Review of tc-2020-297 A novel approach to map the intensity of surface melting in the Antarctica Ice Sheet using SMAP L-Band microwave radiometry

Abstract. Perennial firn aquifers are subsurface meltwater reservoirs formed from a water-saturated firn layer. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting and high snow accumulation. Widespread perennial firn aquifers have been identified within the Greenland Ice Sheet (GrIS) via field expeditions, airborne ice-penetrating radar surveys, and satellite microwave sensors. In contrast, ice slabs are nearly-continuous ice layers that form on spatial scales of kilometers as a result of surface and subsurface water-saturated snow and firn layers sequentially refreezing following multiple melting seasons. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting, but in areas where snow accumulation is at least ~25 % lower as compared to perennial firn aquifer areas. Widespread ice slabs have recently been identified within the GrIS via field expeditions and airborne ice-penetrating radar surveys, specifically in areas where perennial firn aquifers typically do not form. However, ice slabs have yet to be inferred from space. Together, these two ice sheet features represent distinct, but related, sub-facies within the broader percolation facies of the GrIS that can be defined primarily by differences in snow accumulation, which influences the englacial hydrology and thermal characteristics of firn layers at depth. Here, for the first time, we use enhanced-resolution vertically-polarized L-band brightness temperature (TBV) imagery (2015–2019) generated using observations collected over the GrIS by NASA’s Soil Moisture Active Passive (SMAP) satellite to map both perennial firn aquifer and ice slab areas as a continuous system over the percolation facies. We also map “perched” firn aquifer areas, which we define as areas where shallow water-saturated firn layers transiently form on top of buried ice slabs, or other semi-impermeable layers within the snow and firn. An empirical algorithm previously developed to map the extent of Greenland’s perennial firn aquifers via fitting exponentially decreasing temporal L-band signatures to a set of sigmoidal curves is recalibrated to also map the extent of ice slab and perched firn aquifer areas using airborne ice-penetrating radar surveys collected by NASA’s Operation Ice Bridge (OIB) campaigns (2010–2017). Our SMAP-derived maps show that between 2015 and 2019, perennial firn aquifer areas extended over ~64,000 km2, ice slab areas extended over ~76,000 km2, and perched firn aquifer areas extended over ~37,000 km2. Combined together, these three sub-facies are the equivalent of ~24 % of the percolation facies of the GrIS. As Greenland’s climate continues to warm, and seasonal surface melting increases in extent, intensity, and duration, quantifying the possible rapid expansion of each of these sub-facies using satellite L-band microwave radiometry has significant implications for understanding ice sheet-wide variability in englacial firn hydrology resulting in meltwater-induced hydrofracturing and accelerated ice flow as well as high-elevation run-off that can impact the mass balance and stability of the GrIS.

Download Full-text

An empirical algorithm to map perennial firn aquifers and ice slabs within the Greenland Ice Sheet using satellite L-band microwave radiometry

The Cryosphere ◽

10.5194/tc-16-103-2022 ◽

2022 ◽

Vol 16 (1) ◽

pp. 103-125

Author(s):

Julie Z. Miller ◽

Riley Culberg ◽

David G. Long ◽

Christopher A. Shuman ◽

Dustin M. Schroeder ◽

...

Keyword(s):

