SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 Km .

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SMAP Enhanced L3 Radiometer Northern HemisphereDaily 9 km EASE-Grid Freeze/Thaw State, Version 2USER GUIDEHow to Cite These DataAs a condition of using these data, you must include a citation:Xu, X., R. S. Dunbar, C. Derksen, A. Colliander, Y. Kim, and J. S. Kimball. 2018. SMAP Enhanced L3Radiometer Global and Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 2.[Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center DistributedActive Archive Center. https://doi.org/10.5067/1M7OJC7R7VKI. [Date Accessed].FOR QUESTIONS ABOUT THESE DATA, CONTACT NSIDC@NSIDC.ORGFOR CURRENT INFORMATION, VISIT https://nsidc.org/data/SPL3FTP E

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 2TABLE OF CONTENTS1DATA DESCRIPTION . 3Parameters . 3Format . 3File Contents . 3Data Fields . 41.4.1Ancillary Data .51.4.2Freeze/Thaw Retrieval Data .51.4.3Radiometer Data . 5Metadata Fields . 5File Naming Convention . 5File Size. 6Volume . 6Spatial Coverage . 61.9.1Resolution .6EASE-Grid 2.0 . 7Temporal Coverage. 721.11.1Satellite and Processing Events . 81.11.2Latencies .81.11.3Temporal Resolution .8DATA ACQUISITION AND PROCESSING . 8Background . 8Acquisition . 9Derivation Techniques and Algorithms. 92.3.1Baseline Algorithm .92.3.2Ancillary Data . 11Processing Steps . 12Quality, Errors, and Limitations . 142.5.1Error Sources . 142.5.2Quality Assessment .152.5.3Quality Overview . 162.5.4Instrumentation .163SOFTWARE AND TOOLS . 164VERSION HISTORY . 165RELATED DATA SETS . 176RELATED WEBSITES . 177CONTACTS AND ACKNOWLEDGMENTS . 178REFERENCES . 189DOCUMENT INFORMATION. 20Publication Date . 20National Snow and Ice Data Centernsidc.orgPage 1 of 20

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 2Date Last Updated . 20National Snow and Ice Data Centernsidc.orgPage 2 of 20

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 21DATA DESCRIPTIONParametersFreeze/thaw state and the direction of diurnal freeze/thaw transitions (frozen in the morning tothawed in the afternoon and vice versa) derived from brightness temperatures are output on a fixedNorthern Hemisphere azimuthal EASE-Grid 2.0 at 36 km. Freeze/thaw state, the occurrence offreeze/thaw transitions, and the direction of transitions are expressed in boolean values (0 or 1).For freeze/thaw state, 0 indicates thawed conditions and 1 indicates frozen. For freeze/thawtransition state, 0 indicates the a.m. and p.m. FT states are the same (thawed/thawed orfrozen/frozen). The transition direction flag is only meaningful if there is a transition (transition state 1), and is set to 0 for a.m. frozen/p.m. thawed and 1 for a.m. thawed/p.m. frozen.Also included are brightness temperatures (TBs) in kelvin for a 36 km EASE-Grid 2.0 cell.Refer to the Product Specification Document for details on all parameters.FormatData are in HDF5 format. For software and more information, including an HDF5 tutorial, visit theHDF Group's HDF5 Web site.File ContentsAs shown in Figure 1, each HDF5 file is organized into the following main groups, which containadditional groups and/or data sets:National Snow and Ice Data Centernsidc.orgPage 3 of 20

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 2Figure 1. Subset of File Contents. For a complete list of file contents for the SMAP enhanced Level-3freeze/thaw product, refer to the Product Specification Document.Data FieldsEach file contains the main data groups summarized in this section. For a complete list anddescription of all data fields within these groups, refer to the Product Specification Document.Data element arrays are three dimensional, with the exceptionof transition direction and transition state flag arrays, which are two dimensional. All arrays have2000 rows and 2000 columns in each a.m. and p.m. layer. For the a.m./p.m. index of the array, thea.m. layer is assigned to the index value 0 and the p.m. layer is assigned to index value 1.National Snow and Ice Data Centernsidc.orgPage 4 of 20

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 21.4.1 Ancillary DataIncludes all ancillary data, such as landcover classification and open water body fraction.1.4.2 Freeze/Thaw Retrieval DataIncludes freeze/thaw data, latitude and longitude arrays, and quality assessment flags.1.4.3 Radiometer DataIncludes all enhanced radiometer data and quality assessment flags.Metadata FieldsIncludes all metadata that describe the full content of each file. For a description of all metadatafields for this product, refer to the Product Specification Document.File Naming ConventionFiles are named according to the following convention, which is described in Table 1:SMAP L3 FT P E yyyymmdd RLVvvv NNN.[ext]For example:SMAP L3 SM P E 20170117 R14010 001.h5Table 1. File Naming ConventionsVariable DescriptionSMAPIndicates SMAP mission dataL3 FTP EIndicates specific product (L3: Level-3; FT: Freeze/Thaw; P: Passive; E: Enhanced)yyyymmdd4-digit year, 2-digit month, 2-digit day; date in Universal Coordinated Time (UTC) ofthe first data element that appears in the product.National Snow and Ice Data Centernsidc.orgPage 5 of 20

