Ice Freeze-up and Break-up Detection of Shallow Lakes in Northern Alaska with Spaceborne SAR

Abstract

Shallow lakes, with depths less than ca. 3.5-4 m, are a ubiquitous feature of the Arctic Alaskan Coastal Plain, covering up to 40% of the land surface. With such an extended areal coverage, lakes and their ice regimes represent an important component of the cryosphere. The duration of the ice season has major implications for the regional and local climate, as well as for the physical and biogeochemical processes of the lakes. With day and night observations in all weather conditions, synthetic aperture radar (SAR) sensors provide year-round acquisitions. Monitoring the evolution of radar backscatter (σ°) is useful for detecting the timing of the beginning and end of the ice season. Analysis of the temporal evolution of C-band σ° from Advanced Synthetic Aperture Radar (ASAR) Wide Swath and RADARSAT-2 ScanSAR, with a combined frequency of acquisitions from two to five days, was employed to evaluate the potential of SAR to detect the timing of key lake-ice events. SAR observations from 2005 to 2011 were compared to outputs of the Canadian Lake Ice Model (CLIMo). Model simulations fall within similar ranges with those of the SAR observations, with a mean difference between SAR observations and model simulations of only one day for water-clear-of-ice (WCI) from 2006 to 2010. For freeze onset (FO), larger mean differences were observed. SAR analysis shows that the mean FO date for these OPEN ACCESS Remote Sens. 2015, 7 6134 shallow coastal lakes is 30 September and the mean WCI date is 5 July. Results reveal that greater variability existed in the mean FO date (up to 26 days) than in that of melt onset (MO) (up to 12 days) and in that of WCI (6 days). Additionally, this study also identifies limitations and provides recommendations for future work using C-band SAR for monitoring the lake-ice phenology of shallow Arctic lakes.

Monitoring Bedfast Ice and Ice Phenology in Lakes of the Lena River Delta Using TerraSAR-X Backscatter and Coherence Time Series

“…Initial freeze-up (or ice-on) is characterized by low backscatter due to specular reflection from a thin and smooth undeveloped ice cover. At the beginning of ice formation, C-band backscatter intensity values for thin, smooth ice have been reported to range from −22 to −16 dB (ERS-1, VV) [24], −16 to −13 dB (RADARSAT-1, HH) [27] and −15.9 dB to −4.8 dB (ASAR, HH) and −17.4 dB to −11.7 dB (RADARSAT-2, HH) [20]. However, it can be difficult to detect initial ice formation due to similarly low backscatter of newly formed ice and open water under calm wind conditions.…”

“…Howell et al [18] state a difference of 6 dB between the stable stage and freeze onset for QuikSCAT (Ku-band, HH, 46 • incidence angle) time series of Great Bear and Great Slave lakes (Canada). Threshold values of the same order of magnitude are given in [20] for ASAR (HH, 15 • -42 • incidence angle) (5.5 dB) and RADARSAT-2 (HH, 20 • -46 • incidence angle) (6.5 dB) time series.…”

“…Geldsetzer et al [42] report RADARSAT-2 (HH) backscatter values of about −21 dB from lake ice following melt onset. Surdu et al [20] indicate backscatter values ranging from −20.4 to −15.7 dB (ASAR) and from −22.9 to −16.1 dB (RADARSAT-2) at the beginning of break up period.…”

“…Optical remote sensing in the Arctic is limited due to polar night and often persistent cloud cover [15,16]. Active microwave radar signals, on the other hand, penetrate through cloud cover and allow for systematic monitoring of lake ice phenology [17][18][19][20]. Moreover, the difference in radar backscatter intensities between grounded (bedfast) and floating lake ice allows for mapping of these areas and estimation of the timing of grounding.…”

“…Advancements in technology have allowed for lake ice studies using higher spatial resolution (100 m or better) data from C-band RADARSAT-1, ERS-1 and ENVISAT ASAR [12,13,[27][28][29]. One of the main constraints for SAR-based investigations to date has been the insufficient temporal resolution of time series, so that acquisitions obtained with different incidence angles or a multi-sensor approach must be employed [20,27]. Monitoring lake ice conditions using X-band SAR data has mostly been limited to the early airborne studies mentioned above.…”