Brooke Medley

Antarctic Firn Compaction Rates from Repeat-Track Airborne Radar Data: I. Methods

Abstract While measurements of ice-sheet surface elevation change are increasingly used to assess mass change, the processes that control the elevation fluctuations not related to ice-flow dynamics (e.g. firn compaction and accumulation) remain difficult to measure. Here we use radar data from the Thwaites Glacier (West Antarctica) catchment to measure the rate of thickness change between horizons of constant age over different time intervals: 2009–10, 2010–11 and 2009–11. The average compaction rate to ~25 m depth is 0.33 m a−1, with largest compaction rates near the surface. Our measurements indicate that the accumulation rate controls much of the spatio-temporal variations in the compaction rate while the role of temperature is unclear due to a lack of measurements. Based on a semi-empirical, steady-state densification model, we find that surveying older firn horizons minimizes the potential bias resulting from the variable depth of the constant age horizon. Our results suggest that the spatio-temporal variations in the firn compaction rate are an important consideration when converting surface elevation change to ice mass change. Compaction rates varied by up to 0.12 m a−1 over distances <6 km and were on average >20% larger during the 2010–11 interval than during 2009–10.

Antarctic firn compaction rates from repeat-track airborne radar data : II. Firn model evaluation

Abstract The thickness and density of the Antarctic firn layer vary considerably in time and space, thereby contributing to ice-sheet volume and mass changes. Distinguishing between these mass and volume changes is important for ice-sheet mass-balance studies. Evolution of firn layer depth and density is often modeled, because direct measurements are scarce. Here we directly compare modeled firn compaction rates with observed rates obtained from repeat-track airborne radar data over a 2 year interval (2009–11) in West Antarctica. Spatially, the observed compaction rates exhibit significant variability, but when averaged to scales comparable to the model resolution (20–50 km), the measurements and model results qualitatively agree. A colder and drier period preceding the 2009 survey led to lower compaction rates during the 2009–10 interval, when compared to 2010–11, which is partly captured by the firn model. Spatially, higher compaction rates are observed and modeled in warmer regions with higher accumulation.

Climate and surface mass balance of coastal West Antarctica resolved by regional climate modelling

ABSTRACT West Antarctic climate and surface mass balance (SMB) records are sparse. To fill this gap, regional atmospheric climate modelling is useful, providing that such models are employed at sufficiently high horizontal resolution and coupled with a snow model. Here we present the results of a high-resolution (5.5 km) regional atmospheric climate model (RACMO2) simulation of coastal West Antarctica for the period 1979–2015. We evaluate the results with available in situ weather observations, remote-sensing estimates of surface melt, and SMB estimates derived from radar and firn cores. Moreover, results are compared with those from a lower-resolution version, to assess the added value of the resolution. The high-resolution model resolves small-scale climate variability invoked by topography, such as the relatively warm conditions over ice-shelf grounding zones, and local wind speed accelerations. Surface melt and SMB are well reproduced by RACMO2. This dataset will prove useful for picking ice core locations, converting elevation changes to mass changes, for driving ocean, ice-sheet and coupled models, and for attributing changes in the West Antarctic Ice Sheet and shelves to changes in atmospheric forcing.

Temperature and Snowfall in Western Queen Maud Land Increasing Faster Than Climate Model Projections

East Antarctic Ice Sheet (EAIS) mass balance is largely driven by snowfall. Recently, increased snowfall in Queen Maud Land led to years of EAIS mass gain. It is difficult to determine whether these years of enhanced snowfall are anomalous or part of a longer-term trend, reducing our ability to assess the mitigating impact of snowfall on sea level rise. We determine that the recent snowfall increases in western Queen Maud Land (QML) are part of a long-term trend (+5.2 ± 3.7% decade-1) and are unprecedented over the past two millennia. Warming between 1998 and 2016 is significant and rapid (+1.1 ± 0.7°C decade-1). Using these observations, we determine that the current accumulation and temperature increases in QML from an ensemble of global climate simulations are too low, which suggests that projections of the QML contribution to sea level rise are potentially overestimated with a reduced mitigating impact of enhanced snowfall in a warming world.

Development of a multilayer modis IST-albedo product of Greenland

A new multilayer IST-albedo Moderate Resolution Imaging Spectroradiometer (MODIS) product of Greenland was developed to meet the needs of the ice sheet modeling community. The multiple layers of the product enable the relationship between IST and albedo to be evaluated easily. Surface temperature is a fundamental input for dynamical ice sheet models because it is a component of the ice sheet radiation budget and mass balance. Albedo influences absorption of incoming solar radiation. The daily product will combine the existing standard MODIS Collection-6 ice-surface temperature, derived melt maps, snow albedo and water vapor products. The new product is available in a polar stereographic projection in NetCDF format. The product will ultimately extend from March 2000 through the end of 2017.