Michiel van Den Broeke

We present 4 years of radiation observations and derived cloud properties from the ablation zone of the west Greenland ice sheet (67°N). Data were collected using three automatic weather stations located at 6, 38, and 88 km from the ice sheet margin at elevations of 490, 1020, and 1520 m asl. This part of Greenland is characterized by a ∼150 km wide tundra, a ∼100 km wide ablation zone and an average equilibrium line altitude of ∼1500 m asl. At the lowest station, snow is redistributed by the wind in crevasses and gullies, leading to very little measured winter accumulation. As a result, glacier ice (albedo ≈ 0.55) is at the surface throughout the melting season. At 1020 m asl, the winter snow cover typically disappears in mid to end June. At 1520 m asl, superimposed ice briefly reaches the surface at the end of the ablation season. The combined effect of decreasing surface albedo and increasing cloud optical thickness as summer progresses causes absorbed shortwave radiation to peak in June at low elevations but progressively later at higher elevations. Incoming longwave radiation peaks in August, in response to increased cloud optical thickness and heating of the atmosphere over the snow and ice‐free surroundings. The ice sheet margin experiences continuous melting in June, July, and August. This limits the emission of longwave radiation, causing net longwave radiation to peak in August, further enhancing melt. The sum of these effects is that summer net radiation increases sharply toward the ice sheet margin. To resolve this correctly requires high‐resolution climate models, in the order of 10 km or better.

The depth and density of the Antarctic firn layer is modeled, using a combination of regional climate model output and a steady-state firn densification model. The modeled near-surface climate (temperature, wind speed, and accumulation) and the depth of two critical density levels (550 kg m−3 and 830 kg m−3) agree well with climate and firn density observations selected from >50 Antarctic coring sites (r = 0.90–0.99, p < 0.0001). The wide range of near-surface climate conditions in Antarctica forces a strong spatial variability in the depth and density of the Antarctic firn pack. In the calm, dry, and very cold interior, densification is slow and the firn-layer thickness exceeds 100 m and the firn age at pore close-off 2000 years. In the windier, wetter, and milder coastal zone, densification is more rapid and the firn layer shallower, typically 40–60 m, and younger, typically <50 years.

We present the summertime daily cycle of the Antarctic surface energy balance (SEB) and its sensitivity to cloud cover. We use data of automatic weather stations (AWS) located in four major Antarctic climate zones: the coastal ice shelf, the coastal and interior katabatic wind zone and the interior plateau. Absorbed short wave radiation drives the daily cycle of the SEB, in spite of the high surface albedo (0.84–0.88). The dominant heat sink is the cooling by long wave radiation, but this flux is distributed more evenly throughout the day so that a pronounced daily cycle in net all‐wave radiation remains with all‐sky night‐time heat losses of 20–30 W m⁻² and noontime heat gains of 30–40 W m⁻². During the night, heat is re‐supplied to the snow surface by the sensible heat flux, especially in the katabatic wind zone, and the sub‐surface heat flux. Daytime radiative energy excess is removed from the surface by sublimation (except at the high plateau) and sub‐surface heat transport. Daytime convection occurs at all sites around solar noon but is generally weak. Spatial differences in the SEB are largely controlled by differences in cloud cover. Clouds are associated with higher surface temperatures and near‐surface wind speeds. This especially limits nocturnal cooling, so that the strongest daytime convection is found during overcast conditions on the interior plateau. Copyright © 2006 Royal Meteorological Society.

A new Antarctic accumulation distribution, based on regional model output calibrated with 1900 in‐situ observations, is used to re‐assess accumulation in 24 Antarctic ice drainage basins. When compared to the previous compilation, good agreement is found for 19 of the 24 basins, representing 93% of the ice sheet that is reasonably well covered with observations. In contrast, the Amundsen Sea sector of West Antarctica and the western Antarctic Peninsula, both data sparse regions, are found to receive 80–96% more accumulation than previously assumed. For the Pine Island and Thwaites Glaciers (West Antarctica), which have recently undergone rapid acceleration and thinning, this means a downward adjustment of their contribution to global sea level rise from 0.24 to 0.14 mm per year. Model time series do not show a significant change in Antarctic accumulation over the period 1980–2004.

During the austral summer of 2001/02, melting at the surface of Larsen Ice Shelf in the Antarctic Peninsula was three times greater than the average of five previous summers. This exceptional melt event lasted for three months and was followed by the collapse of Larsen B Ice Shelf, during which 3,200 km² of ice shelf surface was lost. The strong melting was caused by a persistent atmospheric circulation anomaly, which depleted sea ice concentrations in front of Larsen Ice Shelf and transported warm air to the ice shelf throughout the 2001/02 summer. This supports the theory that large meltwater fluxes accelerate the retreat of Antarctic Peninsula ice shelves.

