Abstract
Current methods for retrieving SWE (snow water equivalent) from space rely on passive microwave sensors. Observations are limited by poor spatial resolution, ambiguities related to separation of snow microstructural properties from the total snow mass, and signal saturation when snow is deep (~>80 cm). The use of SAR (Synthetic Aperture Radar) at suitable frequencies has been suggested as a potential observation method to overcome the coarse resolution of passive microwave sensors. Nevertheless, suitable sensors operating from space are, up to now, unavailable. Active microwave retrievals suffer, however, from the same difficulties as the passive case in separating impacts of scattering efficiency from those of snow mass. In this study, we explore the potential of applying active (radar) and passive (radiometer) microwave observations in tandem, by using a dataset of co-incident tower-based active and passive microwave observations and detailed in situ data from a test site in Northern Finland. The dataset spans four winter seasons with daily coverage. In order to quantify the temporal variability of snow microstructure, we derive an effective correlation length for the snowpack (treated as a single layer), which matches the simulated microwave response of a semi-empirical radiative transfer model to observations. This effective parameter is derived from radiometer and radar observations at different frequencies and frequency combinations (10.2, 13.3 and 16.7 GHz for radar; 10.65, 18.7 and 37 GHz for radiometer). Under dry snow conditions, correlations are found between the effective correlation length retrieved from active and passive measurements. Consequently, the derived effective correlation length from passive microwave observations is applied to parameterize the retrieval of SWE using radar, improving retrieval skill compared to a case with no prior knowledge of snow-scattering efficiency. The same concept can be applied to future radar satellite mission concepts focused on retrieving SWE, exploiting existing methods for retrieval of snow microstructural parameters, as employed within the ESA (European Space Agency) GlobSnow SWE product. Using radar alone, a seasonally optimized value of effective correlation length to parameterize retrievals of SWE was sufficient to provide an accuracy of <25 mm (unbiased) Root-Mean Square Error using certain frequency combinations. A temporally dynamic value, derived from e.g., physical snow models, is necessary to further improve retrieval skill, in particular for snow regimes with larger temporal variability in snow microstructure and a more pronounced layered structure.
Highlights
The mass of seasonal snow cover, or snow water equivalent (SWE) remains difficult to estimate on a global scale
This study investigated the seasonal behavior of snow bulk scattering properties by means of retrieving a proxy variable, the effective snow correlation length, from active and passive microwave observations
Snow depth was used as the only temporally variable ancillary input, mimicking a similar scheme applied for satellite passive microwave SWE retrievals, in which conventional snow grain size is used as a proxy [4]
Summary
The mass of seasonal snow cover, or snow water equivalent (SWE) remains difficult to estimate on a global scale. Recent efforts have focused on simulating snow as a bicontinuous medium, simulating the resulting active and passive microwave response with some success [24] Statistical parameters such as autocorrelation length of the snow structure in different axial directions are able to describe the snow microstructure with higher fidelity than the conventional measure of grain size [25]. We present the retrieval of an effective correlation length, which describes the radiative transfer properties of snow by a single parameter. The effective correlation length is retrieved from active and passive microwave observations of naturally accumulated snow over four winter seasons at a test site in northern Finland. We examine the interchangeability of the retrieved correlation length (derived independently from active and passive measurements at different frequencies) for the purpose of initializing the retrieval of SWE from radar observations.
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