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Hubert Gallée

Abstract

In this study the mesoscale atmospheric circulation over the southwestern Ross Sea sector during winter is examined. The hydrostatic meso-γ-scale atmospheric model MAR (Modè1e Atmosphérique Régional) is used. Polar night is assumed, and an idealized large-scale situation is prescribed, with zero geostrophic forcing. The impact of a partial sea-ice cover on the atmospheric circulation is assessed by prescribing lead fractions in the range of the observed values (i.e., between 0% and 30%). Simulations show that the propagation of katabatic airstreams over Terra Nova Bay is facilitated by the presence of leads because the identity of cold, dense, katabatic air is better marked in warmer environmental maritime conditions. Boundary layer fronts and meso-cyclonic activity are associated with the katabatic airstreams. They are enhanced by the presence of leads. In particular, when the lead fraction is prescribed to be between 20% and 30%, the model simulates mesocyclone intensities comparable to those observed. Taking into account that such a lead fraction is situated in the upper range of the observed values in the central Ross Sea during winter, these results suggest that winter Ross Sea mesocyclones could not always result from a pure mesoscale forcing. In contrast to the summer situation, no snow precipitation occurs for the simulated winter case, probably because of the too-low absolute humidity content of the air. Such model behavior is in agreement with the observations, which reveal a summer precipitation maximum at McMurdo Station on Ross Island. It is also found that the position of the simulated mesocyclone over Terra Nova Bay is not sensitive to the lead fraction. Furthermore, this meteorological situation favors the advection of relatively mild and moist maritime air over a long distance in the ice-sheet interior. This process, which is referred to as a moist-air intrusion, could affect the Antarctic ice-sheet mass balance.

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Hubert Gallée

Abstract

The mesocyclonic activity in the southwestern Ross Sea is examined, with emphasis on its forcing by katabatic winds. The three-dimensional version of the meso-γ-scale primitive equation model Modèle Atmosphérique Régional is used in which a representation of cloud microphysical processes has been introduced. Idealized boundary conditions are prescribed. In particular, the ocean is assumed to be completely ice-free or partially ice-covered. The former case corresponds to a fall situation that coincides with the climatological maximum of estimated precipitation at McMurdo Station on Ross Island. Due to the propagation of katabatic winds over the ocean, boundary layer fronts form. Clouds are generated in the fronts. A surface pressure trough also forms and extends northeastward from Terra Nova Bay. Mesocyclones are simulated in the trough. When the ocean is ice-free, strong sensible heat fluxes toward the atmosphere amplify the strength of the trough up to typically observed values. In this case, snow precipitation is associated with the boundary layer fronts and occurs in particular over McMurdo Sound. It is also found that latent heat release due to precipitation formation in the fronts does not significantly affect the deepening of the trough.

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Hubert Gallée and Paul Pettré

Abstract

The katabatic wind events observed in the coastal zone of Adélie Land, Antarctica, on 27 November and 3 December 1985 are simulated with a hydrostatic mesoscale atmospheric model coupled to a snow model. The diurnal cycle of insolation is strong. The main difference in the forcing between the two events is the large-scale wind, which is weak on 27 November and moderate on 3 December. In both cases temperature and wind are characterized by well-marked diurnal cycles. In particular, katabatic winds blow during nighttime and upslope winds during daytime. In both cases the katabatic airstream slows down progressively over the ocean. Consequently, continental air piles up and this generates a pool of cold air responsible for a pressure gradient force opposing the katabatic wind. An amplification of the slowing down results. When, in the morning, insolation increases, the surface inversion weakens but the influence of the cold air pool increases. The katabatic flow starts to decay over the coastal zone and then retreats progressively toward the ice sheet interior. When the large-scale wind is weak, the surface warming is sufficient for generating an additional upslope bouyancy force, and anabatic flow develops over the ice sheet in the afternoon. When the large-scale wind is moderate and downslope, the pilling up of cold air is important and this has a dramatic impact on the flow. A sharp spatial transition is generated between downslope and upslope winds over the ocean. This discontinuity moves toward the ice sheet interior in the morning and is responsible for the sudden cessation of the katabatic flow seen by static observers. The results of the simulations are used in order to refine a simple parameterization of Antarctic katabatic winds.

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Hubert Gallée and Guy Schayes

Abstract

The spatial evolution of Antarctic katabatic winds in the area of Terra Nova Bay is examined using the three-dimensional version of the Université Catholique de Louvain-Modèle Atmosphérique Régional (UCL-MAR) mesoscale primitive equation models. The ability of the model to replicate classical linear mountain wave simulations is verified. Then, three-dimensional experiments are performed for the terrain configuration of Terra Nova (Ross Sea coastal zone) using different horizontal resolutions (5, 10, and 20 km). The model converges for resolutions lower than 10 km. Results are in qualitative agreement with available observations and previous modeling work. Strong katabatic winds are simulated with a jet over Terra Nova Bay. The model seems able to initiate the mesocyclonic activity in the Ross Sea due to the katabatic circulation.