Spatial Scales ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Melting ◽

Snow Accumulation ◽

Rapid Expansion ◽

Microwave Radiometry ◽

Subsurface Water ◽

L Band ◽

Water Saturated

Abstract. Perennial firn aquifers are subsurface meltwater reservoirs consisting of a meters-thick water-saturated firn layer that can form on spatial scales as large as tens of kilometers. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting and high snow accumulation. Widespread perennial firn aquifers have been identified within the Greenland Ice Sheet (GrIS) via field expeditions, airborne ice-penetrating radar surveys, and satellite microwave sensors. In contrast, ice slabs are nearly continuous ice layers that can also form on spatial scales as large as tens of kilometers as a result of surface and subsurface water-saturated snow and firn layers sequentially refreezing following multiple melting seasons. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting but in areas where snow accumulation is at least 25 % lower as compared to perennial firn aquifer areas. Widespread ice slabs have recently been identified within the GrIS via field expeditions and airborne ice-penetrating radar surveys, specifically in areas where perennial firn aquifers typically do not form. However, ice slabs have yet to be identified from space. Together, these two ice sheet features represent distinct, but related, sub-facies within the broader percolation facies of the GrIS that can be defined primarily by differences in snow accumulation, which influences the englacial hydrology and thermal characteristics of firn layers at depth. Here, for the first time, we use enhanced-resolution vertically polarized L-band brightness temperature (TVB) imagery (2015–2019) generated using observations collected over the GrIS by NASA's Soil Moisture Active Passive (SMAP) satellite to map perennial firn aquifer and ice slab areas together as a continuous englacial hydrological system. We use an empirical algorithm previously developed to map the extent of Greenland's perennial firn aquifers via fitting exponentially decreasing temporal L-band signatures to a set of sigmoidal curves. This algorithm is recalibrated to also map the extent of ice slab areas using airborne ice-penetrating radar surveys collected by NASA's Operation IceBridge (OIB) campaigns (2010–2017). Our SMAP-derived maps show that between 2015 and 2019, perennial firn aquifer areas extended over 64 000 km2, and ice slab areas extended over 76 000 km2. Combined together, these sub-facies are the equivalent of 24 % of the percolation facies of the GrIS. As Greenland's climate continues to warm, seasonal surface melting will increase in extent, intensity, and duration. Quantifying the possible rapid expansion of these sub-facies using satellite L-band microwave radiometry has significant implications for understanding ice-sheet-wide variability in englacial hydrology that may drive meltwater-induced hydrofracturing and accelerated ice flow as well as high-elevation meltwater runoff that can impact the mass balance and stability of the GrIS.

Download Full-text

Evaluation of Surface Melt on the Greenland Ice Sheet using SMAP L-Band Microwave Radiometry

IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing ◽

10.1109/jstars.2021.3124229 ◽

2021 ◽

pp. 1-1

Author(s):

Mohammad Mousavi ◽

Andreas Colliander ◽

Julie Z. Miller ◽

Dara Entekhabi ◽

J. Johnson ◽

...

Keyword(s):

Ice Sheet ◽

Greenland Ice Sheet ◽

Microwave Radiometry ◽

L Band ◽

Surface Melt

Download Full-text

A Novel Approach to Map the Intensity of Surface Melting on the Antarctica Ice Sheet using SMAP L-Band Microwave Radiometry

10.5194/tc-2020-297 ◽

2020 ◽

Author(s):

Seyedmohammad Mousavi ◽

Andreas Colliander ◽

Julie Z. Miller ◽

John S. Kimball

Keyword(s):

Brightness Temperature ◽

Ice Sheet ◽

Antarctic Ice Sheet ◽

West Antarctic Ice Sheet ◽

Microwave Brightness Temperature ◽

Geophysical Model ◽

West Antarctic ◽

Novel Approach ◽

L Band ◽

Satellite Microwave Measurements

Abstract. The polar ice sheets have undergone unprecedented melt events in the recent past, which have consequences for ice sheet mass balance, stability, and sea level. In this study, we employed L-band (1.4 GHz) brightness temperature observations collected by NASA's Soil Moisture Active Passive (SMAP) mission to investigate the extent, duration and intensity of melt events on the Antarctic Ice Sheet from 2015 to 2020. Satellite microwave measurements have long been used to detect melt events because of their sensitivity to the presence of liquid water in snow and ice. The observed microwave response depends on the sensor measurement frequency. Our hypothesis for this study is that the relatively long wavelength SMAP observations can detect a wider range of surface wetness conditions relative to shorter wavelength microwave observations that attain signal saturation at relatively lower wetness levels and within shallower surface layers. SMAP provides nearly all-weather surface monitoring over all of Antarctica twice daily with morning and evening overpasses at about 40 km spatial resolution. We applied an empirical threshold algorithm using horizontally and vertically polarized microwave brightness temperature differences to detect surface melt events over Antarctica from 2015 through 2020. The results show that the SMAP empirical algorithm can be used to detect melt extent and duration, and the geophysical model-based algorithm can be used to detect snow wetness, which can be used as an indicator of melt intensity. Analysis of the melt seasons between 2015 and 2020 show that the even though the melt extent in 2019–2020 was not as large as during the 2015–2016 melt season, it was significantly more intense, particularity on the West Antarctic Ice Sheet.

