Greenland ice sheet (GrIS) melting has been a significant concern in the warming climate. Accurate quantification of total surface and subsurface meltwater amount across the pan-Greenland scale is crucial to understanding GrIS mass balance, thus better projecting global sea level rise. We used multi-year L-band observations from the NASA Soil Moisture Active Passive (SMAP) mission to quantify the GrIS surface and sub-surface meltwater amount and examine their spatiotemporal variability. We employed an empirical algorithm to detect surface and subsurface melt events. Then, we applied a physics-based retrieval algorithm to estimate the intensity and physical properties of the melt events. Finally, we validated the retrieval by meltwater derived from a locally calibrated energy balance model with in situ observations from the PROMICE automatic weather station (AWS) network. The retrieval and validation results are presented, which demonstrate generally a good agreement with the meltwater amounts derived from in situ observations.

Long-term studies of climate change can benefit from a retrospective analysis of past satellite missions. To facilitate such studies, in this paper we describe two new radar backscatter (σo) products derived from the Seasat-A Scatterometer System (SASS) and the NASA Scatterometer (NSCAT). Operating at Ku-band (14.6 and 13.995 GHz, respectively), the two sensors provided global σomeasurements in 1978 and 1996-1997 and are unique benchmarks for studying long-term climate change, particularly over the polar ice sheets. The products are compatibly gridded to be consistent with other existing climate data record products of σoand microwave brightness temperature (TB) that span multiple decades.

We use enhanced-resolution L-band radar backscatter and brightness temperature image time series generated from observations collected by NASA’s Soil Moisture Active Passive (SMAP) mission to map the extent and physical characteristics of an expansive perennial firn aquifer recently identified in the Wilkins Ice Shelf, Antarctic Peninsula. Empirical algorithms to map extent are derived from a continuous logistic model. Distinct physical characteristics of the firn overlying the perennial firn aquifer are distinguished using images of the slope in radar backscatter versus incidence angle. An inverted two-layer L-band brightness temperature model parameterized by the optical thickness is used to map seasonal firn saturation. Our results demonstrate the potential for using low-frequency L-band satellite microwave missions to map large englacial meltwater reservoirs in the firn of Antarctic ice shelves that may trigger hydrofracture and ice shelf instability year-round.

For understanding englacial hydrology and its impact on ice sheet mass balance, observations of the liquid water content (LWC) within the ice sheets are needed. Earlier studies have shown the complementary nature of multi-frequency microwave radiometer measurements to detect subsurface LWC distribution in addition to surface LWC, which is critical for understanding the seasonal melt dynamics of ice sheets. In this study, we used 1.4 GHz brightness temperature (TB) measurements from the NASA Soil Moisture Active Passive (SMAP) satellite, and 6.9, 10.7, 18.9, and 36.5 GHz TB measurements from the JAXA Global Change Observation Mission-Water Shizuku (GCOM-W) satellite to investigate the multi-frequency response at pan-Greenland scale. The melt indications derived at different frequencies show trends consistent with persistent seasonal subsurface melt water and delayed subsurface refreezing of the seasonal melt water. The result suggests that the seasonal subsurface persistent melt water occurrences that are not captured by the high-frequency retrievals are both temporally and spatially very significant.

The MEaSUREs Calibrated Enhanced-Resolution Passive Microwave Daily EASE-Grid 2.0 Brightness Temperature (CETB) Earth Science Data Record and the SMAP Twice-Daily rSIR-Enhanced EASE-Grid 2.0 Brightness Temperature (SETB) Data Set provide an extensive multi-instrument, multi-decadal, time series of global enhanced-resolution microwave radiometer image products. The Earth-based CETB and SETB archives are generated using swath-based multi-frequency micro-wave brightness temperature (TB) observations collected by the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR), the Special Sensor Microwave/Imager (SSM/I) and Special Sensor Microwave Imager/Sounder (SSMIS) series, the Advanced Microwave Scanning Radiometer–Earth Observing System (AMSR-E), and the Soil Moisture Active Passive (SMAP) microwave radiometer. In this paper, we describe new global SMAP enhanced-resolution scatterometer and synthetic aperture radar (SAR) image products that augment the CETB and SETB archives. Earth-based, morning and evening images are generated using swath-based L-band (1.26 GHz) radar backscatter (σ°) observations and previous developed image reconstruction techniques. The available image products include cylindrical and azimuthal projection scatterometer and SAR σ° images with 1- and 3-day imaging intervals, and azimuthal projection scatterometer σ° images with an 8-day imaging interval that have been further corrected for incidence and azimuth angle variation over the Greenland and Antarctic ice sheets. These σ° image time series are compatibly-gridded with CETB and SETB TBimage time series to support a wide variety of geophysical applications.

