C. H. Reijmer

Large and rapid melt-induced velocity changes in the ablation zone of the Greenland Ice Sheet.

Continuous Global Positioning System observations reveal rapid and large ice velocity fluctuations in the western ablation zone of the Greenland Ice Sheet. Within days, ice velocity reacts to increased meltwater production and increases by a factor of 4. Such a response is much stronger and much faster than previously reported. Over a longer period of 17 years, annual ice velocities have decreased slightly, which suggests that the englacial hydraulic system adjusts constantly to the variable meltwater input, which results in a more or less constant ice flux over the years. The positive-feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades.

Assessing and Improving the Quality of Unattended Radiation Observations in Antarctica

Abstract The quality of atmospheric radiation measurements made at automatic weather stations (AWSs) in Antarctica is assessed. The AWSs are placed on the coastal ice shelf in the katabatic wind zone and on the high Antarctic plateau, and they measure shortwave and longwave radiation fluxes using unheated/unventilated Kipp and Zonen (KZ) CM3/CG3 sensors. During three summertime Antarctic experiments, the AWS sensors were directly compared to instruments of a higher standard, the KZ CM11 for shortwave and Eppley PIR for longwave radiation. It was found that the single-domed KZ CM3 is less sensitive to riming than the double-domed KZ CM11. With an accuracy better than 5% for daily averages, the KZ CM3 and CG3 perform better than their specifications. Net shortwave radiation calculated from individual pairs of incoming and reflected fluxes shows large relative errors, and a method is presented to remedy this. Summertime longwave fluxes measured with the KZ CG3 show very good agreement with ventilated Eppley ...

Surface mass-balance observations and automatic weather station data along a transect near Kangerlussuaq, West Greenland

Abstract Surface mass-balance data from the Kangerlussuaq transect (K-transect) located on the western part of the Greenland ice sheet near 67° N are presented. The series covers the period 1990-2003 and is the longest series of surface mass-balance measurements in Greenland. The surface mass-balance measurements cover an altitude range of 390-1850 m and show a linear increase of the specific mass balance, with a mass-balance gradient of 3.7 × 10–3 m m–1 and a mean equilibrium-line altitude of 1535 ma.s.l. Interannual variability shows a weak 4 yearly periodicity. In addition to the surface mass-balance data, automatic weather station data at an elevation of approximately 1010m are available for the period 1997-2002. These data are used to explain observed surface mass-balance anomalies over the same 5 years. It is shown that variations in shortwave radiation dominate interannual variability. The mean annual cycle of temperature is characterized by a maximum in summer around the melting point, leading to a mean summer outgoing longwave radiation of approximately 314 W–2. The mean annual cycle in wind speed shows a maximum in winter (on average around 8 m s–1) and a minimum in summer (on average around around 5 m s–1), which is characteristic for a katabatic forcing. During summer the net radiation is on average about 61 Wm–2, which is used for ice melting at a rate of typically 2 cm w.e.d-1. Net radiation contributes 84% of the total energy used for summer melting averaged over the 5 years.

Air Parcel Trajectories and Snowfall Related to Five Deep Drilling Locations in Antarctica Based on the ERA-15 Dataset*

Abstract Five-day backward air parcel trajectories are used to define potential moisture sources of snow falling at five Antarctic deep drilling locations: Byrd, DML05, Dome C, Dome F, and Vostok. The trajectory calculations are based on European Centre for Medium-Range Weather Forecasts reanalysis data, ERA-15 (1979–93). Based on model precipitation, a distinction is made between cases with and without snowfall at the point of arrival. ERA-15 precipitation is in reasonable agreement with measured accumulation at Byrd, but seriously underestimates the amount of precipitation on the East Antarctic ice sheet. The trajectories show that the oceans closest to the site contribute the most moisture. The latitude band contributing most (∼30% of the total annual precipitation) is 50°–60°S, that is, the area just north of the sea-ice edge. The calculated trajectories show seasonal dependency, resulting in a seasonal cycle in the moisture sources, which is further enhanced by a seasonal cycle in the amount of preci...

Characteristics of the Antarctic surface mass balance, 1958-2002, using a regional atmospheric climate model

Abstract Temporal and spatial characteristics of the Antarctic specific surface mass balance (SSMB) are presented, including its components solid precipitation, sublimation/deposition and melt. For this purpose, we use the output of a regional atmospheric climate model (RACMO2/ANT, horizontal resolution of ~55 km) for the period 1958–2002. RACMO2/ANT uses European Centre for Medium-Range Weather Forecasts (ECMWF) 40 year re-analysis (ERA-40) fields as forcing at the lateral boundaries. RACMO2/ANT underestimates SSMB in the high interior of East and West Antarctica and overestimates SSMB on the steep coastal slopes. Otherwise, the modeled spatial pattern of SSMB is in good qualitative agreement with recent compilations of in situ observations. Large-scale patterns, like the precipitation shadow effect of the Antarctic Peninsula, are well reproduced, and mesoscale SSMB patterns, such as the strong precipitation gradients on Law Dome, are well represented in the model. The integrated SSMB over the grounded ice sheet is 153mmw.e. a–1 for the period 1958–2002, which agrees within 5% with the latest measurement compilations. Sublimation and melt remove 7% and <1% respectively of the solid precipitation. We found significant seasonality of solid precipitation, with a maximum in autumn and a minimum in summer. No meaningful trend was identified for the SSMB, because the time series of solid precipitation and SSMB are affected by an inhomogeneity in 1980 within the ERA-40 fields that drive RACMO2/ANT. Sublimation, melt and liquid precipitation increase in time, which is related to a modeled increase in 2m temperature.

