Microwave L-band emission of freezing soil

Abstract

Abstract-We report on field-measured microwave emission in a period of frost penetration into a grassland soil. The measurements were recorded with a high temporal resolution using an L-band radiometer mounted on a 7-m high tower. The observation period (December 2002 to March 2003) included two cycles of soil freezing and thawing with maximum frost depth of 25 cm. In situ soil temperature and liquid water content were measured at five depths down to 45 cm. Soil moisture profiles were calculated using the COUP numerical soil water and heat model in combination with measured soil properties and meteorological data monitored at the site. The L-band radiation data clearly showed the penetration and thawing of seasonal soil frost. We calculated soil reflectivities based on in situ measured and modeled soil moisture profiles by applying a coherent radiative transfer model. The calculated reflectivities were compared with the radiometrically determined soil reflectivities. It was demonstrated that the quantitative consistency between these reflectivities was significantly improved by applying an impedance matching approach accounting for surface effects. In this particular case, the dielectric structure of the uppermost soil horizon was largely influenced by soil roughness, vegetation, and snow cover. The radiometrically measured soil reflectivities were fitted using a radiative transfer model in combination with a roughness model assuming a soil surface roughness of 25 mm. The analysis during a period of frost penetration shows coherent behavior of the soil reflectivity. Temporal oscillation of the measured L-band radiation appears to be a coherent effect. This effect has the potential to be used for estimating the frost penetration velocity.

Testing a New Model for the L-Band Radiation of Moist Leaf Litter

“…Since 2002, this instrument has been successfully deployed in numerous campaigns which provided information that is important for interpreting L-band radiance in terms of soil moisture and other scene parameters like roughness and vegetation covers [10]- [12], [19]- [23].…”

Theoretical Modeling and Analysis of L- and P-band Radar Backscatter Sensitivity to Soil Active Layer Dielectric Variations

“…Previous studies based on multi-temporal L-band radiometer observations demonstrate the potential of retrieving the penetration velocity of the frost front within the soil active layer [34]. With additional P-band sensor observations, the progressive thawing or freezing process and associated change in soil liquid water content are more likely to be captured and characterized.…”

“…For this purpose, a two-layer soil scattering (FTSS) model is developed for L-and P-band microwave observations based on first-order solution of the Radiative Transfer Equation. The soil dielectric profile during the FT transition period can be described by a Fermi-distribution function and dielectric constants for frozen and unfrozen conditions of relatively moist soils [34]. As a simplification, we separate the soil profile into two adjacent layers, representing predominantly frozen and thawed conditions.…”

Microwave remote sensing for soil moisture monitoring: synergy of active and passive observations and validation of retrieved products

“…Both active and passive microwave signals show a clear dependence on the water state in the soil (i.e. liquid versus frozen), due to the contrasting dielectric properties of the soil (Rignot and Way 1994;Schwank et al 2004;Ulaby et al 1982). Therefore, particular attention should be applied in the modelling of permittivity and microwave signal of frozen soils (Mironov et al 2010).…”

“…The ECH2O probes measure the unfrozen water content of the frozen soils, as demonstrated by Yoshikawa and Overduin (2005), but a specific calibration is needed. Active and passive microwave signatures can be used to monitor frozen soils, as demonstrated by previous studies (Rignot and Way 1994;Schwank et al 2004;Wang et al 2015). However, the relationship between the microwave satellite observations and frozen/liquid water content in frozen soils needs to be quantitatively analysed and the potential to retrieve the liquid water content needs to be investigated.…”