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Michiel R. van den Broeke

Abstract

In the summer of 1994, meteorological measurements were performed on Pasterze Glacier in the eastern Alps. One of the most remarkable observations concerning the observed climate was the persistent glacier wind. On the relatively large glacier, which has a length of 9.6 km, the gravity wind at the lower parts of the glacier is well developed and has a thickness of about 100 m. To determine the mechanisms that cause the steady-state glacier wind, the author calculated its vertically integrated budgets of momentum, heat, and moisture at a spot on the lower glacier tongue. It was found that the sum of interfacial and surface friction account for only half of the momentum dissipation of the glacier wind; the rest of the katabatic force is most probably balanced by the mesoscale pressure gradient that drives the valley wind above the katabatic layer. The heat and moisture budgets of the boundary layer show simple two-term balances: heat is lost through the turbulent flux of sensible heat at the surface (on average 50 W m−2) and gained by the downward transport of air. Moisture is added to the katabatic layer by entrainment and lost through condensation at the surface, but on average the moisture fluxes are small. The entrainment velocity, averaged over the length of the glacier, was estimated to be 2.4 cm s−1; it reduces to 0.7 cm s−1 at the glacier tongue, which was estimated by closing the local moisture budget.

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Michiel R. van den Broeke

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Determining the rate of atmospheric warming in Antarctica is hampered by the brevity of the temperature records (<50 years), which still contain signals of decadal circulation variability in the Southern Hemisphere. In this note it is demonstrated that Antarctic warming trends have been regionally modified by slow circulation changes and associated changes in sea-ice cover: decadal weakening of the semiannual oscillation since the mid-1970s has limited the meridional heat exchange between Antarctica and its surroundings, so that warming trends have leveled out since then. In contrast, northerly circulation anomalies in combination with decreased sea-ice cover have regionally enhanced low-level warming, for instance in the region of the Antarctic Peninsula. Based on this knowledge, the authors propose a background Antarctic warming trend of 1.30 ± 0.38°C (century)−1, representative of the period 1957–95.

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Richard Bintanja and Michiel R. Van Den Broeke

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Little is known about the surface energy balance of Antartic blue-ice areas although there have been some studies of the surface energy balance of snow surfaces. Therefore, a detailed meteorological experiment was carded out in the vicinity of a blue-ice area in the Heimefrontfjella, Dronning Maud Land, Antarctica, during the austral summer of 1992/93. Since not all the surface fluxes could be measured directly, the use of a model was necessary. The main purpose of the model is to calculate the surface and subsurface temperatures from which the emitted longwave radiation and the turbulent fluxes can be calculated. The surface energy balance was evaluated at four locations: one on blue ice, and three on snow. Differences are due mainly to the fact that ice has a lower albedo (0.56) than snow (0.80). To compensate for the larger solar absorption of ice, upward fluxes of longwave radiation and turbulent fluxes are larger over ice. Moreover, the energy flux into the ice is larger than into snow due to the differences in the radiative and conductive properties. Surface temperatures, snow subsurface temperatures, and ice sublimation rates evaluated with the model compare well with the measurements, which yields confidence in the surface energy balance results. The latent heat flux is particularly important since the spatial variability of the sublimation rate largely influences the extent of a blue-ice area. This study helps to explain the heat exchange processes over Antarctic surfaces.

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Michiel van den Broeke, Dirk van As, Carleen Reijmer, and Roderik van de Wal

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 PIR measurements [root-mean-square difference (rmsd) about 1%], but a larger systematic difference is found when comparison is made with unventilated Eppley PIR measurements. Upward extrapolation of snow temperatures suggest that the unventilated Eppley PIR measurements have a systematic offset, but additional measurements are necessary to confirm this. Wintertime riming of the unventilated/unheated KZ CG3 sensor window leads to rejection of 25%–28% of the LW↓ data for the AWS on the ice shelf and the plateau. Replacing these data with parameterized values removes the systematic offset but introduces an uncertainty of 10%–15%.

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Michiel R. van den Broeke, Roderik S. W. van de Wal, and Martin Wild

Abstract

A high-resolution GCM (ECHAM-3 T106, resolution 1.1° × 1.1°) is found to simulate many characteristic features of the Antarctic climate. The position and depth of the circumpolar storm belt, the semiannual cycle of the midlatitude westerlies, and the temperature and wind field over the higher parts of the ice sheet are well simulated. However, the strength of the westerlies is overestimated, the annual latitudinal shift of the storm belt is suppressed, and the wintertime temperature and wind speed in the coastal areas are underestimated. These errors are caused by the imperfect simulation of the position of the subtropical ridge, the prescribed sea ice characteristics, and the smoothened model topography in the coastal regions. Ice shelves in the model are erroneously treated as sea ice, which leads to a serious overestimation of the wintertime surface temperature in these areas. In spite of these deficiencies, the model results show much improvement over earlier simulations. In a climate run, the model was forced to a new equilibrium state under enhanced greenhouse conditions (IPCC scenario A, doubled CO2), which enables us to cast a preliminary look at the climate sensitivity of Antarctic katabatic winds. Summertime katabatic winds show a decrease of up to 15% in the lower parts of the ice sheet, as a result of the destruction of the surface inversion by increased absorption of solar radiation (temperature–albedo feedback). On the other hand, wintertime near-surface winds increase by up to 10% owing to a deepening of the circumpolar trough. As a result, the model predicts that the annual mean wind speed remains within 10% of its present value in a doubled CO2 climate, but with an increased amplitude of the annual cycle.

