introduce a new climate data record (CDR) of year-round, global, daily sea-ice drift vectors covering 1991–2020. The motion vectors are computed from series of passive microwave imagery in the winter months and from a parametric free-drift model in the summer months. An evaluation against on-ice buoy trajectories reveals that the RMSEs of the sea-ice drift CDR are small and vary with hemisphere and seasons (2.1 km for Arctic winters, 2.6 km for Arctic summer, 3 to 4 km for the Antarctic sea ice). The CDR is un-biased for Arctic winter conditions. The bias is larger for Antarctic and for summer sea-ice motion. The CDR consists of daily product files holding the dX and dY components of the drift vectors on an Equal-Area Scalable Earth (EASE2) grid with 75 km spacing as well as associated uncertainties and flags. It is prepared in the context of the EUMETSAT Ocean and Sea Ice Satellite Application Facility (OSI SAF) and is readily available at https://doi.org/10.15770/EUM_SAF_OSI_0012 (OSI SAF, 2022).

Design, Modeling, and Analysis of IEEE Std 80 Earth Grid Design Refinement Methods Using ETAP

The design of a compliant, safe, and reliable substation earth grid is not a straightforward process; in most cases, it requires some additional measures to be taken due to various constraints that differ from environment to environment, such as soil resistivity, a high fault level, a limited surface area, construction budget, etc. The IEEE Std 80-2013 proposes various refinement methods that can be applied to address different situations. For this study, the current limiting method, current diversion method, and touch and step voltage increment method were applied using the Electrical Transient Analysis Program (ETAP). A power system was designed, where a fault current generated by the supply transformers and back-fed by the power system’s motors was exported to the earth grid. Using this fault current, various simulations were conducted to assess the performance of the earth grid. The analysis results show that the application of the current limiting method using neutral earthing resistors has a great impact on the design of the earth grid as this method significantly reduces the fault current injected into the earth grid. Furthermore, by applying the current diversion method, the amount of fault current injected into the earth grid is reduced by a fair amount, which improves the performance of the earth grid. Lastly, increasing the tolerable limits of touch and step voltages by reducing the fault clearance times significantly improves the compliance of the earth grid as the clearance time is reduced. From this study, it is therefore concluded that, by implementing the refinement methods depending on the design requirements and feasibility of the application, one can improve the compliance state of an earth grid.

제한된 면적에서 심타식 접지봉의 경제성 및 안전전압 유지성능에 관한 연구

In this paper, in order to design an effective grounding electrode in a confined space, the application of a deep-drilled ground rod together with a mesh grounding electrode is proposed. The CDEGS program was used to calculate the maximum safe current of the earthing electrode for each case, and the performance and economic feasibility of the earthing electrode were compared and analyzed through this data. In addition, ground electrode characteristics according to frequency changes were also compared. As a result of the study, it was confirmed that the role of the vertical earth electrode is very important when designing the earth electrode in a small space, and that the deep-drilled earth rod can effectively reduce the area of the earth grid. However, it was found that the ability to secure the safety voltage of the vertical earth electrode was greatly reduced in the frequency domain.

Evaluating the effective resolution of enhanced resolution SMAP brightness temperature image products

The MEaSUREs Calibrated Enhanced-Resolution Passive Microwave Daily Equal-Area Scalable Earth Grid 2.0 Brightness Temperature (CETB) Earth System Data Record (ESDR) includes conventional- and enhanced-resolution radiometer brightness temperature (TB) images on standard, compatible grids from calibrated satellite radiometer measurements collected over a multi-decade period. Recently, the CETB team processed the first 4 years of enhanced resolution Soil Moisture Active Passive (SMAP) L-band (1.41 GHz) radiometer TB images. The CETB processing employs the radiometer form of the Scatterometer Image Reconstruction (rSIR) algorithm to create enhanced resolution images, which are posted on fine resolution grids. In this paper, we evaluate the effective resolution of the SMAP TB image products using coastline and island crossings. We similarly evaluate the effective resolution of the SMAP L1C_TB_E enhanced resolution product that is based on Backus-Gilbert processing. We present a comparison of the spatial resolution of the rSIR and L1C_TB_E enhanced resolution products with conventionally-processed (gridded) SMAP data. We find that the effective resolution of daily CETB rSIR SMAP TB images is slightly finer than that of L1C_TB_E and about 30% finer than conventionally processed data.

