Abstract
Abstract. We use the CloudSat 2006–2016 data record to estimate snowfall over the Greenland Ice Sheet (GrIS). We first evaluate CloudSat snowfall retrievals with respect to remaining ground-clutter issues. Comparing CloudSat observations to the GrIS topography (obtained from airborne altimetry measurements during IceBridge) we find that at the edges of the GrIS spurious high-snowfall retrievals caused by ground clutter occasionally affect the operational snowfall product. After correcting for this effect, the height of the lowest valid CloudSat observation is about 1200 m above the local topography as defined by IceBridge. We then use ground-based millimeter wavelength cloud radar (MMCR) observations obtained from the Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit, Greenland (ICECAPS) experiment to devise a simple, empirical correction to account for precipitation processes occurring between the height of the observed CloudSat reflectivities and the snowfall near the surface. Using the height-corrected, clutter-cleared CloudSat reflectivities we next evaluate various Z–S relationships in terms of snowfall accumulation at Summit through comparison with weekly stake field observations of snow accumulation available since 2007. Using a set of three Z–S relationships that best agree with the observed accumulation at Summit, we then calculate the annual cycle snowfall over the entire GrIS as well as over different drainage areas and compare the derived mean values and annual cycles of snowfall to ERA-Interim reanalysis. We find the annual mean snowfall over the GrIS inferred from CloudSat to be 34±7.5 cm yr−1 liquid equivalent (where the uncertainty is determined by the range in values between the three different Z–S relationships used). In comparison, the ERA-Interim reanalysis product only yields 30 cm yr−1 liquid equivalent snowfall, where the majority of the underestimation in the reanalysis appears to occur in the summer months over the higher GrIS and appears to be related to shallow precipitation events. Comparing all available estimates of snowfall accumulation at Summit Station, we find the annually averaged liquid equivalent snowfall from the stake field to be between 20 and 24 cm yr−1, depending on the assumed snowpack density and from CloudSat 23±4.5 cm yr−1. The annual cycle at Summit is generally similar between all data sources, with the exception of ERA-Interim reanalysis, which shows the aforementioned underestimation during summer months.
Highlights
The Greenland Ice Sheet (GrIS) is currently losing mass at a rate of roughly 240 Gt yr−1, translating into a sea level rise of 0.47 ± 0.23 mm yr−1, which corresponds to roughly 15 %–20 % of the total annual mean sea level rise
We use millimeter wavelength cloud radar (MMCR) observations from ICECAPS to estimate the impact of CloudSat’s observation height on estimated surface snowfall, and we propose a simple GrISspecific empirical correction to account for the difference between actual surface snowfall and the CloudSat observations made at about 1200 m height above the surface
As discussed below, KB09_LR3 is likely more representative of the light snowfall observed over the high GrIS than other Z–S relationships, which apply more to the midlatitudes
Summary
The Greenland Ice Sheet (GrIS) is currently losing mass at a rate of roughly 240 Gt yr−1, translating into a sea level rise of 0.47 ± 0.23 mm yr−1 (van den Broeke et al, 2016), which corresponds to roughly 15 %–20 % of the total annual mean sea level rise. Precipitation is the sole source of mass of the GrIS. The inter-annual precipitation variability appears to be the main driver of inter-annual variability in the mass balance of the GrIS (van den Broeke et al, 2009). R. Bennartz et al.: Spatial and temporal variability of snowfall over Greenland
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