Evaluating Passive Microwave Radiometry for the Dynamical Transition From Dry to Wet Snowpacks

The microwave response of an idealized snowpack is evaluated for a change of the liquid water content (LWC) ranging from 0.0 m³ m^-3 to 0.1 m³ m^-3 , a realistic range for a dry to a wet snowpack transition. Next, the microwave radiometric behavior of a snowpack as a function of LWC is investigated using a coupled snow hydrology-microwave emission model that consists of a direct combination of multi-layer snow hydrology model and a forward model of microwave emission based on the simulated multi-layer snowpack. The microwave response during the transition is characterized by an initial increase of the brightness temperature (Tb) followed by a monotonic attenuation of Tb with a linear increase of LWC. Thus, the microwave response Tb to the increase in LWC exhibits a convex shape. The early amplification of Tb is caused by a sharp increase of the absorption coefficient attributed by a relatively small amount of LWC within the snowpack. This peak of Tb can be explained by the decrease in layer reflectivity and transmissivity in the Microwave Emission Model of Layered Snowpacks. The decrease of snow layer transmissivity indicates that the snowpack operates as an opaque medium in the microwave spectrum for small values of LWC. However, when LWC continues to increase, the increase of interface reflectivity at the atmosphere-snowpack interface begins to suppress the increasing Tb trend. As a result, the Tb subsequently decreases because the interface transmissivity, a complement of reflectivity, decreases asymptotically. This arched (convex) behavior of the Tb signal with increasing LWC is also exhibited by the Special Sensor Microwave Imager and the ground-based microwave radiometer observations concurrent with the presence of LWC in the simulated snowpack. For the validation of the Tb response to the LWC, simulations using a coupled snow hydrology-microwave emission model are compared against both passive microwave satellite and- ground-based radiometer observations during the 2002-2003 Cold Land Processes Experiment (CLPX). Two Meso-cell Study Areas from CLPX are selected to evaluate the coupled model performances of Tb and snowpack physical properties in response to changes in LWC.