Informing Improvements in Microwave Freeze/Thaw Products using
High-Resolution Temperature Data Over North America
Abstract
Existing global FT records, derived from the Soil Moisture Active
Passive (SMAP), the Advanced Scanning Microwave Radiometer (AMSR), and
the Special Sensor Microwave Imager (SSM/I) produce relatively course
spatial resolution (25-36km) binary FT classifications. These
classifications can vary widely depending on the microwave bands used,
topography, and land cover, leading to a somewhat ambiguous definition
of ‘frozen’ and ‘thawed’ states. In this study, we assess the
relationship between satellite observation derived FT products over
North America compared to modeled near-surface temperatures and land
surface temperature (LST) from the Geostationary Operational
Environmental Satellite system (GOES). Utilizing the higher spatial
resolution of these products (~4.5km), sub-grid scale
variability and its relationship to courser microwave FT classifications
was assessed. Through an analysis of spatial variability and uncertainty
across North America, five focus study pixels each representing unique
FT profiles were examined. These included pixels in: (1) Southern Plains
(36, -97), (2) Tundra (61, -76), (3) Northern Forest (47, -74), (4)
Northern Plains (52, -103), and Mountainous (38.9, -107.9). The model
ensemble adequately captured near surface temperatures as they relate to
FT classifications in Tundra, the Northern Plains, and Northern Forest
regions. On average, 85.3% to 99.6% of sub-grid cells were below
freezing when FT products classified the associated pixels as frozen.
GOES - LST observations were shown to have the highest proportion of
sub-grid cells below freezing on average, when classified as frozen by
FT products (97.3% - 100%) across the same 3 focus locations. However,
we also find that fractional FT products utilizing higher resolution
data inputs, such as LST, would provide a considerable improvement in
mountainous regions with high inter-grid cell heterogeneity, in regions
characterized by ephemeral FT events (Southern Plains), as well as
during freeze and thaw onset periods. These locations showed a
significant reduction in the average temperature product frozen
proportion associated with frozen classifications (as low as 5.8%).
This study provides insight to improving representation of FT state and
providing a clearer meaning of what constitutes a ‘frozen’
classification.