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 2Variable DescriptionRLVvvvComposite Release ID (CRID), where:RReleaseLLaunch Indicator (1: Post-launch standard data)V1-Digit Major CRID Version Numbervvv3-Digit Minor CRID Version NumberRefer to the SMAP Data Versions page for version information.NNNNumber of times the file was generated under the same version for a particulardate/time interval (002: 2nd time).[ext]File extensions include:.h5HDF5 data file.qaQuality Assurance file.xmlXML Metadata fileFile SizeEach file is approximately 65 MB.VolumeThe daily data volume is approximately 65 MB.Spatial CoverageCoverage for this data set spans the Northern Hemisphere for all land regions north of 45 Nlatitude, and from 180 W to 180 E. The gap in coverage at the North Pole, called a pole hole, hasa radius of approximately 400 km. The swath width is approximately 1000 km, enabling nearlycomplete coverage of the Northern Hemisphere every three days.1.9.1 ResolutionThe native spatial resolution of the radiometer footprint is 36 km. Data are then interpolated usingthe Backus-Gilbert optimal interpolation algorithm into the Northern Hemisphere azimuthal EASEGrid 2.0 projection with 9 km spacing.National Snow and Ice Data Centernsidc.orgPage 6 of 20

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 2EASE-Grid 2.0These data are provided on the Northern Hemisphere azimuthal EASE-Grid 2.0 (Brodzik et al.2012). Each grid cell has a nominal area of approximately 9 x 9 km2 regardless of longitude andlatitude. Using this projection, all data arrays have dimensions of 2000 rows x 2000 columns.EASE-Grid 2.0 has a flexible formulation. By adjusting a single scaling parameter, a family of multiresolution grids that nest within one another can be generated. The nesting can be adjusted so thatsmaller grid cells can be tessellated to form larger grid cells. Figure 3 shows a schematic of thenesting.This feature of perfect nesting provides SMAP data products with a convenient common projectionfor both high-resolution radar observations and low-resolution radiometer observations, as well asfor their derived geophysical products. For more on EASE-Grid 2.0, refer to the EASE-Grid 2.0Format Description.Figure 2. Perfect Nesting in EASE-Grid 2.0Temporal CoverageCoverage spans from 31 March 2015 to the 27 August 2020National Snow and Ice Data Centernsidc.orgPage 7 of 20

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 21.11.1 Satellite and Processing EventsDue to instrument maneuvers, data downlink anomalies, data quality screening, and other factors,small gaps in the SMAP time series will occur. Details of these events are maintained on twomaster lists:SMAP On-Orbit Events List for Instrument Data UsersMaster List of Bad and Missing DataA significant gap in coverage occurred between 19 June and 23 July 2019 after the SMAP satellitewent into Safe Mode. A brief description of the event and its impact on data quality is available inthe SMAP Post-Recovery Notice.1.11.2 LatenciesFAQ: What are the latencies for SMAP radiometer data sets?1.11.3 Temporal ResolutionEach Level-3 file is a daily composite of half-orbit files/swaths.To ensure complete coverage of the freeze/thaw domain in each daily file, a.m. and p.m. data forthe current day are combined with a.m. and p.m. data from previous days. A maximum of threedays of past data is used, and is necessary only near the southern margin of the freeze/thawdomain.2DATA ACQUISITION AND PROCESSINGBackgroundThe SPL3FTP E product is derived using a temporal change detection approach that has beenpreviously developed and successfully applied using time-series satellite remote sensing radarbackscatter and radiometric brightness temperature data from a variety of sensors and spectralwavelengths. The approach is to identify the landscape freeze/thaw (F/T) state via the temporalresponse of the normalized polarization ratio (NPR) of the brightness temperature, which issensitive to changes in the dielectric constant of the landscape that occur as the water within thecomponents transitions between frozen and non-frozen conditions.This approach assumes that the large changes in dielectric constant occurring between frozen andnon-frozen conditions dominates the corresponding NPR temporal dynamics across the seasons,National Snow and Ice Data Centernsidc.orgPage 8 of 20