Data of four automatic weather stations (AWSs) are used to calculate the turbulent exchange of sensible heat at the Antarctic snow surface for a 4 year period (1998–2001). The AWSs are situated on the ice shelf, in the coastal/inland katabatic wind zone and on the interior plateau in Dronning Maud Land, East Antarctica. Sensible heat flux (SHF) is calculated using the aerodynamic ‘bulk’ method between a single AWS sensor level and the surface, in combination with surface temperature derived from upwelling longwave radiation and surface roughness derived from eddy correlation measurements. Good agreement is found between calculated and directly measured SHF. All AWS sites show a downward‐directed average sensible heat transport, but otherwise the differences between the various zones are large. The surface roughness for momentum differs by an order of magnitude between the interior plateau (0.02 mm) and the katabatic wind zone (0.16 mm). On the ice shelf, frequent clouds limit surface cooling, and annual mean SHF is small (8 W m⁻²). In contrast, clear skies prevail on the interior plateau, but weak winds, an aerodynamically smooth surface and stability effects limit annual mean SHF to an equally low value (8 W m⁻²). The most favourable conditions for sensible heat exchange are found in the katabatic wind zone, where a combination of strong winds, relatively little cloud cover and a rougher surface results in annual mean SHF values of 22 to 24 W m⁻². Copyright © 2005 Royal Meteorological Society

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 –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.

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 ... 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 ...

We present 4 years of near‐surface radiation balance observations of four Antarctic automatic weather stations (AWS). The AWS are situated along a traverse line in Dronning Maud Land, connecting the coastal ice shelf and the inland plateau via the katabatic wind zone, covering the three major climate regimes of East Antarctica. Important differences in the radiation balance of the three regions are found. Clouds not only limit atmospheric transmissivity for shortwave radiation but also strongly enhance the albedo for the shortwave radiation that reaches the surface. As a result, the snow surface of the coastal ice shelves absorbs up to 65% less shortwave radiation in high summer than at the high plateau, where cloudy episodes and precipitation events are less frequent. In winter, over the slopes, katabatic winds maintain a continuous turbulent transport of sensible heat toward the surface, which enhances outgoing longwave radiation. As a result, the katabatic wind zone shows the largest longwave and all‐wave radiation loss in winter and over the year. Clear‐sky effective emissivity for incoming longwave radiation shows great spatial variability resulting from differences in vertical temperature and moisture profiles among the various climate zones.

Using data from the regional atmospheric climate model RACMO/ANT1 the momentum budget of the Antarctic atmospheric surface layer (SL; taken as the lowest model layer located at 6–7 m above the surface) is presented. In July (winter), the katabatic pressure gradient force (PGF) dominates the SL momentum budget over the steep coastal slopes, which results in strong (10–15 m s−1) and directionally constant katabatic winds. Farther inland, where surface slopes are more gentle, the large-scale PGF may become equally important. With the circumpolar pressure trough north of the Antarctic coastline, the large-scale PGF acts along with the katabatic PGF in the downslope direction. In Wilkes Land, Dronning Maud Land, and on the western Ross Ice Shelf, the large-scale PGF causes equivalent geostrophic winds in excess of 10 m s−1. Thermal wind effects that oppose the downslope acceleration are especially strong in areas where large-scale forcing is weak, which allows cold air to pile up over the flat ice she... Abstract Using data from the regional atmospheric climate model RACMO/ANT1 the momentum budget of the Antarctic atmospheric surface layer (SL; taken as the lowest model layer located at 6–7 m above the surface) is presented. In July (winter), the katabatic pressure gradient force (PGF) dominates the SL momentum budget over the steep coastal slopes, which results in strong (10–15 m s−1) and directionally constant katabatic winds. Farther inland, where surface slopes are more gentle, the large-scale PGF may become equally important. With the circumpolar pressure trough north of the Antarctic coastline, the large-scale PGF acts along with the katabatic PGF in the downslope direction. In Wilkes Land, Dronning Maud Land, and on the western Ross Ice Shelf, the large-scale PGF causes equivalent geostrophic winds in excess of 10 m s−1. Thermal wind effects that oppose the downslope acceleration are especially strong in areas where large-scale forcing is weak, which allows cold air to pile up over the flat ice she...