Multi-Scale Assessment of SMAP Level 3 and Level 4 Soil Moisture Products over the Soil Moisture Network within the ShanDian River (SMN-SDR) Basin, China

The Soil Moisture Active Passive (SMAP) mission with high-precision soil moisture (SM) retrieval products provides global daily composites of SM at 3, 9, and 36 km earth grids measured by L-band active and passive microwave sensors. The capability of passive microwave remote sensing has been recognized for the estimation of SM variations. The purpose of this work was to establish an interaction between the highly variable SM spatial distribution on the ground and the SMAP’s coarse resolution radiometer-based SM retrievals. In this work, SMAP Level 3 (L3) and Level 4 (L4) SM products are validated with in situ datasets observed from the different locations of the Soil Moisture Network within the ShanDian River (SMN-SDR) Basin over the period of January 2018 to December 2019. The values of the unbiased root mean square error (ubRMSE) for L3 (SPL3SMP_E) SM retrievals are close to the standard SMAP mission SM accuracy requirement of 0.04 m3/m3 at the 9-km scale, with an averaged ubRMSE value of 0.041 m3/m3 (0.050 m3/m3) for descending (ascending) SM with the correlation (R) values of 0.62 (0.42) against the sparse network sites. The L4 (SPL4SMGP) Surface and Root-zone SM (RZSM) estimates show less error (ubRMSE < 0.04) and high correlation (R > 0.60) values, and are consistent with the previous SMAP-based SM estimations. The SMAP L4 SM products (SPL4SMGP) performed well compared to the L3 SM retrieval products (SPL3SMP_E). In vegetated land, the variability and compatibility of the SMAP SM estimates with the evaluation metrics for both products (L3 and L4) showed a good performance in the grassland, then in the farmland, and worst in the woodlands. Finally, SMAP algorithm parameters sensitivity analysis of the satellite products was conducted to produce time-series and highly precise SM datasets in China.

Evaluation of Global Surface Water Temperature Data Sets for Use in Passive Remote Sensing of Soil Moisture

Inland open water bodies often pose a systematic error source in the passive remote sensing retrievals of soil moisture. Water temperature is a necessary variable used to compute water emissions that is required to be subtracted from satellite observation to yield actual emissions from the land portion, which in turn generates accurate soil moisture retrievals. Therefore, overestimation of soil moisture can often be corrected using concurrent water temperature data in the overall mitigation procedure. In recent years, several data sets of lake water temperature have become available, but their specifications and accuracy have rarely been investigated in the context of passive soil moisture remote sensing on a global scale. For this reason, three lake temperature products were evaluated against in-situ measurements from 2007 to 2011. The data sets include the lake surface water temperature (LSWT) from Global Observatory of Lake Responses to Environmental Change (GloboLakes), the Copernicus Global Land Operations Cryosphere and Water (C-GLOPS), as well as the lake mix-layer temperature (LMLT) from the European Centers for Medium-Range Weather Forecast (ECMWF) ERA5 Land Reanalysis. GloboLakes, C-GLOPS, and ERA5 Land have overall comparable performance with Pearson correlations (R) of 0.87, 0.92 and 0.88 in comparison with in-situ measurements. LSWT products exhibit negative median biases of −0.27 K (GloboLakes) and −0.31 K (C-GLOPS), whereas the median bias of LMLT is 1.56 K. When mapped from their respective native resolutions to a common 9 km Equal-Area Scalable Earth (EASE) Grid 2.0 projection, similar relative performance was observed. LMLT and LSWT data are closer in performance over the 9 km grid cells that exhibit a small range of lake cover fractions (0.05–0.5). Despite comparable relative performance, ERA5 Land shows great advantages in spatial coverage and temporal resolution. In summary, an integrated evaluation on data accuracy, long-term availability, global coverage, temporal resolution, and regular forward processing with modest data latency led us to conclude that LMLT from the ERA5 Land Reanalysis product represents the most optimal path for use in the development of a long-term soil moisture product.

