Detecting Greenland Melt with the SMAP L-band Radiometer

<p>Complex processes within the ice govern and record the evolution of the Greenland Ice Sheet. Low frequency microwave measurements have been used to gain insight into what happens deep inside the ice for some time now. NASA’s SMAP mission offers a valuable additional set of observations. SMAP covers virtually the entire ice sheet twice a day with its L-band radiometer. The overpasses center on morning and evening hours as the satellite is on a 6 AM/6PM equator-crossing orbit, and the spatial resolution of the instrument is about 40 km.</p><p>In this study, we investigated the response of L-band (1.4 GHz) measurements to surface melting of the ice sheet from the 2015 through 2019 melt seasons. The changes in brightness temperature caused by surface melt differs in the ablation zone, the active melt areas, and the interior’s dry snow zone. The melt area can be tracked with SMAP when accounting for these differences. SMAP’s frequent revisit time enables tracking of the melt events with comparatively high temporal fidelity. The evolution of the seasonal melt area derived from SMAP is consistent with other methods used for tracking ice sheet melt area.</p><p>Most notably, Greenland experienced an unusually strong melt event at the end of July 2019, which extended the melt area to the dry snow zone of the ice sheet over a period of two days. In-situ temperatures measured at Greenland’s Summit station show above-freezing temperatures during this event, and subsequent in-situ ice analyses have revealed ice structure changes associated with melt on these dates and subsequent refreezing. SMAP was able to record the extent of this unusual melt event on both days, and to show the anomalous extent of the melt event compared to the past 4 years of operational measurements.</p><p>This presentation will discuss the SMAP signal sensitivity to ice structure changes, the seasonal melt extent evolution and its inter-annual variation, and the comparison of the results to other data sources.</p>

Drivers of ASCAT C band backscatter variability in the dry snow zone of Antarctica

ABSTRACTC band backscatter parameters contain information about the upper snowpack/firn in the dry snow zone. The wide incidence angle diversity of the Advanced Scatterometer (ASCAT) gives unprecedented characterisation of backscatter anisotropy, revealing the backscatter response to climatic forcing. TheA(isotropic component) andM2(bi-sinusoidal azimuth anisotropy) parameters are investigated here, in conjunction with data from atmospheric and snowpack models, to identify the backscatter response to surface forcing parameters (wind speed and persistence, precipitation, surface temperature, density and grain size). The long-term meanAparameter is successfully recreated with a regression using these drivers, indicating strong links between theAparameter and precipitation on long timescales. While the ASCAT time series is too short to determine which factors drive observed trends, factors influencing the seasonal and short timescale variability are revealed. On these timescales,Astrongly responds to the propagation of surface temperature cycles/anomalies downward through the firn, via direct modulation of the dielectric constant. The influence of precipitation onAis small at shorter timescales. TheM2parameter is controlled by wind speed and persistence, through modification of monodirectionally-aligned surface roughness. This variability indicates that throughout much of coastal Antarctica, a microwave ‘snapshot’ is generally not representative of longer-term conditions.

Spatially extensive estimates of annual accumulation in the dry snow zone of the Greenland Ice Sheet determined from radar altimetry

We present estimates of accumulation rate along a 200 km transect ranging in elevation from 2750 to 3150 m in the dry snow zone on the western slope of the Greenland Ice Sheet. An airborne radar altimeter is used to estimate the thickness of annual internal layers and, in conjunction with ground based snow/firn density profiles, annual accumulation rates between 1998 and 2003 are derived. A clear gradient in the thickness of each layer observed by the radar altimeter and in the associated estimates of annual accumulation is seen along the transect, with a 33.6% ± 16% mean decrease in accumulation from west to east. The observed inter-annual variability is high, with the annual mean accumulation rate estimated at 0.359 m.w.e. yr−1 (s.d. ± 0.049 m.w.e. yr−1). Mean accumulation rates modelled using meteorological models overestimate our results by 16% on average, but by 32% and 42% in the years 2001 and 2002. The methodology presented here demonstrates the potential to obtain accurate and spatially extensive accumulation rates from radar altimeters in regions of ice sheets where field observations are sparse, and accumulation rates greater than several tens of cm.

Relating microwave backscatter azimuth modulation to surface properties of the Greenland ice sheet

Azimuth dependence of a normalized radar cross-section (σº) over the Greenland ice sheet is modeled with a simple surface scattering model. The model assumes that azimuth anisotropy in surface roughness at scales of 3–300 m is the primary mechanism driving the modulation. To evaluate the contribution of azimuth anisotropy in surface roughness to the radar backscatter, the model is compared to models based on isotropic surface roughness. The models are inverted to estimate snow surface properties using σº measurements from the C-band European Remote-sensing Satellite advanced microwave instrument in scatterometer mode. Results indicate that the largest mesoscale rms surface slopes are found in the lower portions of the dry snow zone. Estimates of the preferential direction in surface roughness are highly correlated with katabatic wind fields over Greenland, which is consistent with wind-formed sastrugi as the dominant mechanism causing azimuth modulation of σº. The maximum improvement of the azimuth modulation surface model compared to its isotropic counterparts occurs in the lower regions of the dry snow zone where the azimuth variability of σº is the largest. In regions with azimuth modulation over 1 dB, the mean root-mean-square error estimate of the azimuth-dependent surface scattering model is 0.46 dB compared with 0.70 dB for similar models using isotropic roughness.