Degradation of sub-aquatic permafrost can impact offshore infrastructure, affect coastal erosion and release large quantities of methane, which may reach the atmosphere and function as a positive feedback to climate warming. The degradation rate depends on the duration of inundation, warming rate, sediment characteristics, the coupling of the bottom to the atmosphere through bottom-fast ice, and brine injections into the sediment. We apply the Cryo-GRID2 model, coupled to a salt diffusion model, to near-shore subsea permafrost thawing offshore of the Bykovsky Peninsula in Siberia. We model permafrost through multiple settings, including 1) terrestrial permafrost, 2) shallow sea with ice grounding, and 3) shallow offshore sea (<= 5.3m depth) without ice grounding. The model uses a terrestrial permafrost temperature of -10 °C at the depth of zero annual amplitude, based on borehole observations, and a coastal erosion rate of 0.5 m/year, based on historical remote sensing imagery dating back to 1951. The seawater salinity prior to ice formation is based on a series of conductivity, temperature, and depth (CTD) measurements from summer 2017, as well as from Soil Moisture and Ocean Salinity (SMOS) satellite data. Water depth is available from echo-sounding surveys made in parallel with floating electrode electrical resistivity surveys in summer 2017. The model outputs are compared to the depth of the ice-bearing permafrost table (IBPT) determined from an electrical resistivity survey perpendicular to the shoreline. The floating electrode survey was combined with a terrestrial resistivity survey to show the transition from undisturbed terrestrial permafrost to submerged permafrost. The geoelectric surveys show a gently inclining IBPT table perpendicular to the coastline, which can be explained by a decreasing rate of degradation with increasing period of inundation. As the inundation period increases, the diffusive (heat and salt) gradients become less steep. The IBPT is located 20 m below the seabed 300 m offshore, which corresponds to 600 years of coastal erosion and an average IBPT degradation rate of 0.33 m per decade. The modelling results show an IBPT 18 m below the seabed and salty sediment up to 14 m below the seabed 300 m offshore. Therefore, the modelling results agree, at least qualitatively, with the sediment
state inferred from the geoelectric data. Coupled heat and salt diffusion produces profiles of temperature and salt concentration in sediment as a function of time. The inclusion of salt flow in thermal models is particularly important in shallow waters where cryotic sediments form due to negative benthic water temperatures or ice grounding, because the depressed freezing point produced by salt diffusion can delay or prevent ice formation in the sediments and enhance the IBPT degradation rate.
With current remote sensing technologies, it is not possible to directly measure the thermal state of the ground from spaceborne platforms. Here, we demonstrate that such limitations can be overcome by exploiting the combined information content of several remote sensing products in a data fusion approach. For this purpose, time series of remotely sensed land surface temperature, as well as snow cover and snow water equivalent, are employed to force ground thermal models which deliver ground temperatures and thaw depths.
First, we present a semi-empirical model approach based on remotely sensed land surface temperatures and reanalysis products from which mean annual ground temperatures (MAGT) can be estimated at a spatial resolution of 1 km at continental scales. The approach is tested for the unglacierized land areas in the North Atlantic region, an area of more than 5 million km2. The results are compared to in-situ temperature measurements in more than 100 boreholes from which the accuracy of the scheme is estimated to approximately 2.5 °C.
Furthermore, we explore transient modeling of ground temperatures driven by remotely sensed land surface temperature, snow cover and snow water equivalent. The permafrost model CryoGrid 2 is applied to the Lena River Delta in NE Siberia (~25,000 km2) at 1 km spatial and weekly time resolution for the period 2000-2014. A comparison to in-situ measurements suggests a possible accuracy of around 1 °C for annual average ground temperatures, and around 0.1 m for thaw depths. However, information on subsurface stratigraphies including the distribution of ground ice is required to achieve this accuracy which is currently not available from remote sensing products alone.
Finally, we discuss the potential and limitations of such schemes and give a feasibility assessment for both mountain and lowland permafrost regions.
The thermal regime in steep and snow-free rock slopes is crucial for understanding rock slope stability, frost weathering and the associated material production in steep mountain areas. In this study, we model heat flow and explore the hypothesis that strong thermal gradients are maintained in transition areas between snow-free rock walls and snow-covered talus slopes. The results of our 2D heat transfer modelling experiments indicate that, under the assumption of snow-free steep rock walls, conductive heat flow can cool the upper parts of an adjacent talus slope with low conductivity and induce strong thermal gradients in the solid bedrock. The modelled conductive cooling effect may be relevant for both frost weathering processes and subsequent geomorphological implications and for the thermal regime of complex surface material in rock wall-talus systems in alpine areas.