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Hubert Gallée, Olivier Fontaine de Ghélin, and Michiel R. van den Broeke

Abstract

A meso-γ-scale atmospheric model has been used to simulate atmospheric circulations observed during the Greenland Ice Margin EXperiment (GIMEX). The simulations shown here are two-dimensional and cover the 12–13 July 1991 period, a typical summer situation in this area. The synoptic-scale wind forcing is included. The tundra topography is assumed to be either flat, or averaged over a 50-km-wide cross section centered on the GIMEX transect. Simulated wind, temperature, humidity, and turbulent fluxes compare reasonably well with available observations. The simulated heat used to melt snow or ice is also shown. The sensitivity of the model results to the synoptic-scale wind forcing is significant. The impact of a tundra much warmer than the ocean on the ice sheet melting is discussed. It is found that weak easterly synoptic-scale winds are able to overwhelm this impact, especially when the tundra is assumed to be flat.

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Miren Vizcaíno, William H. Lipscomb, William J. Sacks, and Michiel van den Broeke

Abstract

This study presents the first twenty-first-century projections of surface mass balance (SMB) changes for the Greenland Ice Sheet (GIS) with the Community Earth System Model (CESM), which includes a new ice sheet component. For glaciated surfaces, CESM includes a sophisticated calculation of energy fluxes, surface albedo, and snowpack hydrology (melt, percolation, refreezing, etc.). To efficiently resolve the high SMB gradients at the ice sheet margins and provide surface forcing at the scale needed by ice sheet models, the SMB is calculated at multiple elevations and interpolated to a finer 5-km ice sheet grid. During a twenty-first-century simulation driven by representative concentration pathway 8.5 (RCP8.5) forcing, the SMB decreases from 372 ± 100 Gt yr−1 in 1980–99 to −78 ± 143 Gt yr−1 in 2080–99. The 2080–99 near-surface temperatures over the GIS increase by 4.7 K (annual mean) with respect to 1980–99, only 1.3 times the global increase (+3.7 K). Snowfall increases by 18%, while surface melt doubles. The ablation area increases from 9% of the GIS in 1980–99 to 28% in 2080–99. Over the ablation areas, summer downward longwave radiation and turbulent fluxes increase, while incoming shortwave radiation decreases owing to increased cloud cover. The reduction in GIS-averaged July albedo from 0.78 in 1980–99 to 0.75 in 2080–99 increases the absorbed solar radiation in this month by 12%. Summer warming is strongest in the north and east of Greenland owing to reduced sea ice cover. In the ablation area, summer temperature increases are smaller due to frequent periods of surface melt.

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Miren Vizcaíno, William H. Lipscomb, William J. Sacks, Jan H. van Angelen, Bert Wouters, and Michiel R. van den Broeke

Abstract

The modeling of the surface mass balance (SMB) of the Greenland Ice Sheet (GIS) requires high-resolution models in order to capture the observed large gradients in the steep marginal areas. Until now, global climate models have not been considered suitable to model ice sheet SMB owing to model biases and insufficient resolution. This study analyzes the GIS SMB simulated for the period 1850–2005 by the Community Earth System Model (CESM), which includes a new ice sheet component with multiple elevation classes for SMB calculations. The model is evaluated against observational data and output from the regional model Regional Atmospheric Climate Model version 2 (RACMO2). Because of a lack of major climate biases, a sophisticated calculation of snow processes (including surface albedo evolution) and an adequate downscaling technique, CESM is able to realistically simulate GIS surface climate and SMB. CESM SMB agrees reasonably well with in situ data from 475 locations (r = 0.80) and output from RACMO2 (r = 0.79). The simulated mean SMB for 1960–2005 is 359 ± 120 Gt yr−1 in the range of estimates from regional climate models. The simulated seasonal mass variability is comparable with mass observations from the Gravity Recovery and Climate Experiment (GRACE), with synchronous annual maximum (May) and minimum (August–September) and similar amplitudes of the seasonal cycle. CESM is able to simulate the bands of precipitation maxima along the southeast and northwest margins, but absolute precipitation rates are underestimated along the southeastern margin and overestimated in the high interior. The model correctly simulates the major ablation areas. Total refreezing represents 35% of the available liquid water (the sum of rain and melt).

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Jan T. M. Lenaerts, Michiel R. van den Broeke, Jan M. van Wessem, Willem Jan van de Berg, Erik van Meijgaard, Lambertus H. van Ulft, and Marius Schaefer
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Jan Melchior van Wessem, Carleen H. Reijmer, Willem Jan van de Berg, Michiel R. van den Broeke, Alison J. Cook, Lambertus H. van Ulft, and Erik van Meijgaard

Abstract

The latest polar version of the Regional Atmospheric Climate Model (RACMO2.3) has been applied to the Antarctic Peninsula (AP). In this study, the authors present results of a climate run at 5.5 km for the period 1979–2013, in which RACMO2.3 is forced by ERA-Interim atmospheric and ocean surface fields, using an updated AP surface topography. The model results are evaluated with near-surface temperature and wind measurements from 12 manned and automatic weather stations and vertical profiles from balloon soundings made at three stations. The seasonal cycle of near-surface temperature and wind is simulated well, with most biases still related to the limited model resolution. High-resolution climate maps of temperature and wind showing that the AP climate exhibits large spatial variability are discussed. Over the steep and high mountains of the northern AP, large west-to-east climate gradients exist, while over the gentle southern AP mountains the near-surface climate is dominated by katabatic winds. Over the flat ice shelves, where katabatic wind forcing is weak, interannual variability in temperature is largest. Finally, decadal trends of temperature and wind are presented, and it is shown that recently there has been distinct warming over the northwestern AP and cooling over the rest of the AP, related to changes in sea ice cover.

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