Detection of soil permittivity and soil freezing using satellite microwave radars

• Main Author: Smolander, Tuomo
• Other Authors: Helsingin yliopisto, Matemaattis-luonnontieteellinen tiedekunta, Fysiikan laitos, University of Helsinki, Faculty of Science, Department of Physics, Helsingfors universitet, Matematisk-naturvetenskapliga fakulteten, Institutionen för fysik
• Format: Master Thesis
• Language: English
• Published: Helsingin yliopisto 2018
• Subjects: Theoretical Physics; Teoreettinen fysiikka; Teoretisk fysik; Asar; Northern Finland
• Online Access: http://hdl.handle.net/10138/273616

Remote sensing of soil permittivity and soil freezing was investigated using two different satellite based microwave radars: ASCAT and ASAR. ASCAT is a scatterometer with a good temporal resolution but coarse spatial resolution. ASAR is a synthetic aperture radar and has fine spatial resolution, but lacks good temporal coverage. Soil permittivity is related to soil moisture, which is considered an essential climate vari- able since it has an effect on both weather and climate. Soil freezing affects hydrological and carbon cycles, surface energy balance, photosynthesis of vegetation and the activity of soil microbes. A semi-empirical model for backscattering of forested land was used to acquire soil permittivity retrievals from satellite measurements using the method of least squares. The onset of soil freezing was determined from the permittivity retrievals using a simple threshold method. A five year time series of satellite observations from July 2007 to June 2012 (April 2012 for ASAR) was investigated in Sodankylä in Northern Finland. The satellite based retrievals were compared against in situ measurements of soil permittivity, soil temperature, soil frost and snow depth. According to the results the satellite permittivity retrievals correlate with each other, but not with in situ permittivity measurements. ASCAT retrieval shows some correlation with in situ temperature measurements, which could impair its correlation with in situ permittivity. The explanation for this phenomenon needs further research. Comparison of soil freezing onset dates from satellite retrievals with in situ soil temperature and soil frost measurements showed quite good agreement for most years, and did not seem to be affected by first snowfall, even though the permittivity retrievals appeared to react in a similar way to snow cover and soil freezing. This indicates that with better calibration of the permittivity threshold limit this method could be used for soil freeze detection. Auxiliary information about air temperature and snow ...