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 2rather than other potential sources of temporal variability such as changes in canopy structure andbiomass, large precipitation events, or changes in soil moisture.However, in lower-latitude areas where the seasonal difference of the NPR is too small to beeffectively used to discriminate F/T state, the V-polarization (V-pol) brightness temperature iscompared to a threshold value to retrieve the F/T state. At very low latitudes where no F/Ttransitions occur, no algorithm is applied.AcquisitionSMAP enhanced Level-3 radiometer freeze/thaw data (SPL3FTP E) are derived from SMAPEnhanced L1C Radiometer Half-Orbit 9 km EASE-Grid Brightness Temperatures, Version 1(SPL1CTB E).Derivation Techniques and AlgorithmsThis section has been adapted from Dunbar et al. (2016), the Algorithm Theoretical BasisDocument (ATBD) for this data set.This product (SPL3FTP E) is an enhanced version of the SMAP L3 Radiometer NorthernHemisphere Daily 36 km EASE-Grid Freeze/Thaw State (SPL3FTP) product. Both products arederived sing the same techniques and algorithms described in this section.For information regarding the Backus-Gilbert optimal interpolation algorithm used to enhance theinput data for this product, refer to the SPL1CTB E user guide.The SMAP enhanced Level-3 radiometer freeze/thaw product is derived from SMAP L1CEnhanced Radiometer Half-Orbit 36 km EASE-Grid Brightness Temperatures, Version 3(SPL1CTB E). The derivation of freeze/thaw from SMAP brightness temperature measurementsoccurs during an intermediate Level-2 processing step of the input Level-1 brightness temperaturedata. During the Level-2 processing step, the freeze/thaw algorithm utilizes a seasonal thresholdapproach to convert SMAP brightness temperature measurements to freeze/thaw state. For anoverview of the steps involved in processing this data product, refer to Figure 4 in the ProcessingSteps section.2.3.1 Baseline AlgorithmThe SPL3FTP E freeze/thaw baseline algorithm examines the time series progression of thebrightness temperature signature relative to signatures acquired during seasonal reference frozenNational Snow and Ice Data Centernsidc.orgPage 9 of 20

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 2and thawed states. The algorithm is applied to the normalized polarization ratio (NPR) of SMAPradiometer measurementsNPR (TBV-TBH)/(TBV TBH)(Equation 1)A seasonal scale factor D(t) is defined for an observation acquired at time t asDt (NPR(t)-NPR(fr))/(NPR(th)-NPR(fr))(Equation 2)where NPR(t) is the normalized polarization ratio calculated at time t, for which a freeze/thawclassification is sought, and NPR(th) and NPR(fr) are normalized polarization ratios correspondingto the frozen and thawed reference states, respectively. The twenty highest (lowest) NPR valuesfrom SMAP radiometer measurements during July and August 2015 (thaw) and January andFebruary 2016 (freeze) for the northern ( 45 N) domain were retained and averaged to create thethaw (freeze) reference. Data were separated by ascending and descending orbit. Themethodological approach to NPR freeze and thaw references will be refined in future productreleases. In addition, the reference values will be updated following each transition season.Reprocessing of the SMAP data record incorporating annual variations in the SMAP freeze/thawreference states should improve product accuracy over the use of static reference conditions. Theinitial SMAP freeze and thaw NPR references are shown in Figure 4 below (and Section 4.2.2 ofthe ATBD).Figure 2. Initial SMAP Freeze and Thaw NPR References. SMAP radiometer (a) freeze and (b) thawreferences; (c) reference difference between panels (a) and (b). Units are NPR scaled by 100.A threshold level T is then defined such thatD(t) TD(t) T(Equation 3)National Snow and Ice Data Centernsidc.orgPage 10 of 20

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 2defines the thawed and frozen landscape states, respectively. This series of equations (1-3) arerun on a grid cell-by-cell basis for unmasked portions of the FT domain. The output from Equation(3) is a dimensionless binary state variable designating either frozen or thawed conditions for eachunmasked grid cell. The threshold values can be optimized on a grid cell by grid cell basis, but arefixed at 0.5 for this release. Optimization approaches will be evaluated in advance of future productreleases.Following the pixel-wise determination of freeze/thaw state, two additional processing steps areapplied to mitigate summer season false freeze and winter season false thaw retrievals. First, if thebrightness temperature magnitude at either V or H pol is greater than 273 K, the pixel is set to thawregardless of the retrieval. Second a temporally fixed 'never frozen' mask calculated from dailyAMSR-E freeze/thaw maps (using the approach described in Kim et al., 2012) is applied to removeobviously false summer freeze flags. False freeze retrievals occur in some regions of the F/Tdomain because of small differences between the reference freeze and thaw values (see Figure4c). Implemen

USER GUIDE: SMAP Enhanced L3 Radiometer Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State, Version 2 Page 3 of 20 National Snow and Ice Data Center nsidc.org 1 DATA DESCRIPTION Parameters Freeze/thaw state and the direction of diurnal freeze/thaw transitions (frozen in the morning to

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