Download Full-text

An L-Band Radar System for Ice Sheet Measurements

IGARSS 2020 - 2020 IEEE International Geoscience and Remote Sensing Symposium ◽

10.1109/igarss39084.2020.9324401 ◽

2020 ◽

Author(s):

J.-B. Yan ◽

S. Kolpuke ◽

J. Nunn ◽

L. Li ◽

S. Gogineni ◽

...

Keyword(s):

Ice Sheet ◽

Radar System ◽

L Band

Download Full-text

Greenland land-terminating glaciers velocity trends during the last two decades

10.5194/egusphere-egu21-2084 ◽

2021 ◽

Author(s):

Paul Halas ◽

Jeremie Mouginot ◽

Basile de Fleurian ◽

Petra Langebroek

Keyword(s):

Water Pressure ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Melting ◽

Limited Area ◽

Ice Dynamics ◽

Basal Sliding ◽

Surface Melt ◽

Ice Sheet Dynamics ◽

The Impact

<div> <p>Ice losses from the Greenland Ice Sheet have been increasing in the last two decades, leading to a larger contribution to the global sea level rise.&#160;Roughly 40% of the contribution comes from ice-sheet dynamics, mainly regulated by basal sliding.&#160;The sliding component&#160;of glaciers has been observed to be strongly related to surface melting, as water&#160;can eventually&#160;reach the bed and impact&#160;the subglacial water pressure, affecting the basal sliding.&#160;&#160;</p> </div><div> <p>The link between ice velocities and surface melt on multi-annual time scale is still not totally understood&#160;even though it&#160;is of major importance with expected increasing surface melting.&#160;Several studies showed&#160;some&#160;correlation between an increase in surface melt and a slowdown in&#160;velocities, but&#160;there is no&#160;consensus&#160;on those trends.&#160;Moreover&#160;those&#160;investigations&#160;only&#160;presented results&#160;in a limited area over&#160;Southwest&#160;Greenland.&#160;&#160;</p> </div><div> <p>Here we present the ice motion over many land-terminating glaciers on the Greenland Ice Sheet for the period 2000 - 2020. This type of glacier is ideal for studying processes at the interface between the bed and the ice since they are exempted from interactions with the sea while still being relevant for all glaciers since they share the same basal friction laws. The velocity data was obtained using optical Landsat 7 & 8 imagery and feature-tracking algorithm. We attached importance keeping the starting date of our image pairs similar, and avoided stacking pairs starting before and after melt seasons, resulting in multiple velocity products for each year.&#160;&#160;</p> </div><div> <p>Our results show similar velocity trends for previously studied areas with a slowdown until 2012 followed by an acceleration.&#160;This trend however does not seem to be observed on the whole ice sheet and is probably&#160;specific&#160;to&#160;this region&#8217;s&#160;climate forcing.&#160;</p> </div><div> <p>Moreover comparison between ice velocities from different parts of Greenland allows us to observe the impact of different climatic trends on ice dynamics.</p> </div>

Download Full-text

Geospatial simulations of airborne ice-penetrating radar surveying reveal elevation under-measurement bias for ice-sheet bed topography

Annals of Glaciology ◽

10.1017/aog.2020.35 ◽

2020 ◽

Vol 61 (81) ◽

pp. 46-57 ◽

Cited By ~ 1

Author(s):

Oliver T. Bartlett ◽

Steven J. Palmer ◽

Dustin M. Schroeder ◽

Emma J. MacKie ◽

Timothy T. Barrows ◽

...

