Ice sheet climate in the Community Earth System Model

terra novaOne of the primary targets of the Community Earth System Model (CESM) towards the Sixth Assessment Report of the International Panel on Climate Change (IPCC) is to represent the mutual interaction between the Greenland Ice Sheet and the remainder the climate system. This will be included in CESM2, to be available in 2017, which makes this model ideally suitable to study ice sheet - climate interactions, and to estimate future sea level contribution of ice sheet mass loss. This not only required substantial software development, including allowing for solid ice discarge into the ocean and varying land type, but also requires a realistic ice sheet climate: the balance between incoming mass, in the form of precipitation, and surface melting and runoff at the edges determines how the ice sheet responds to the climate, and is a direct forcing to the ice sheet model. Our group uses state-of-the-art observational datasets of atmospheric and surface conditions from remote sensing (snow, clouds, precipitation), atmospheric reanalyses, and high-resolution regional climate models to evaluate and improve CESM's climate over Greenland and Antarctica.

We use CESM to study sea ice - ice sheet interactions over Antarctica, assess the impact of ice sheet mass loss to oceanic circulations, and to estimate regional sea level rise. And much more!

 


High-resolution modelling of Antarctic ice sheet SMB

In this work, we aim to map the Antarctic Ice Sheet (AIS) surface mass balance (SMB) at an unprecedentedly high resolution (1 km), using a unique combination of elevation data derived from radar and laser altimetry (CryoSat-2 and ICESat-2), atmospheric reanalyses, and IceBridge radar data. A remaining challenge, however, is in partitioning these elevation changes into ice-dynamical and surface-induced processes, complicating the inversion from elevation (volume) to mass changes, the latter of which ultimately determine the ice sheet’s contribution to sea lBlowing snow saltation and sunset, East Antarctica, December 2014evel change. The surface mass balance, one of the major components of surface-induced elevation change, can be mapped in space and time using atmospheric reanalyses, which are models that assimilate observational data. State-of-the-art atmospheric reanalyses, currently working at grid resolutions of approximately 50 to 100 km, provide a realistic picture of large-scale Antarctic SMB. At this scale, SMB is predominantly driven by precipitation, the spatial patterns of which are mostly determined by large-scale orography that is well resolved by climate models. In contrast, unresolved fine-scale topography causes wind-driven redistribution and sublimation of snow that significantly affects SMB at length scales smaller than the model grid size (so-called sub-grid drifting snow processes), but larger than the small-scale and highly mobile drifts and sastrugi that are not considered here. These subtle topographic variations of a few meters can span hundreds of meters to a few kilometers and can lead to SMB variations of a factor of six. We propose to map Antarctic ice sheet surface topography at sub-grid resolution (~1 km) by combining ICESat-2 laser and CryoSat-2 radar altimetry data. In conjunction with this new elevation model, surface winds and climate simulated by NASA’s MERRA-2 reanalysis will provide the input for a snow redistribution model (SnowModel) that simulates SMB at the elevation model scale. The results will be evaluated and refined using Operation Ice Bridge snow radar derived SMB and ICESat-2 drifting snow frequency. 

 


Antarctic ice shelf hydrology

Surface lakes along the grounding line of Roi Baudouin ice shelf, East AntarcticaIce shelves are the gatekeepers of Antarctica: they control how much ice flows off the ice sheet and melts into the ocean. If ice shelves become unstable, they can break apart and invoke major ice loss from upstream glaciers. Previous research has shown that an efficient process to destabilize ice shelves is so-called hydrofracturing: the ponding of surface meltwater on the ice shelf surface deepens and widens crevasses, eventually up to the ice shelf bottom, and breaking up the ice shelf. That has happened on the warmest part of Antarctica, the Antarctic Pensinula, already in 1995 and 2002. We are wondering how sensitive other Antarctic ice shelves are to hydrofracturing, and how that might change in a future climate. During two field seasons we have collected weather and snow data to understand the climate of an ice shelf in East Antarctica in detail. These field data are complemented and put into a larger perspective by remote sensing and climate model analyses. Understand present and future hydrofractruing potential of Antarctic ice shelves will shed light on the magnitude and dynamics of the anticipated contribution of Antarctica to future sea level rise.