Observing and Modeling Ice‐Sheet Surface Mass Balance

Surface mass balance (SMB) provides mass input to the surface of the Antarctic and Greenland Ice Sheets and therefore comprises an important control on ice sheet mass balance and resulting contribution to global sea level change. As ice sheet SMB varies highly across multiple scales of space (meters to hundreds of kilometers) and time (hourly to decadal), it is notoriously challenging to observe and represent in models. In addition, SMB consists of multiple components, all of which depend on complex interactions between the atmosphere and the snow/ice surface, large-scale atmospheric circulation and ocean conditions, and ice sheet topography. In this review, we present the state-of-the-art knowledge and recent advances in ice sheet SMB observations and models, highlight current shortcomings, and propose future directions. Novel observational methods allow mapping SMB across larger areas, longer time periods, and/or at very high (subdaily) temporal frequency. As a recent observational breakthrough, cosmic ray counters provide direct estimates of SMB, circumventing the need for accurate snow density observations upon which many other techniques rely. Regional atmospheric climate models have drastically improved their simulation of ice sheet SMB in the last decade, thanks to the inclusion or improved representation of essential processes (e.g., clouds, blowing snow, and snow albedo), and by enhancing horizontal resolution (5–30 km). Future modeling efforts are required in improving Earth system models to match regional atmospheric climate model performance in simulating ice sheet SMB, and in reinforcing the efforts in developing statistical and dynamic downscaling to represent smaller-scale SMB processes.Physical and Space Geodes

Representing Greenland ice sheet freshwater fluxes in climate models

Here we present a long-term (1850–2200) best estimate of Greenland ice sheet (GrIS) freshwater runoff that improves spatial detail of runoff locations and temporal resolution. Ice discharge is taken from observations since 2000 and assumed constant in time. Surface meltwater runoff is retrieved from regional climate model output for the recent past and parameterized for the future based on significant correlations between runoff and midtropospheric (500 hPa) summer temperature changes over the GrIS. The simplicity of this approach enables assimilation of meltwater runoff into coupled climate models, which is demonstrated here in a case study with the medium-resolution (1?) Community Earth System Model. The model results suggest that the decrease in Atlantic Meridional Overturning Circulation (AMOC) is dominated by warming of the surface ocean and enhanced GrIS freshwater forcing leads to a slightly enhanced (?1.2 sverdrup in the 21st century) weakening of the AMOC.Geoscience & Remote SensingCivil Engineering and Geoscience

Empirical estimation of present-day Antarctic glacial isostatic adjustment and ice mass change

This study explores an approach that simultaneously estimates Antarctic mass balance and glacial isostatic adjustment (GIA) through the combination of satellite gravity and altimetry data sets. The results improve upon previous efforts by incorporating a firn densification model to account for firn compaction and surface processes as well as reprocessed data sets over a slightly longer period of time. A range of different Gravity Recovery and Climate Experiment (GRACE) gravity models were evaluated and a new Ice, Cloud, and Land Elevation Satellite (ICESat) surface height trend map computed using an overlapping footprint approach. When the GIA models created from the combination approach were compared to in situ GPS ground station displacements, the vertical rates estimated showed consistently better agreement than recent conventional GIA models. The new empirically derived GIA rates suggest the presence of strong uplift in the Amundsen Sea sector in West Antarctica (WA) and the Philippi/Denman sectors, as well as subsidence in large parts of East Antarctica (EA). The total GIA-related mass change estimates for the entire Antarctic ice sheet ranged from 53 to 103 Gt yr?1, depending on the GRACE solution used, with an estimated uncertainty of ±40 Gt yr?1. Over the time frame February 2003– October 2009, the corresponding ice mass change showed an average value of ?100±44 Gt yr?1 (EA: 5±38, WA:?105±22), consistent with other recent estimates in the literature, with regional mass loss mostly concentrated in WA. The refined approach presented in this study shows the contribution that such data combinations can make towards improving estimates of present-day GIA and ice mass change, particularly with respect to determining more reliable uncertainties.Geoscience & Remote SensingCivil Engineering and Geoscience

Combining satellite altimetry and gravimetry data to improve Antarctic mass balance and gia estimates

This study explores an approach that simultaneously estimates Antarctic mass balance and glacial isostatic adjustment (GIA) through the combination of satellite gravity and altimetry data sets. The results improve upon previous efforts by incorporating reprocessed data sets over a longer period of time, and now include a firn densification model to convert the altimetry volume estimates into mass. When the GIA models created from the combination approach were compared to insitu GPS ground station displacements, the vertical rates estimated showed good agreement after a systematic bias was removed from the computed GIA models. The new models suggest the potential for GIA uplift in the Amundsen Sea Sector, as well as the possible subsidence in large parts of East Antarctica.Geoscience & Remote SensingCivil Engineering and Geoscience