The Soil Moisture Active Passive (SMAP) “Freeze/Thaw 9 km (L3, Passive, Night)” layer displays freeze/thaw state for Global land surface areas and land surface areas north of 45°N latitude from the SMAP radiometer. To enhance the grid resolution, Backus-Gilbert optimal interpolation techniques are used to extract maximum information from SMAP antenna temperatures and convert them to brightness temperatures, which are posted to the 9 km EASE-Grid 2.0. Freeze/thaw is detected by identifying the temporal response of the normalized polarization ratio (NPR) of the brightness temperature, which is sensitive to changes in the dielectric constant of the landscape components that occur as the water within the components transitions between frozen and non-frozen conditions.

The SMAP spacecraft carries two instruments, a radar (active) and a radiometer (passive), that together make global measurements of land surface soil moisture and freeze/thaw state. It is useful for monitoring and predicting natural hazards such as floods and droughts, understanding the linkages between Earth’s water, energy and carbon cycles, and reducing uncertainties in predicting weather and climate.

References: SMAP Enhanced L3 Radiometer Global and Northern Hemisphere Daily 9 km EASE-Grid Freeze/Thaw State

Data field: freeze_thaw