## 3 Results
### Spatial Frozen Season Characteristics  
The annual frozen season was estimated by summing up the daily FT records for each grid cell, including frozen and transitional conditions, from the 36 km and 9 km grid products. The resulting frozen season map for 2016 from the 9 km grid product is presented (Figure 6), showing a mean frozen season of \(112\pm 79\) [spatial-SD] days for the global domain. The 2017 frozen season map (not shown) has a consistent global pattern and mean frozen season (\(112\pm 82\) days). The SMAP FT data captures characteristic frozen season increases at higher elevations, including Tibetan Plateau, Andes and Rocky Mountain areas. The frozen season is generally shorter along coastal margins relative to inland areas, which is consistent with more moderate maritime climate conditions in the coastal zone. The regional depictions of the 2016 annual frozen season over the continental United States are also presented (Figure 6), highlighting differences between the 36 km and 9 km FT products. The SMAP 9 km grid FT data shows a general enhancement of the frozen season pattern relative to the 36 km product. The mean and spatial SD of the frozen season distributions are \(57\pm 33\) and \(62\pm 42\) days for respective 36 km and 9 km products. The relatively large SD is associated with the wide distribution of annual frozen days over the continental US, ranging from relatively few (<9) frozen days across the warmer southern states to a much longer frozen season in the northeastern states and in higher elevation mountain areas (Figure 6). In addition, the relatively large variation in the frozen season may be associated with heterogeneous radiometric properties of the land surface in neighboring grid cells and within the satellite footprint during FT transitions. The larger SD in the 9 km product reflects the fact that greater frozen season heterogeneity was represented, including improved delineation of characteristic elevation and regional climate-driven patterns over the intermountain west. The larger SD in the 9 km product also reflects an anomalous frozen season increase along coastal boundaries and large inland water bodies (inset map in Figure 6) that is greatly reduced in the 36 km product. The larger frozen season along coastal boundaries is attributed to water contamination of the adjacent Tb retrievals over land, which may be exacerbated by the BG interpolation used in the 9 km product. The associated water contamination accounts for the much stronger FT accuracy decline in the 9 km  
Figure 5: Selected landscape factors affecting the 2016–2017 PM overpass FT classification agreement for individual grid cells, including open water fraction (fw), terrain heterogeneity, and percent tree cover within a grid cell. The number of weather stations used for FT accuracy assessment within each landscape category and product grid is represented by black (36 km) and grey (9 km) bars. The grey shading denotes grid cells assigned lower product quality flags for having significant open water fraction (\(20\%<\text{fw}\leq 50\%\)) or large elevation spatial heterogeneity (SD \(>\) 250-m). Higher fw levels represent grid cells dominated by large water bodies identified from the ESA water body map in this study, but missing from MOD44W-based screening of water-contaminated grid cells during SMAP operational processing. product in areas with higher fw levels relative to the 36 km product (Figure 5). However, the L3_FT products assign a lower quality flag to grid cells with fw levels ranging from 20-50% cover from an ancillary static (MOD44W) water mask, while a majority of grid cells with higher fw levels are excluded from the FT processing. Figure 6: Annual frozen season (frozen, transitional or inverse transitional status) for 2016 at a 9 km resolution (top). The white and grey shades denote, respectively, open water bodies and land areas outside of the SMAP global FT data domain. The annual frozen season over the United States at 36 km and 9 km resolutions for 2016.