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Abstract
Detailed angular ground-based L-band brightness temperature (TB) measurements over snow covered frozen soil in a prairie environment were used to parameterize and evaluate an electromagnetic model, the Wave Approach for LOw-frequency MIcrowave emission in Snow (WALOMIS), for seasonal snow. WALOMIS, initially developed for Antarctic applications, was extended with a soil interface model. A Gaussian noise on snow layer thickness was implemented to account for natural variability and thus improve the TB simulations compared to observations. The model performance was compared with two radiative transfer models, the Dense Media Radiative Transfer-Multi Layer incoherent model (DMRT-ML) and a version of the Microwave Emission Model for Layered Snowpacks (MEMLS) adapted specifically for use at L-band in the original one-layer configuration (LS-MEMLS-1L). Angular radiometer measurements (30°, 40°, 50°, and 60°) were acquired at six snow pits. The root-mean-square error (RMSE) between simulated and measured TB at vertical and horizontal polarizations were similar for the three models, with overall RMSE between 7.2 and 10.5 K. However, WALOMIS and DMRT-ML were able to better reproduce the observed TB at higher incidence angles (50° and 60°) and at horizontal polarization. The similar results obtained between WALOMIS and DMRT-ML suggests that the interference phenomena are weak in the case of shallow seasonal snow despite the presence of visible layers with thicknesses smaller than the wavelength, and the radiative transfer model can thus be used to compute L-band brightness temperature.
Highlights
Three spaceborne L-band passive microwave radiometer missions were successfully launched in recent years for global monitoring of soil moisture and sea surface salinity
In cases of wet snow, absorption is considered by LS-Microwave Emission Model for Layered Snowpacks (MEMLS), whereas dry snow is assumed to be fully transparent, which is reasonable for seasonal snowpacks with thicknesses much smaller than L-band emission depth in dry snow (>300 m [20])
This emission model is based on parts of MEMLS, [12] with the assumptions of no absorption and no volume scattering in dry snow, which are applicable to the L-band frequencies in dry snow
Summary
Three spaceborne L-band passive microwave radiometer missions were successfully launched in recent years for global monitoring of soil moisture and sea surface salinity. The NASA Aquarius instrument on board the Aquarius/Satélite de Aplicaciones Científicas (SAC-D) mission, developed collaboratively between the U.S National Aeronautics and Space Administration (NASA) and Argentina’s space agency, Comisión Nacional de Actividades Espaciales (CONAE) acquired L-band observations between September 2012 and July 2015 [2], and the NASA Soil Moisture Active Passive (SMAP) satellite was launched in January 2015 [3] These missions provide useful information for cryosphere applications including monitoring the freeze/thaw (F/T) state of the land surface [4,5,6,7], estimating snow density and ground permittivity [8,9], and retrieving the thickness of thin sea ice [10]. The model parameterizations are presented, after which we present results and a comparison of the model performance
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