Keyword(s):

Measurement Errors ◽

Survey Design ◽

Ice Sheet ◽

Cross Sectional Area ◽

Sectional Area ◽

Cross Sectional ◽

Ice Dynamics ◽

Outlet Glacier ◽

Bed Topography ◽

Novel Approach

AbstractAirborne radio-echo sounding (RES) surveys are widely used to measure ice-sheet bed topography. Measuring bed topography as accurately and widely as possible is of critical importance to modelling ice dynamics and hence to constraining better future ice response to climate change. Measurement accuracy of RES surveys is influenced both by the geometry of bed topography and the survey design. Here we develop a novel approach for simulating RES surveys over glaciated terrain, to quantify the sensitivity of derived bed elevation to topographic geometry. Furthermore, we investigate how measurement errors influence the quantification of glacial valley geometry. We find a negative bias across RES measurements, where off-nadir return measurement error is typically −1.8 ± 11.6 m. Topographic highlands are under-measured an order of magnitude more than lowlands. Consequently, valley depth and cross-sectional area are largely under-estimated. While overall estimates of ice thickness are likely too high, we find large glacier valley cross-sectional area to be under-estimated by −2.8 ± 18.1%. Therefore, estimates of ice flux through large outlet glaciers are likely too low when this effect is not taken into account. Additionally, bed mismeasurements potentially impact our appreciation of outlet-glacier stability.

Download Full-text

Surface melting over the Greenland ice sheet from enhanced resolution passive microwave brightness temperatures (1979–2019)

10.5194/tc-2020-250 ◽

2020 ◽

Author(s):

Paolo Colosio ◽

Marco Tedesco ◽

Xavier Fettweis ◽

Roberto Ranzi

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Spatial Resolution ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Surface Melting ◽

Passive Microwave ◽

High Temporal Resolution ◽

Ice Dynamics ◽

Enhanced Resolution

Abstract. Surface melting is a major component of the Greenland ice sheet (GrIS) surface mass balance, affecting sea level rise through direct runoff and the modulation on ice dynamics and hydrological processes, supraglacially, englacially and subglacially. Passive microwave (PMW) brightness temperature observations are of paramount importance in studying the spatial and temporal evolution of surface melting in view of their long temporal coverage (1979–to date) and high temporal resolution (daily). However, a major limitation of PMW datasets has been the relatively coarse spatial resolution, being historically of the order of tens of kilometres. Here, we use a newly released passive microwave dataset (37 GHz, horizontal polarization) made available through the NASA MeASUREs program to study the spatiotemporal evolution of surface melting over the GrIS at an enhanced spatial resolution of 3.125 Km. We assess the outputs of different detection algorithms through data collected by Automatic Weather Stations (AWS) and the outputs of the MAR regional climate model. We found that surface melting is well captured using a dynamic algorithm based on the outputs of MEMLS model, capable to detect sporadic and persistent melting. Our results indicate that, during the reference period 1979–2019 (1988–2019), surface melting over the GrIS increased in terms of both duration, up to ~4.5 (2.9) days per decade, and extension, up to 6.9 % (3.6 %) of the GrIS surface extent per decade, according to the MEMLS algorithm. Furthermore, the melting season has started up to ~4 (2.5) days earlier and ended ~7 (3.9) days later per decade. We also explored the information content of the enhanced resolution dataset with respect to the one at 25 km and MAR outputs through a semi-variogram approach. We found that the enhanced product is more sensitive to local scale processes, hence confirming the potential interest of this new enhanced product for studying surface melting over Greenland at a higher spatial resolution than the historical products and monitor its impact on sea level rise. This offers the opportunity to improve our understanding of the processes driving melting, to validate modelled melt extent at high resolution and potentially to assimilate this data in climate models.

Download Full-text