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A three-dimensional dynamic downscaling model of the Greenland atmospheric boundary layer is usedto investigate the surface energy balance of the Greenland ice sheet for the summer ablation season of1998. The model is forced by ECMWF analysis data and has a horizontal resolution of 20 km. Theresults show that the turbulent heat fluxes contribute significantly (32%) to the total surface energy fluxin the ablation region. A climate sensitivity experiment is carried out with the model by increasing thefree atmospheric forcing temperature by 1 K. The resulting change, known as climate sensitivity, in2 m temperature, 2 m wind speed and the surface energy balance components are discussed. The 2 mtemperature is shown to have a climate sensitivity significantly less than unity, as low as 0.3, in regionswhere melt prevails for most of the ablation season. The surface energy flux in the ablation region isfound to increase by 21% for a 1 K increase in atmospheric temperature. Wind speeds also increase, particularly near the margin of the ice sheet, as a result of enhanced katabatic forcing. Enhanced windspeeds are shown to account for roughly 12% of the climate sensitivity of the sensible heat flux whenaveraged over the ablation region. Albedo feedback is also shown to play an important role in the totalclimate sensitivity of the surface energy balance.

Pure katabatic flow is studied with a Prandtl-type model allowing eddy diffusivity/conductivity to varywith height. Recently we obtained an asymptotic solution to the katabatic flow assuming the validity ofthe WKB method, which solves the fourth-order governing equation coupling the momentum and heattransfer. The WKB approximation requires that eddy diffusivity may vary only gradually compared tothe calculated quantities, i.e., potential temperature and wind speed. This means that the scale heightfor eddy diffusivity must be higher than that for the calculated potential temperature and wind speed. The ratio between the maximum versus the mean eddy diffusivity should be less than that for thescale heights of the diffusivity versus the calculated quantities (temperature and wind). Here we justify(a posteriori) the WKB method independently based on two arguments: (i) a scaling argument and (ii )a philosophy behind a higher-order closure turbulence modeling. Both the eddy diffusivity maximumand the level of the relevant maximum turbulent kinetic energy are above the strongest part of the nearsurfaceinversion and the low-level jet which is required for the WKB validity. Thus, the numericalmodeling perspective lends credibility to the simple WKB modeling. This justification is importantbefore other data sets are analyzed and a parameterization scheme written.

A three-dimensional dynamic downscaling model of the Greenland atmospheric boundary layer, witha horizontal resolution of 20 km, is descibed and applied to the Greenland ice sheet for the 1998ablation season. The model uses ECMWF analysis data fields of synoptic pressure, free atmospherictemperature, cloud cover, humidity and sea surface temperature to force the model. The model calculatesthe perturbation component of the temperature and pressure field to describe the atmospheric boundarylayer dynamics. The aim of this study, the first of two papers, is to investigate the role of the turbulentheat fluxes in the surface energy balance of the ice sheet and their response to changes in atmospherictemperature. In this first paper, results from the simulation are compared with observations fromsix automatic weather stations situated on the ice sheet, three in the ablation zone and three in theaccumulation zone. The comparison shows that the boundary layer model can reproduce the nearsurfacemeteorological variables of wind, temperature and specific humidity quite well and improvesignificantly on 2 m values taken directly from the ECMWF analysis. The increased spatial resolutionof the model is essential in order to model accurately katabatically forced winds near the margin ofthe ice sheet. The calculated and observed melts at two sites in the ablation zone are also compared.At one site close to the margin, which is situated in a well drained ice region, the comparison withobservations is very good, within 1%. At a higher site, where subsurface processes not included in themodel are important for the total ablation, the calculated melt is 35% larger than the observed ablation.

