Abstract
Abstract. The landscape freeze–thaw (FT) signal determined from satellite microwave brightness temperature (Tb) observations has been widely used to define frozen temperature controls on land surface water mobility and ecological processes. Calibrated 37 GHz Tb retrievals from the Scanning Multichannel Microwave Radiometer (SMMR), Special Sensor Microwave Imager (SSM/I), and SSM/I Sounder (SSMIS) were used to produce a consistent and continuous global daily data record of landscape FT status at 25 km grid cell resolution. The resulting FT Earth system data record (FT-ESDR) is derived from a refined classification algorithm and extends over a larger domain and longer period (1979–2014) than prior FT-ESDR releases. The global domain encompasses all land areas affected by seasonal frozen temperatures, including urban, snow- and ice-dominant and barren land, which were not represented by prior FT-ESDR versions. The FT retrieval is obtained using a modified seasonal threshold algorithm (MSTA) that classifies daily Tb variations in relation to grid-cell-wise FT thresholds calibrated using surface air temperature data from model reanalysis. The resulting FT record shows respective mean annual spatial classification accuracies of 90.3 and 84.3 % for evening (PM) and morning (AM) overpass retrievals relative to global weather station measurements. Detailed data quality metrics are derived characterizing the effects of sub-grid-scale open water and terrain heterogeneity, as well as algorithm uncertainties on FT classification accuracy. The FT-ESDR results are also verified against other independent cryospheric data, including in situ lake and river ice phenology, and satellite observations of Greenland surface melt. The expanded FT-ESDR enables new investigations encompassing snow- and ice-dominant land areas, while the longer record and favorable accuracy allow for refined global change assessments that can better distinguish transient weather extremes, landscape phenological shifts, and climate anomalies from longer-term trends extending over multiple decades. The dataset is freely available online (doi:10.5067/MEASURES/CRYOSPHERE/nsidc-0477.003).
Highlights
The freeze–thaw (FT) signal detected from moderate frequency (37 GHz) satellite microwave brightness temperature (Tb) observations is sensitive to changes in the relative abundance of liquid water at the land surface between frozen and non-frozen conditions
The FT Earth system data record (FT-ESDR) results show generally favorable agreement with daily FT estimates defined from global WMO weather station surface air temperature (SAT) measurements; the FT classification agreement was generally stronger for vegetated land areas but was favorable where other land cover classes were dominant within a grid cell, including urban (89.5–96.4 %), barren land (91.8–94.3 %), water bodies (75.0–77.5 %) and snowand ice-covered areas (81.1–83.1 %)
The FT classification agreement was directly proportional to the correlation between the satellite Tb retrievals and corresponding ERA-Interim global reanalysis-based SAT values used for grid-cell-wise calibration of the modified seasonal threshold algorithm (MSTA) FT thresholds
Summary
The freeze–thaw (FT) signal detected from moderate frequency (37 GHz) satellite microwave brightness temperature (Tb) observations is sensitive to changes in the relative abundance of liquid water at the land surface between frozen and non-frozen conditions. The FT metric defines the predominant landscape frozen or non-frozen status within a sensor footprint and is insensitive to potential degradation from solar illumination variations and atmosphere cloud/aerosol contamination effects. These properties enable consistent global coverage and daily monitoring from available satellite microwave sensors (Kim et al, 2011; Zhao et al, 2014). The FT-ESDR-defined frozen season was used to estimate permafrost extent and stability over the pan-Arctic (Park et al, 2016a) In this investigation, we utilize calibrated daily (morning (AM) and evening (PM) overpass) 37 GHz Tb retrievals from SMMR, SSM/I and SSMIS sensor series to refine and extend the FT-ESDR. The FT-ESDR methods, database description and performance assessment are provided
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