Calculation of 220kV Transformer Bias Current Based on the Earth and Power Grid Model in the Metro Area

The metro stray current will be generated during subway operation, which will affect the electromagnetic environment near the subway, and then affect the surrounding 220kV transformer. In order to analyze the influence of stray current on the DC bias of the transformer, this paper calculates the magnitude of the stray current through the resistance network model, and then analyzes the changes in the surrounding ground potential caused by the stray current based on the earth resistivity model. Finally, the 220kV transformer network topology model of the city network was established, and the relationship between the neutral point current of the transformer and time was obtained. It provided a quantitative method for research the correlation between subway stray current and 220kV transformer bias current phenomenon and calculate the bias current of 220kV transformer in urban network near subway.

Analysis of the Antenna Array Orientation Performance of the Interferometric Microwave Radiometer (IMR) Onboard the Chinese Ocean Salinity Satellite.

The Chinese Ocean Salinity Satellite is designed to monitor global sea-surface salinity (SSS). One of the main payloads onboard the Chinese Ocean Salinity Satellite, named the Interferometric Microwave Radiometer (IMR), is a two-dimensional interferometric radiometer system with an L-band, Y-shaped antenna array. The comparison of two different array orientations is analyzed by an end-to-end simulation based on the configuration of the IMR. Simulation results of the different array orientations are presented and analyzed, including the brightness temperature (TB) images, the distribution of the incidence angles in the field of view, the TB radiometric resolutions, the spatial resolutions, the number of measurements in the Earth grid and the expected SSS accuracy. From the simulations we conclude that one of the array orientations has better performance for SSS inversion than the other one. The advantages mainly result in wider swath and better SSS accuracy at the edge of the swath, which then improve the accuracy of the monthly SSS after averaging. The differences of the Sun’s effects for two different array orientations are also presented. The analysis in this paper provides the guidance and reference for the in-orbit design of the array orientation for the IMR.

Effects of cost optimised grid configuration on earthing system performance: a comparative assessment

This study presents the comparative analysis of earthing grid configurations to determine the optimum design in terms of safety parameters and cost for 66/11 kV substation. For different grid configurations, a fixed earthing area of 9000 m2 was considered. Rectangular, triangular, L-shaped and T-shaped grids buried in two-layer soil model were considered for this study. Three methods, i.e. ANSI/IEEE standard 80-2000, finite element method and genetic algorithm, were used to design the grids. All safety constraints as mentioned in the IEEE standard were followed for designing the earthing grid. A new mathematical cost function (CF) was proposed for designing an effective and economic L-shaped, T-shaped and triangular earthing systems. The CFs used in this study include grid depth, dimensions of earthing rod and horizontal conductor, number of rods and conductors, revetment and excavation area. The economic earthing grid obtained through the proposed method reduces the total cost related to grid materials, installation and excavation. Analysis also showed that the rectangular grid showed preferred results in terms of safety parameters while the triangular grid showed optimum cost design for the earthing system under study. Good agreements were found with the rectangular earthing grid in comparison with triangular, T-shape and L-shape configurations.

Effects of Various Earth Grid Configurations on Ground Potential Rise Caused by Lightning Strike

Ground Potential Rise (GPR) caused by lightning strike is a potential hazard for electrical equipment inside an oil and gas refinery plant. In order to mitigate the risk, horizontal grounding grid is applied. The best mitigation is to install a grounding grid with mesh size as small as possible. This condition requires a high cost. In order to obtain the optimal mesh size, a series of simulation of a grounding grid with mesh size variations on GPR caused by lightning strike has been carried out. CDEGS software was used to observe the GPR with various mesh size from 6.5 x 6.5 m to 20 x 20 m. Simulation results show that the maximum transient GPR rises as the grounding grid mesh size is increased, while the GPR distribution throughout the grounding grid area does not change much for different mesh sizes. In the other hand, decreasing the grid size would mean that more conductors are required, hence the cost would increase accordingly. The result shows that grid sizes from 6.5 x 6.5 m up to 20 x 20 m have no significant difference in term of GPR. In term of cost, 10 x 10 m does not show significant difference with 20 x 20 m, on the other hand, there is a significant difference for grid sizes 1 x 1 m to 10 x 10 m. From the results, grid sizes between 10 x 10 m up to 20 x 20 m are still applicable as stated in Petronas Technical. To comply with proper GPR value, additional protection devices are needed to protect the electrical equipment from potential damage.Keywords –GPR, grounding grid, mesh size