This study focuses on the differences between the present-day climate and the climate of the last glacial maximum (LGM) of 18 000 y BP using a zonally averaged energy balance climate model. The ocean is represented by a 2-D model with prescribed overturning pattern in which the overturning velocities can be adjusted freely. We discuss what influence the use of ice-age conditions (i.e. enhanced land-ice cover, reduced CO2-concentration and reduced oceanic overturning rate) has on the differences between ice-age and present-day climate. When compared to LGM sea-surface temperatures derived from proxy data, the model is able to simulate fairly well the important features of the meridional distribution of these temperature differences. Applying reduced ocean overturning rates during the LGM significantly decreases poleward heat transport in the oceans, thereby allowing for additional cooling of the polar regions and less cooling of the equatorial region. As a result, the agreement with CLIMAP proxy temperature differences increases, especially in the equatorial region. This mechanism can explain the slight differences in the CLIMAP proxy equatorial surface temperatures between the LGM and the present-day climate.

Possible future changes in land ice volume are mentioned frequently as an important aspect of the greenhouse problem. This paper deals with the response of the Antarctic ice sheet and presents a tentative projection of changes in global sea level for the next few hundred years, due to changes in its surface mass balance. We imposed a temperature scenario, in which surface air temperature rises to 4.2° C in the year 2100 AD and is kept constant afterwards. As GCM studies seem to indicate a higher temperature increase in polar latitudes, the response to a more extreme scenario (warming doubled) has also been investigated. The mass balance model, driven by these temperature perturbations, consists of two parts: the accumulation rate is derived from present observed values and is consequently perturbed in proportion to the saturated vapour pressure at the temperature above the inversion layer. The ablation model is based on the degree-day method. It accounts for the daily temperature cycle, uses a different degree-day factor for snow and ice melting and treats refreezing of melt water in a simple way. According to this mass balance model, the amount of accumulation over the entire ice sheet is presently 24.06 × 1011 m3 of ice, and no runoff takes place. A 1°C uniform warming is then calculated to increase the overall mass balance by an amount of 1.43 × 1011 m3 of ice, corresponding to a lowering of global sea level with 0.36 mm/yr. A temperature increase of 5.3°C is needed for the increase in ablation to become more important than the increase in accumulation and the temperature would have to rise by as much as 11.4°C to produce a zero surface mass balance. Imposing the Bellagio-scenario and accumulating changes in mass balance forward in time (static response) would then lower global sea level by 9 cm by 2100 AD. In a subsequent run with a high-resolution 3-D thermomechanic model of the ice sheet, it turns out that the dynamic response of the ice sheet (as compared to the direct effect of the changes in surface mass balance) becomes significant after 100 years or so. Ice-discharge across the grounding-line increases, and eventually leads to grounding-line retreat. This is particularly evident in the extreme case scenario and is important along the Antarctic Peninsula and the overdeepened outlet glaciers along the East Antarctic coast. Grounding-line retreat in the Ross and Ronne-Filchner ice shelves, on the other hand, is small or absent.

Summary In this paper the wind-driven flow of water through the Straits of Dover is studied with a numerical model. The main goal is to get some insight into the effect of the specific configuration of the Straits of Dover region on the wind-driven flow from the Channel into the North Sea. For 18 wind directions the steady-state solutions and response times are presented. The results of a 21-day model run are compared with those of computing the transport with the steady-state solutions. Except for periods with a very rapidly changing wind field, the steady-state approximation appears to be good.

During the Climate Conference 2001, which was held in August 2001 at Utrecht University, the Netherlands, a workshop was devoted to katabatic flows over glaciers and ice sheets.In this issue of Tellus a number ofpapers presented at this workshop have been brought together. In recent years the mass balance of glaciersand ice sheets and its sensitivity to climate change has received considerable attention. The exchange of mass, momentum and sensible heat between the glacier surface and atmosphere is determined by turbulence in thelower atmospheric layers. Extensive field experiments have shown that persistent katabatic flow dominates thedynamics of the lower atmosphere over ice sheets and glaciers. Even over such exposed glaciers like Vatnajökull, Iceland, katabatic flow prevails most of the time.