Identification of Antarctic ablation areas using a regional atmospheric climate model

The occurrence of Antarctic ablation areas in Dronning Maud Land, the Lambert Glacier Basin, Victoria Land, the Transantarctic Mountains and the Antarctic Peninsula is realistically predicted by the regional atmospheric climate model RACMO2/ANT, with snowdrift-related processes calculated offline. Antarctic ablation areas are characterized by a low solid precipitation flux in combination with strong sublimation, snowdrift erosion and/or melt. The strong interaction between atmospheric circulation and topography plays a decisive role in the precipitation distribution and hence that of ablation areas. Three types of Antarctic ablation areas can be distinguished, all occurring in dry regions: Type 1 is the erosion-driven ablation area, caused by 1-D and/or 2-D divergence in the katabatic wind field at high elevations (2000–3200 m asl). Type 2 is the sublimation-driven ablation area. This type occurs at lower elevations (

Internal accumulation on Storglaciären, Sweden, in a multi-layer snow model coupled to a distributed energy- and mass balance model

To investigate the internal accumulation on Storglaciaren, Sweden, we couple a multilayer snow model to a distributed energy- and mass-balance model. The snow model describes the temperature, density and water-content evolution of the snow/ice pack and includes the processes of percolation and refreezing of water. The model is run for the period 9 May-2 September 1999 and validated against weather station and mass-balance observations on the glacier. The model performs reasonably well, with an average summer mass balance for the location of stake observations of −1.56 m w.e. compared to −1.59 m w.e. observed. However, the amount of melt is overestimated in the higher parts of the accumulation area and underestimated in the lower parts of the ablation area. The modelled mass balance is most sensitive to the albedo parameterization, the chosen momentum and scalar roughness lengths and all parameters related to snowfall. The modelled internal accumulation is +0.25 m w.e., which amounts to about 20% of the winter accumulation and results in a positive net balance for 1999 of +0.23 m w.e. The modelled internal accumulation is most sensitive to the initial subsurface temperature profile and the irreducible water content.

Narrowband to broadband conversion of Landsat TM glacier albedos

In this paper we present an empirical relationship between the broadband glacier albedo (alpha) and the narrowband glacier albedos in Landsat TM bands 2 and 4 (alpha2 and alpha4, respectively). The relationship was established on the basis of multiple linear regression analysis of 112 ground-based simultaneous measurements of alpha, alpha2 and alpha4 made at 32 sites on the tongue of the Morteratschgletscher, Switzerland. The measurements were carried out over a representative set of glacier surface types ranging from completely debris-covered glacier ice (alpha=0.08)to dry snow (alpha=0.86). The regression model explains more than 99% of the variance of the broadband albedo and the root-mean-square value of the residuals is only 0.009. The relationship enables users of Landsat TM data to make an accurate estimate of the broadband albedo on the basis of narrowband albedos without having to classify the glacier surface.

Moisture source of precipitation in Western Dronning Maud Land, Antarctica

Moisture sources for snow falling in Dronning Maud Land (DML), Antarctica, are calculated for 1998 using three dimensional 5-days backward air parcel trajectories. The drilling site of the European Project on Ice Coring in Antarctica (EPICA) in DML is chosen as the main arrival point (75.0°S, 0.01°E). A distinction is made between trajectories with and without snowfall at arrival. Of the snowfall trajectories, 40–80% are located in the Atlantic Ocean within four days before arrival. Evaporation along these trajectories is largest three to four days before arrival. The air parcels are then located between 40° and 60°S in the Atlantic Ocean where surface temperatures range between 0° and 20°C. A case study for May 1998 shows that when snow falls exceptionally high temperatures and wind speeds prevail in the atmospheric boundary layer. The position of the trajectories in the boundary layer suggests a source region for this event between 40° and 50°S and 20° and 60°W in the Atlantic Ocean, where sea surface temperatures vary between 5° and 15°C.

Seasonal cycles of Antarctic surface energy balance from automatic weather stations

Abstract We present the seasonal cycle of the Antarctic surface energy balance (SEB) using 4 years (1998–2001) of automatic weather station (AWS) data. The four AWSs are situated on an ice shelf, in the coastal and inland katabatic wind zone and the interior plateau of Dronning Maud Land. To calculate surface temperature we use a SEB closure assumption for a surface skin layer. Modelled and observed surface have a root-mean-square difference of 1.8 K at the plateau AWS (corresponding to an uncertainty in the SEB of 5Wm–2) and <1 K (3Wm–2) at the other sites. The effect of wind-speed sensor freezing on the calculated SEB is discussed. At all sites the annual mean net radiation is negative and the near-surface air is on average stably stratified. Differences in the seasonal cycle of the SEB are mainly caused by the different wind climates at the AWS sites. In the katabatic wind zone, a combination of clear skies and strong winds forces a large wintertime turbulent transport of sensible heat towards the surface, which in turn enhances the longwave radiative heat loss. On the coastal ice shelf and on the plateau, strong winds are associated with overcast conditions, limiting the radiative heat loss and sensible heat exchange. During the short Antarctic summer, the net radiation becomes slightly positive at all sites. Away from the cold interior, the main compensating heat loss at the surface is sublimation. In the interior, summer temperatures are too low to allow significant sublimation to occur; here, surface heat loss is mainly due to convection.