Enhanced-Resolution SMAP Brightness Temperature Image Products

The NASA-sponsored Calibrated Passive Micro- wave Daily Equal-Area Scalable Earth Grid 2.0 Brightness Temperature (CETB) Earth System Data Record Project team has generated a multisensor, multidecadal time series of high-resolution radiometer products designed to support climate studies. This project uses image reconstruction techniques to generate conventional and enhanced-resolution daily brightness temperature images on a standard set of map projections. Sensors included in CETB are the Aqua Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E), Scanning Multichannel Microwave Radiometer, and all Special Sensor Microwave/Imager and Special Sensor Microwave Imager/Sounder radiometers. These span frequencies between 6 and 89 GHz. This paper considers the issues of adding the L-band (1.6 GHz) Soil Moisture Active Passive (SMAP) radiometer measurements to the CETB climate record, with emphasis on optimizing the reconstruction to provide the highest possible spatial resolution at the lowest noise level. SMAP radiometer reconstruction on SMAP-standard grids is also considered. Simulation is used to optimize the reconstruction, and the results confirmed using actual data. A comparison of the performance of the Backus–Gilbert approach and the radiometer form of the Scatterometer Image Reconstruction algorithm is provided. These are compared to the conventional drop-in-the-bucket gridded imaging.

The Texas Soil Observation Network:A Comprehensive Soil Moisture Dataset for Remote Sensing and Land Surface Model Validation

Core Ideas TxSON is a Core Cal/Val Site for the NASA SMAP, SMOS, and Sentinel programs. TxSON consists of 40 in situ locations nested within a 36‐km EASE2 grid cell. Locations monitor soil moisture, temperature, and precipitation at 5, 10, 20, and 50 cm. Hourly, quality‐controlled data from 2015 to 2018 are available. The spatiotemporal variability of soil water content (SWC) at the remote sensing scale requires dense monitoring for calibration and validation. Here, we present an overview of the Texas Soil Observation Network (TxSON), an intensively monitored area in the semiarid rangelands of the central Texas Hill Country. TxSON is a dense network consisting of 40 in situ locations nested at 36, 9, and 3 km within the Equal‐Area Scalable Earth Grid and serves as a Core Calibration and Validation Site for NASA's Soil Moisture Active Passive mission. The 4‐yr dataset consists of hourly SWC measured at 5, 10, 20, and 50 cm. The SWC data are upscaled using arithmetic, Voronoi, and inverse distance weighting for 36‐, (2) 9‐, and (5) 3‐km grid cells. We present the site selection, environmental characteristics, network design, quality assurance (QA), upscaling algorithms, and data structure. Ancillary data include bulk density, particle size, and carbon content for each site and depth. We also provide automated QA for each location and scripts to use our binary flagging in upscaling methods. To summarize, the 36‐km grid cell has a mean bulk density of 1.34 ± 0.18 g cm−3 and a loam textural class. The in situ SWC has a root mean square error of 0.029 m3 m−3 against gravimetric data from 14 field campaigns. TxSON continues to add new locations with additional dense networks planned in other ecotones of the Edwards Plateau. The time series data along with scripts to import, plot, and upscaled SWC are available at https://doi.org/10.18738/T8/JJ16CF.

