Satellite observations of snowfall regimes over the Greenland Ice Sheet

Abstract

Abstract. The mass of the Greenland Ice Sheet (GrIS) is decreasing due to increasing surface melt and ice dynamics. Snowfall both adds mass to the GrIS and has the capacity to reduce surface melt by increasing surface brightness, reflecting additional solar radiation back to space. Modeling the GrIS’s current and future mass balance and potential contribution to future sea level rise requires reliable observational benchmarks for current snowfall accumulation as well as robust connections between individual snowfall events and the large-scale atmospheric circulation patterns that produce them. Previous work using ground-based observations showed that, for one research station on the GrIS, two distinct snowfall regimes exist: those associated with exclusively ice-phase cloud processes (IC) and those involving mixed-phase processes indicated by the presence of supercooled liquid water (CLW). The two regimes have markedly different accumulation characteristics and dynamical drivers. This study leverages the synergy between two satellite instruments, CloudSat's Cloud Profiling Radar (CPR) and CALIPSO's Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), to identify snowfall cases over the full GrIS and partition them into the IC and CLW regimes. We find that, overall, most CPR observations of snowfall over the GrIS come from IC events (70 %); however, during the summer months, close to half of the snow observed is produced in CLW events (45 %). IC snowfall plays a dominant role in adding mass to the GrIS, producing ∼ 80 % of the total estimated 399 Gt yr−1 accumulation. Beyond the cloud phase that defines the snowfall regimes, the macrophysical cloud characteristics are distinct as well; the mean IC geometric cloud depth (∼ 4 km) is deeper than the CLW geometric cloud depth (∼ 2 km), consistent with previous studies based on surface observations. Two-dimensional histograms of the vertical distribution of CPR reflectivities show that IC events demonstrate consistently increasing reflectivity toward the surface while CLW events do not. Analysis of ERA5 reanalyses shows that IC events are associated with cyclone activity and CLW events generally occur under large-scale anomalously high geopotential heights over the GrIS. When combined with future climate predictions, this snapshot of GrIS snowfall characteristics may shed light on how this source of ice sheet mass might respond to changing synoptic patterns in a warming climate.

Controls on surface aerosol particle number concentrations and aerosol-limited cloud regimes over the central Greenland Ice Sheet

“…SBIs are most common in the winter (Oct-Mar) where they occur over 70 % of the time with a typical magnitude of ∼ 5 • C between 10 and 2 m a.s.l. (Miller et al, 2013). In the summer (JJA), the amplitude of SBIs is weaker, and they only occur ∼ 30 % of the time (Miller et al, 2013).…”

“…In mixed-phase clouds this process is more complex, since changes in the cloud droplet size distribution can have both positive and negative effects on the efficiency of ice production (Cheng et al, 2010;Lance et al, 2011;Possner et al, 2017). Cloud phase partitioning is also important since ice phase clouds have markedly different precipitation characteristics to those containing super-cooled liquid water (Pettersen et al, 2018;McIlhattan et al, 2020). Model simulations generally overestimate precipitation accumulation over the GrIS (McIlhattan et al, 2017;Kay et al, 2018;Lenaerts et al, 2020) and in particular the contribu- To date, all observations of the CCN-limited regime (Mauritsen et al, 2011;Leaitch et al, 2016), and INP concentrations (Wex et al, 2019), in the Arctic are located at marine or coastal sites.…”

Controls on surface aerosol number concentrations and aerosol-limited cloud regimes over the central Greenland Ice Sheet

“…In addition, the descent of aerosols to the surface from the upper troposphere, and the fact that precipitation in the accumulation zone of the GrIS occurs as ice phase year-round (e.g. Shupe et al, 2013b;Lenaerts et al, 2020;McIlhattan et al, 2020) and is less efficient at scavenging aerosol than liquid phase precipitation (Henning et al, 2004), suggests that aerosol losses to wet deposition during transport is reduced compared to sea level Arctic sites. Wet deposition is the main driver of the extremely low summer aerosol concentrations at sea level in the Arctic (Garrett et al, 2010;Browse et al, 2012), so the reduced wet deposition over the GrIS could explain the relatively high summertime aerosol concentrations.…”

“…Both Hirdman et al (2009) and Schmeisser et al (2018) conclude that because on average the majority of air arriving at Summit has only been in contact with the surface over the ice sheet itself, aerosols measured at Summit must have descended from the free troposphere after transportation at high altitudes over time scales > 20 days. Persistent anomalously high geopotential heights over central Greenland are also associated with the occurrence of precipitating low-level mixed-phase stratocumulous clouds (McIlhattan et al, 2020) that can encourage the transport of aerosols from the free tropospheric into the boundary layer (Igel et al, 2017).…”

“…In mixed-phase clouds this process is more complex, since changes in the cloud droplet size distribution can have both positive and negative effects on the efficiency of ice production (Cheng et al, 2010;Lance et al, 2011;Possner et al, 2017). Cloud phase partitioning is also important since ice phase clouds have markedly different precipitation characteristics to those containing super-cooled liquid water (Pettersen et al, 2018;McIlhattan et al, 2020). Model simulations generally overestimate precipitation accumulation over the GrIS (McIlhattan et al, 2017;Kay et al, 2018;Lenaerts et al, 2020), and in particular the contribution from mixed-phase clouds.…”