Feasibility of Satellite Observations to Monitor Snow and Soil Frost Processes: Campaign Activities at Sodankylä
Pulliainen, Jouni1; Lemmetyinen, Juha1; Kontu, Anna1; Heinilä, Kirsikka2; Salminen, Miia1; Rautiainen, Kimmo1; Cohen, Juval1; Leppänen, Leena1; Hannula, Henna-Reetta1; Vehviläinen, Juho1
1Finnish Meteorological Institute, FINLAND; 2Finnish Environment Institute, FINLAND
Intensive campaign activities for the remote sensing of snow cover and soil processes have been carried out in the Sodankylä region, North Finland, since 2006. The objective has been to investigate the feasibility of space-borne instruments operating at optical and microwave regions for the monitoring of snow and soil processes in boreal forest and sub-arctic environment. The Sodankylä in situ observations also facilitate the analysis of the effect of snow and soil processes to carbon cycling (as CO2 and CH4 flux measurement time-series are also available for different ecosystems). The Sodankylä site is equipped with tower-based reference instruments of present and planned cryosphere-observing satellites. The systems provide time-series of reference observations on a continuous basis. They include multi-channel microwave radiometers (SodRad systems since 2009; reference to AMSR and SSM/I), the L-band ESA Elbara-II microwave radiometer (since 2010; reference to SMOS), the ESA SnowScat X/Ku-band scatterometer (during 2009-2013; reference to CoReH2O) and VIS/NIR spectroradiometer (since 2006; reference to AATSR, MODIS, VIIRS, Sentinel-2/3 etc.). An essential part in experimental activities has been the production of comprehensive in situ reference data sets from automatic sensor networks, including a full set of atmospheric profile observations, and from regular manual observations (e.g. snow pit observations for the snow pack vertical stratigraphy).
The time-series from tower-based observations are accompanied with airborne and space-borne data sets. The employed airborne instruments include AISA imaging spectrometer, X/Ku-band SnowSAR (ESA's airborne SAR for simulating CoReH2O) and HUT-2D L-band airborne imaging synthetic aperture radiometer. A major campaign activity in Sodankylä has been ESA's multi-year NoSREx that focused to the analysis of the feasibility of CoReH2O mission to retrieve information on snow water equivalent (SWE).
Sodankylä experiments have enabled the development and validation modeling approaches to describe space-borne microwave and optical observations of ground-snow-forest canopy system. In general, the parametrization of models has been investigated concerning the influence of soil frost (or soil dielectric constant), snowpack characteristics (density, gain size, SWE, temperatature), and forest canopy characteristics (closure, tree height). As an outcome, novel models to describe e.g. scene brightness temperature at the microwave region and scene reflectance at the visible region have been developed. For example, in case of optical data the combined use of high-resolution airborne lidar data and AISA-imaging spectrometer data enabled the development of model to describe the quantitative effect forest closure and tree height to optical-range reflectance observations over snow covered landscapes, which is relevant for the feasibility of Sentinel-2 and Sentinel-3 for the snow monitoring. Analogously, tower-based Elbara-II data facilitated the development of models to predict the influence of soil frost to L-band brightness temperature. This led to the development of a methodology to map the evolution of seasonal frost from SMOS data for a hemispheric application (both northern Eurasia and North America). Another example is the analysis of the sensitivity of passive and active microwave observations to SWE, snow grain size and snow density. This indicates the overall feasibility of microwave observations to SWE retrieval. Additionally, the passive microwave and optical data sets from Sodankylä have been used to rectify forward modeling approaches in the ESA DUE GlobSnow project for the needs of the improvement of the global mapping of SWE and snow extent.