Castles in the Clouds: LiDAR for Historical Study and Terrain Analysis

Examples of castles and fortresses seen in publicly available LiDAR data by the national mapping agencies in Finland, the Netherlands, Norway, Slovenia, the United Kingdom, and United States demonstrate the value of LiDAR data for understanding military history. The data sets, which include derived grids and the original point clouds with densities from 2–24 points/m², always identify the ground points, generally include LiDAR return intensity values, and sometimes include point classifications that discriminate vegetation and buildings. Grids with 0,5- or one-metre resolution could be created from the highest density point clouds in this study, while grids with one- or two-metre resolution can be created from the lowest density clouds. The digital surface model, which includes everything seen by the sensor, notably ground, buildings, and vegetation, can be created with higher resolution than the bare earth grids. The surface model provides the best visual representation of the castle and its surroundings. Viewed in interactive 3D, the data allows familiarisation with the landscape. Optimal displays depend on the desired scale and the terrain characteristics, but hillshade or shaded reflectance maps, reverse greyscale slope maps, and openness maps all work effectively. Further analysis may include functions such as viewsheds, which enhance the understanding of key terrain.

Northern Hemisphere surface freeze–thaw product from Aquarius L-band radiometers

Abstract. In the Northern Hemisphere, seasonal changes in surface freeze–thaw (FT) cycles are an important component of surface energy, hydrological and eco-biogeochemical processes that must be accurately monitored. This paper presents the weekly polar-gridded Aquarius passive L-band surface freeze–thaw product (FT-AP) distributed on the Equal-Area Scalable Earth Grid version 2.0, above the parallel 50° N, with a spatial resolution of 36 km × 36 km. The FT-AP classification algorithm is based on a seasonal threshold approach using the normalized polarization ratio, references for frozen and thawed conditions and optimized thresholds. To evaluate the uncertainties of the product, we compared it with another satellite FT product also derived from passive microwave observations but at higher frequency: the resampled 37 GHz FT Earth Science Data Record (FT-ESDR). The assessment was carried out during the overlapping period between 2011 and 2014. Results show that 77.1 % of their common grid cells have an agreement better than 80 %. Their differences vary with land cover type (tundra, forest and open land) and freezing and thawing periods. The best agreement is obtained during the thawing transition and over forest areas, with differences between product mean freeze or thaw onsets of under 0.4 weeks. Over tundra, FT-AP tends to detect freeze onset 2–5 weeks earlier than FT-ESDR, likely due to FT sensitivity to the different frequencies used. Analysis with mean surface air temperature time series from six in situ meteorological stations shows that the main discrepancies between FT-AP and FT-ESDR are related to false frozen retrievals in summer for some regions with FT-AP. The Aquarius product is distributed by the U.S. National Snow and Ice Data Center (NSIDC) at https://nsidc.org/data/aq3_ft/versions/5 with the DOI https://doi.org/10.5067/OV4R18NL3BQR.

Spatiotemporal variability of snow cover and snow water equivalent in the last three decades over Eurasia

Changes in the extent and amount of snow cover in Eurasia are of great interest because of their vital impacts on the global climate system and regional water resource management. This study investigated the spatial and temporal variability of the snow cover extent (SCE) and snow water equivalent (SWE) of the continental Eurasia using the Northern Hemisphere Equal-Area Scalable Earth Grid (EASE-Grid) Weekly SCE data for 1972–2006 and the Global Monthly EASE-Grid SWE data for 1979–2004. The results indicated that, in general, the spatial extent of snow cover significantly decreased during spring and summer, but varied little during autumn and winter over Eurasia in the study period. The date at which snow cover began to disappear in spring has significantly advanced, whereas the timing of snow cover onset in autumn did not vary significantly during 1972–2006. The snow cover persistence period declined significantly in the western Tibetan Plateau as well as partial area of Central Asia and northwestern Russia, but varied little in other parts of Eurasia. “Snow-free breaks” (SFBs) with intermittent snow cover in the cold season were principally observed in the Tibetan Plateau and Central Asia, causing a low sensitivity of snow cover persistence period to the timings of snow cover onset and disappearance over the areas with shallow snow. The averaged SFBs were 1–14 weeks during the study period and the maximum intermittence could even reach 25 weeks in certain years. At a seasonal scale, SWE usually peaked in February or March, but fell gradually since April across Eurasia. Both annual mean and annual maximum SWE decreased significantly during 1979–2004 in most parts of Eurasia except for eastern Siberia as well as northwestern and northeastern China. The possible cross-platform inconsistencies between two passive microwave radiometers may cause uncertainties in the detected trends of SWE here, suggesting an urgent need of producing a long-term, more homogeneous SWE product in future.