Abstract. The future development of ground temperatures in permafrost areas is determined by a number of factors varying on different spatial and temporal scales. For sound projections of impacts of permafrost thaw, scaling procedures are of paramount importance. We present numerical simulations of present and future ground temperatures at 10 m resolution for a 4 km long transect across the lower Zackenberg valley in NE Greenland. The results are based on stepwise downscaling of General Circulation Model-derived future projections using observational data, snow redistribution modeling, remote sensing data and a ground thermal model. Comparison to in-situ measurements of thaw depths at two CALM sites and 10 m ground temperatures in two boreholes suggest agreement within 0.10 m for the maximum thaw depth and 1°C for annual average ground temperature. Until 2100, modeled ground temperatures at 10 m depth warm by about 5° and the active layer thickness increases by about 30%, in conjunction with a warming of average near-surface summer soil temperatures by 2°. While permafrost remains thermally stable until 2100 in most model grid cells, the thaw threshold is exceeded for a few model years and grid cells at the end of this century. The ensemble of all 10 m model grid cells highlights the significant spatial variability of the ground thermal regime which is not accessible in traditional coarse-scale modeling approaches.
In this study the authors present a remote sensing based scheme for transient modelling of the ground surface regime together with the previously published numerical model CryoGrid2.The scheme is applied over a large area in the Lena River Delta (LRD), Siberia.Forcing datasets at 1km and weekly resolution are derived from MODIS LST, MODIS SCE, GlobSnow SWE plus meteo fields from ERA-Interim reanalysis.Spatially distributed ground properties are based on geomorphological observations and mapping drawing on previous studies in the region.Results are compared to insitu observations of ground temperatures from boreholes, CALM active layer depths and measurements from the Samoylov Island Permafrost observatory.The authors conclude that comparison to in-situ measurements shows that the scheme is capable
ABSTRACT Research in geocryology is currently principally concerned with the effects of climate change on permafrost terrain. The motivations for most of the research are (1) quantification of the anticipated net emissions of CO 2 and CH 4 from warming and thaw of near‐surface permafrost and (2) mitigation of effects on infrastructure of such warming and thaw. Some of the effects, such as increases in ground temperature or active‐layer thickness, have been observed for several decades. Landforms that are sensitive to creep deformation are moving more quickly as a result, and Rock Glacier Velocity is now part of the Essential Climate Variable Permafrost of the Global Climate Observing System. Other effects, for example, the occurrence of physical disturbances associated with thawing permafrost, particularly the development of thaw slumps, have noticeably increased since 2010. Still, others, such as erosion of sedimentary permafrost coasts, have accelerated. Geochemical effects in groundwater from trace elements, including contaminants, and those that issue from the release of sediment particles during mass wasting have become evident since 2020. Net release of CO 2 and CH 4 from thawing permafrost is anticipated within two decades and, worldwide, may reach emissions that are equivalent to a large industrial economy. The most immediate local concerns are for waste disposal pits that were constructed on the premise that permafrost would be an effective and permanent containment medium. This assumption is no longer valid at many contaminated sites. The role of ground ice in conditioning responses to changes in the thermal or hydrological regimes of permafrost has re‐emphasized the importance of regional conditions, particularly landscape history, when applying research results to practical problems.
In permafrost regions, there is a strong coupling between a soil’s moisture content and its thermal dynamics.
However, dynamic changes in soil moisture have not been given much attention in permafrost monitoring, partially due to a previous shortage of observations. The questions hence arises: can novel remotely-sensed soil moisture estimates improve permafrost monitoring? Data assimilation seems a promising avenue, as it can improve the predicted temperatures and soil moisture by exploiting their complex, model-predicted coupling while accounting for uncertainties in both modelled and observed soil moisture. To explore its potential benefit, we conduct synthetic and real-world (Radarsat-2 soil moisture estimates over the Mackenzie River Delta Uplands, Canada) data assimilation experiments. We use an Ensemble Kalman Filter to ingest surface soil moisture into the state-of-the art CryoGrid-3 permafrost model, which has a flexible two-layer hydrology scheme. We address two questions.
1) Where can surface soil moisture information improve modelled temperatures? We find that it mainly does so for porous, organic soils, but not for mineral soils. As organic soils dry, the cooling effect by the insulating soil wins out over the competing warming effect induced by decreasing evaporation. Surface soil moisture observations thus provide valuable information on deeper soil temperatures, a finding that is largely consistent with field observations. In mineral soils, by contrast, the thermal conductivity decreases much less upon drying,and surface soil moisture provides little information on deeper soil temperatures.
2) How big are the improvements in organic soils? In our synthetic experiments, we find that estimates of the active layer thickness improve by up to a factor of two (down to 10 cm) upon assimilation, even when soil moisture observations are of limited precision. The modelled soil temperatures improve throughout the entire profile, with the largest improvements below 10 cm. We will compare those synthetic results with the Radarsat-2 observations.
We conclude that satellite soil moisture information can help to reduce one major uncertainty in permafrost monitoring. We predict that advances in remote sensing and models will improve our knowledge of active layer and permafrost dynamics, not just of the water and energy balance, but also of ecological and biogeochemical processes.
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