USING SMOS PASSIVE MICROWAVE DATA TO DEVELOP SMAP FREEZE/THAW ALGORITHMS ADAPTED FOR THE CANADIAN SUBARCTIC |
Paper ID : 1120-SMPR-FULL |
Authors: |
Parvin Kalantari *1, Monique Bernier2, Kyle C. McDonald3, Jimmy Poulin4 1490, Rue de la Couronne,
Québec, QC, G1K 9A9
Canada 2Institut national de la recherche scientifique- Centre Eau Terre Environnement 3CUNY Environmental Crossroads Initiative and CREST Institute
The City College of New York, City University of New York 4Institut national de la recherche scientifique
Centre - Eau Terre Environnement |
Abstract: |
Climate models predict further warming and higher average flows of rivers in northern Quebec (Nunavik, Eastern Canada) for spring and autumn for the 2041-2070 horizon. Also, the frost season has tended to shorten in recent years. The length of the frost season is defined as the period between the first day of frost, when the daily average temperature is below 0°C and the last day of frost, where the daily average temperature is above 0°C. In the subarctic, the air temperature and precipitation are two key climate variables. Numerous processes such as spring melt, the growth of vegetation, and soil Freezing/Thaw (F/T) are sensitive to temperature changes; snow cover, water resources and ecology of the region are sensitive to precipitations. The various sources of data show that the second half of the 20th century was warmer in Nunavik, but there is some uncertainty as to the years when the recent warming period began due to the large climate variability in this region. The air temperature data recorded at weather stations show rapid warming period in the early 1990s. Seasonal soil F/T cycles play an important role in Boreal and Arctic regions, where structure, condition, and distribution of vegetation are strongly regulated by environmental factors such as soil moisture and nutrient availability, permafrost, growing season length, and disturbance. In these seasonally frozen environments, the growing season is determined primarily by the length of the non-frozen period. Variations in both timing of spring thaw and growing season length have a major impact on the atmospheric-terrestrial carbon exchange in Boreal regions. This study was conducted in the context of the SMAP (Soil Moisture Active and Passive) mission of NASA (launched on January 31st 2015), which includes both Radiometer and Synthetic Aperture Radar (SAR) operating at the L-band for soil moisture mapping and its F/T from a 685 km, near-polar, sun-synchronous orbit for a period of 3 years. The SMAP radiometer provides brightness temperature (Tb) measurement at low resolution (~40 km) from the top ~5 cm in regions having vegetation water contents (VWC) up to ~5 kg m–2. SAR backscattering is measured at higher resolution (~1 to 3 km). So, the radar and radiometer measurements could be combined to derive soil moisture estimates with intermediate accuracy and resolution (~9 km). The accuracy, resolution, and global coverage of the SMAP mission allow for a systematic updating of frozen ground maps and for monitoring the seasonal soil F/T cycle. Microwave sensors are well adapted tools to monitor the soil F/T cycle over the Boreal and Arctic regions. The main objective of this study was to develop and validate algorithms to monitor F/T over the Tundra and the Boreal Forest with SMAP data using available time series of passive microwave data. For that purpose, data from the European Space Agency’s Soil Moisture and Ocean Salinity (SMOS) mission launched in November 2nd 2009 was used. SMOS carries a single payload, an L-Band 2D interferometric radiometer operating at 1.4 GHz or ~21 cm wavelength. Also, SMOS retrieves soil moisture over a much higher range of vegetation conditions at a spatial resolution of ~40 km with a sensing depth of ~5 cm. The daily SMOS data used in this project cover a period of 24 months from January 2011 to the end of December 2012. The study area is Northern Quebec (Nunavik). It is a zone of discontinuous permafrost situated at the tree line. This region covers an area of approximately 507 000 km² and includes numerous lakes and large rivers. The main field site is located near Umiujaq (56.55° N, 76.55° O). This site has been the subject of more than 20 years of study by the Center for Northern Studies (CEN). To classify daily F/T state dynamics from SMOS Tb, the seasonal threshold approach (STA) was adapted with the study area where we can find many lakes and rivers. The approach has capability of resolving daily F/T dynamics state and examination of the temporal progressing of the remote sensing F/T states. Since the water presence causes a decrease in Tb value, weights proportional to the percentage of open water (lake and river), determined from the land cover map was applied. The STA approach uses a spatial and seasonal scale factor of defined for an observation acquired at location and time. Then a dynamic threshold was estimated pixel by pixel using last-square linear regression of the seasonal scale factor vs. surface temperature that is independent variable. The dynamic threshold levels for frozen and non-frozen landscape conditions in each EASE-grid cell were determined where the seasonal scale factor corresponded to an air temperature equal 0°C. The results showed a satisfactory mapping pixel by pixel for the daily monitoring of the soil state (F/T) with a success rate exceeding 80% with in situ data for the HH and VV polarizations, and for different territories. The average accuracy is more than 80% during the soil freeze period vs 84% during soil thaw. The comparison is limited by the small number of validation pixels (1 site = 1 pixel). Another limitation is related to the presence of mixed pixels as the presence of open water that influences the results of soil freezing. The delay between the soil freezing and the water freezing, and also this delay in the thawing period cause errors. A passive microwave sensor with a finer spatial resolution (active microwave) would be more appropriate to solve this type of ambiguity. The joint use of passive and active microwave data in the SMAP mission would improve the identification of the soil freezing state on the study area and achieve a better spatial resolution product. It is planned to further expand the measurement stations network of the soil freezing state Nunavik, to improve the calibration and validation of algorithms of the SMAP mission monitoring the soil freeze-thaw on the tundra and the boreal forest using SMOS. |
Keywords: |
Passive Microwave, Freeze-Thaw, Frozen Soil, SMOS, Remote Sensing |
Status : Paper Accepted (Poster Presentation) |