An assessment of the differences between spatial resolution and grid size for the SMAP enhanced soil moisture product over homogeneous sites

Satellite-based passive microwave remote sensing typically involves a scanning antenna that makes measurements at irregularly spaced locations. These locations can change on a day to day basis. Soil moisture products derived from satellite-based passive microwave remote sensing are usually resampled to a fixed Earth grid that facilitates their use in applications. In many cases the grid size is finer than the actual spatial resolution of the observation, and often this difference is not well understood by the user. Here, this issue was examined for the Soil Moisture Active Passive (SMAP) enhanced version of the passive-based soil moisture product, which has a grid size of 9-km and a nominal spatial resolution of 33-km. In situ observations from core validation sites were used to compute comparison metrics. For sites that satisfied the established reliability and scaling criteria, the impact of validating the 9-km grid product with in situ data collected over a 9-km versus a 33-km domain was very small for the sites studied (0.039 m3/m3 unbiased root mean square difference for the 9-km case versus 0.037 m3/m3 for the 33-km case). This result does not mean that the resolution of the product is 9-km but that for the conditions studied here the soil moisture estimated from in situ observations over 9-km is a close approximation of the soil moisture estimated from in situ observations over the 33-km resolution. The implication is that using the enhanced SMAP product at its grid resolution of 9-km should not introduce large errors in most applications.

Multi-Scale Validation of SMAP Soil Moisture Products over Cold and Arid Regions in Northwestern China Using Distributed Ground Observation Data

The Soil Moisture Active Passive (SMAP) mission was designed to provide global mapping of soil moisture (SM) on nested 3, 9, and 36 km earth grids measured by L-band passive and active microwave sensors. The validation of SMAP SM products is crucial for the application of the products and improvement of the retrieval algorithm. Since the SMAP SM products were released, much effort has been invested in the evaluation of the SMAP radiometer SM product (SMAP_P). However, there has been little validation of SMAP radar (SMAP_A) and active/passive combined (SMAP_AP) SM products. This paper presents an evaluation of SMAP_P, SMAP_A and SMAP_AP SM products by using distributed ground observations networks in different landscapes in the Heihe River Basin of northwestern China. The standard error metrics of SMAP products and relative error are applied to measure the products’ performances. The results show that the SMAP SM products exhibit consistent spatial-temporal variation with the ground measurements and typical precipitation events. Three products show various types of performance capability (e.g., active, passive and combined), surface coverage (e.g., bare, vegetated) and climatic region (e.g., cold, arid). Relatively, the SMAP_P shows the best performance, while the SMAP_A performs the worst. The best performances are observed over bare soils but with overestimation and the largest relative error, and unsatisfactory accuracies are observed over cold regions and woody vegetated surfaces with underestimation. The vegetation effect and the freezing-thawing cycle may be major factors that led to an unsatisfactory performance. Efforts on resolving the influence of these factors are expected to improve the accuracy and to promote the application of SMAP SM products over these regions. Overall, this evaluation provides an understanding of SMAP SM products over cold and arid regions, and suggestions for the further refinement of the SMAP SM retrieval algorithms.

Persistent artifacts in the NSIDC ice motion data set and their implications for analysis

AbstractIn this study we evaluate weekly Equal‐Area Scalable Earth Grid (EASE‐Grid) sea ice velocities released by the National Snow and Ice Data Centre (NSIDC). We identify persistent Eulerian and Lagrangian features that arise solely as an artifact of the method used in the incorporation of buoy data. This, in turn, significantly impacts calculation of sea ice motion gradients, including divergence, convergence, and shear. Our numerical experiments and comparison with observations further demonstrate the impact of these artificial features on climatological assessments, including age‐of‐ice studies. In particular, we find that age‐of‐ice studies using this data set significantly underestimate multiyear ice extent by an average of 0.8 × 106 km2 in the month of March.