Comparing Global Passive Microwave Freeze/Thaw Records: Investigating Differences between Ka- and L-Band Products

The recent Soil Moisture Active Passive (SMAP) mission has produced valuable soil moisture and freeze thaw (FT) products at a global scale. While the use of L-band (1.4 GHz) passive microwave radiometry (P-MW) has proven useful in detecting changes in the surface FT state, these classifications have not been comprehensively assessed against similar existing FT products, such as the global FT record from the Special Sensor Microwave Imager/Sounder (SSM/I, Ka-band, 37.0 GHz) as part of the FT Earth System Data Record (FT-ESDR). In order to fill in this gap, this study investigates regions in which FT classifications diverge and identifies potential sources of such variability. The SMAP and SSM/I FT records are compared over an extended period covering multiple seasonal cycles from April 2015 through December 2017. The spatially and temporally varying relationship between these products is examined in relation to climate (Köppen-Geiger climate classes and freezing days), land cover (from the Moderate Resolution Imaging Spectrometer), and elevation (using Global Multi-resolution Terrain Elevation Data). After correcting for seasonal cycles, several land cover and climate classes are found to have significantly varied FT classifications between the SMAP and SSM/I products. Most notably in areas near abundant surface water, snow and ice, and wetlands, although clear variability in agreement is also observed between several vegetation classes. Unique differences in FT classification agreement were also observed between products over cold arid regions and between continental and temperate classes. Additionally, a general decrease in product agreement is observed as regions experienced an increase in days with sub-freezing mean daily temperatures. However, this relationship is most clearly defined in regions in which 0-20% of the study period was below freezing. Similarly, as elevation increases above 50 meters a decreasing trend in agreement between L- and Ka-band FT products is observed. The reduced agreement at elevations <50 meters is shown to be attributed to reduced agreement in coastal regions. This study contributes new insights to the global dynamics of P-MW based FT records and to improved FT classification algorithms moving forward.