Due to their larger penetration and sensing depth, low frequency microwave measurements have been recently employed to detect ice sheet melt events. In this paper, the response of NASA's SMAP (Soil Moisture Active Passive) L-band measurements to surface melting of the Greenland ice sheet from 2015 through 2021 is investigated. SMAP covers virtually the entire Greenland ice sheet twice a day with its L-band (1.4 GHz) radiometer. The results show that the ice sheet experienced unusually strong surface melting on August 14,2021, which extended the melt area across much of dry snow zone over a period of two days. Moreover, the observational results agree well with model simulations conducted using Glacier Energy and Mass Balance (GEMB) module within the Ice-sheet and Sea-level System Model (ISSM).

Radiometric and radar data from NASA’s Soil Moisture Active Passive (SMAP) satellite are presented in a study of the Ross Ice Shelf, Antarctica. L-band brightness temperature (TB) patterns compare favorably to the outflow patterns from East and West Antarctica. CoolerTBis associated with the broad outflow from West Antarctic ice streams and the outflow from narrow outlet glaciers that drain East Antarctica. Aside from outlet glacier discharges, ice from East Antarctica is thinner and radiometrically warmer than that from West Antarctica.TBis stable across the ice shelf over the 6-year period of observations (1-2 K standard deviation). Over shorter times, surface melt events cause pre- and post-meltTBto vary by as much as 5 K in vertical polarization. Radar measurements highlight areas where backscatter is strong from melt related ice lenses and ice layers, consistent with a corresponding decrease inTB. TheTBpolarization ratio on the ice shelf is approximately 1.13 and decreases from the West Antarctic grounding line towards the East Antarctic outlet glaciers. The backscatter polarization ratio increases by several dB from West to East Antarctica, indicating a decreasing influence of volume scatter toward East Antarctica. The decreasingTBpolarization ratio indicates a diminishing role of firn layering on the depth-integrated emission. There is a negative correlation between warming brightness temperature and thinning. An explanation for this observation is that the relatively warm ice shelf has a relatively shallow (500 m or less) penetration depth leading to a warm physical temperature bias.

Low frequency microwave measurements have been used to gain insight into what happens deep inside ice sheets for some time now. In this paper, we used a deep neural network to classify each pixel within SMAP radiometer footprints over the Antarctica ice sheet as melt or no-melt. NASA's SMAP mission offers a valuable additional set of observations. The SMAP L-band (1.4 GHz) radiometer retrievals also cover virtually the entire Antarctica ice sheet twice a day. Consistent morning and evening sampling are provided by 6 AM/PM equator-crossings of the satellite ascending and descending polar orbits. The spatial resolution of the instrument is about 40 km. The cross-entropy loss function is used in our network. To make training and test sets, we used air temperature records from available weather stations to distinguish melt and no-melt ice sheet conditions. Our results show that the ice sheet experienced extensive surface melting during the 2015–2016 melt season, and also intensive melting in 2019–2020, particularity on the West Antarctic Ice Sheet.

Low frequency microwave measurements have been used to gain insight into what happens deep inside ice sheets for some time. In this paper, the response of SMAP (Soil Moisture Active Passive) L-band measurements to surface melting of the ice sheet from 2015 through 2019 is investigated. SMAP covers virtually the entire Antarctica ice sheet twice a day with its L-band (1.4 GHz) radiometer. The overpasses center on morning and evening hours as the satellite is on a 6 AM/6PM equator-crossing orbit. The spatial resolution of the instrument is about 40 km. We applied a newly developed geophysical model-based algorithm to detect snow wetness, which can be used as an indicator of melt extent and intensity. It is shown that the ice sheet experienced extensive surface melting during the 2015-2016 melt season (~10% melt extent), and also underwent intensive melting in the 2019-2020 season (median of 0.3% snow wetness), particularity on the West Antarctic Ice Sheet.

Microwave measurements have been previously used to detect melt events due to their sensitivity to the presence of liquid water in snow. Since NASA's SMAP mission offers a valuable set of low frequency radiometer measurements, SMAP measurements have been used as a tool to detect melt events. SMAP's L-band radiometer also covers virtually the entire Greenland ice sheet twice daily. The overpasses center on morning and evening hours as the satellite is on a 6AM/6PM equator-crossing orbit, and the spatial resolution of the instrument is about 40 km. In this paper, the response of L-band measurements to surface melting of the ice sheet from 2015 through 2019 melt seasons is investigated. It is shown that the Greenland ice sheet experienced an unusually strong melt event at the end of July 2019, which extended the melt area across much of dry snow zone of the ice sheet over a period of two days.

The Ultra-Wideband Software Defined Microwave Radiometer (UWBRAD) measures scene brightness temperatures from 0.5-2 GHz. UWBRAD was deployed in a September 2017 airborne campaign in Greenland, and observed brightness temperatures of the ice sheet as well as sea ice, the ocean surface, and land regions during the transit to and from Calgary, Canada (the aircraft base of operations). This presentation will review the campaign and datasets collected. Spectral features of thermal emissions from the ice sheet and other geophysical regions are also examined to obtain insight into the utility of 0.5-2 GHz thermal emission measurements for remote sensing applications.