Lake-source methane ebullition synthetic aperture radar (SAR) single-polarization regression model
Melanie Engram, University of Alaska, Fairbanks, melanie.engram@alaska.edu (Presenter)
Katey Marion Walter Anthony, University of Alaska, Fairbanks, kmwalteranthony@alaska.edu
Franz Josef Meyer, University of Alaska, Fairbanks, fjmeyer@alaska.edu
Northern lakes, especially lakes impacted by thawing permafrost, are a major source of atmospheric methane, a potent greenhouse gas. However, challenges in upscaling field data, including fluxes by ebullition (bubbling), lead to large uncertainties in emissions estimates. Previous work demonstrated that ebullition increases synthetic aperture radar (SAR) backscatter in the L-band frequency from ice-covered lakes by increasing the roughness of the ice/water interface (Engram et al. 2012, 2013). An empirical regression model using decomposed quadrature-polarized (quad-pol) SAR was created based on year-round flux data from underwater bubble traps, then validated in the Barrow Pen. and Atqasuk regions with independent airborne eddy covariance measurements (Engram et al. 2020). This work established L-band SAR as a useful tool to estimate gas ebullition from lakes that freeze and created the potential for detailed mapping of regional emissions on a landscape scale.
However, too few quad-pol L-band SAR images were acquired to apply this analysis to new regions, creating a data availability problem. Here, we developed a novel regression model for predicting ebullition in ice-covered lakes using single-polarized (single-pol) L-band SAR data. Single-pol data are much more widely available and represent a platform analogous to the NISAR satellite data expected to be routinely provided later this year (2025). Our new L-band single pol regression model utilizes field data of annual ebullition from study lakes from five Alaska regions: Utqia?vik (Barrow Peninsula), Atqasuk, Toolik, northern Seward Peninsula, and Fairbanks. Data were acquired from the Phased Array type L-band Synthetic Aperture Radar (PALSAR-1) instrument on the Japanese Advanced Land Observing Satellite (ALOS-1) at ~39° incidence angle with a horizontal transmit and receive polarization (HH). Field data on lake ebullition were based on a combination of on-ice bubble surveys from 2007-2017 and semi-automated underwater bubble-traps per Engram et al. (2020). Single-pol (HH) L-band SAR backscatter was highly correlated with ebullition field data, consistent with our previous results using the T11 “roughness” component of quad-pol L-band SAR (Engram el al. 2020).
In addition to the polarization difference, several differences between our previous T11 quad-pol and the new single-pol analyses should be noted. The quad-pol data were acquired with a steeper incidence angle (~24°) than the Fine Beam Single-pol data (~39°). Also, the pixel size of the quad-pol data was larger (12.5 m) than that of the Fine Beam Single-pol data (6.25 m). To examine the effect of SAR data variables isolated from polarization differences, we used lakes in the Fairbanks area which exhibit a wide range of gas fluxes. We compared only the HH channel of the quad-pol data for the single early winter scene to the sum of 13-17 winter HH Fine Beam Single-pol scenes for five years to examine the effects of incidence angle and more frequent temporal sampling later in the winter on efficacy of emission prediction.
New L-band data from the upcoming NISAR mission coupled with such a single-pol SAR empirical regression model could provide current methane ebullition estimates from lakes that freeze.
Associated Project(s):
Poster Location ID: 30
Presentation Type: Poster
Session: Carbon Dynamics
Session Date: Tuesday (5/13) 4:30-5:30 PM