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Koen De Ridder and Hubert Gallée

Abstract

Since the mid-1960s, the southern part of Israel has experienced major land use changes following the start of the irrigation scheme and the subsequent intensification of agricultural practice. Several studies, mainly based on the analysis of climatic time series, have shown that this has been followed by a significant change of the local climate, especially during the summer and early fall. They indicate a reduced diurnal amplitude of surface air temperature and wind speed, and a threefold increase of the October (early wet season) convective precipitation. In this paper, these phenomena are investigated by simulating the influence of the land surface on local meteorological variables with a two-dimensional version of a mesoscale atmospheric model containing a detailed land surface scheme. Particular attention is given to the correct estimation of land surface parameters from soil and vegetation maps and remote sensing data. The simulations confirm the observed reduction of the diurnal amplitude of temperature and wind speed when replacing a semiarid surface by a partly irrigated one. Furthermore, it is shown that the potential for moist convection increases with the surface moisture availability and is rather insensitive to the surface roughness.

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Hubert Gallée, Paul Pettré, and Guy Schayes

Abstract

The evolution of summer katabatic wind events over the steep slopes of Adélie Land is examined, with emphasis on the sudden cessation of these events. Different idealized large-scale forcings are considered, including a situation that comes very close to one observed during the IAGO (Interaction Atmosphère Glace Océan) campaign, held in the region in November–December 1985. The hydrostatic meso-γ-scale atmospheric model MAR (Modèle Atmosphérique Régional) is used to assess the sensitivity of the simulated cessation process to a prescribed large-scale forcing.

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Philippe Marbaix, Hubert Gallée, Olivier Brasseur, and Jean-Pascal van Ypersele

Abstract

In gridpoint regional climate models (RCMs), the lateral boundary conditions are usually provided by a procedure called relaxation. The technique was originally studied in the context of numerical weather forecasting. This paper complements the preceding theoretical studies in order to assess the practical choices of model relaxation coefficients. Several profiles of coefficients used in RCMs are then evaluated. The complexity of actual model numerics makes any definite choice of the coefficients out of reach of simple theoretical considerations, but these provide practical guidelines. The latter are confirmed by pragmatic considerations such as minimizing discontinuities and keeping relaxation rates in the range of the represented physical processes. The last part of the paper presents a sensitivity study with the Modèle Atmosphérique Régional (MAR).

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Alexandre Trouvilliez, Florence Naaim-Bouvet, Hervé Bellot, Christophe Genthon, and Hubert Gallée

Abstract

FlowCapt acoustic sensors, designed for measuring the aeolian transport of snow fluxes, are compared to the snow particle counter S7optical sensor, considered herein as the reference. They were compared in the French Alps at the Lac Blanc Pass, where a bench test for the aeolian transport of snow was set up. The two existing generations of FlowCapt are compared. Both seem to be good detectors for the aeolian transport of snow, especially for transport events with a flux above 1 g m−2 s−1. The second-generation FlowCapt is also compared in terms of quantification. The aeolian snow mass fluxes and snow quantity transported recorded by the second-generation FlowCapt are close to the integrative snow particle counter S7 fluxes for an event without precipitation, but they are underestimated when an event with precipitation is considered. When the winter season is considered, for integrative snow particle counter S7 fluxes above 20 g m−2 s−1, the second-generation FlowCapt fluxes are underestimated, regardless of precipitation. In conclusion, both generations of FlowCapt can be used as a drifting snow detector and the second generation can record an underestimation of the quantity of snow transported at one location: over the winter season, the quantity of snow transported recorded by the SPC is between 4 and 6 times greater than the quantity recorded by the second-generation FlowCapt.

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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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Étienne Vignon, Ghislain Picard, Claudio Durán-Alarcón, Simon P. Alexander, Hubert Gallée, and Alexis Berne

Abstract

The offshore extent of Antarctic katabatic winds exerts a strong control on the production of sea ice and the formation of polynyas. In this study, we make use of a combination of ground-based remotely sensed and meteorological measurements at Dumont d’Urville (DDU) station, satellite images, and simulations with the Weather Research and Forecasting Model to analyze a major katabatic wind event in Adélie Land. Once well developed over the slope of the ice sheet, the katabatic flow experiences an abrupt transition near the coastal edge consisting of a sharp increase in the boundary layer depth, a sudden decrease in wind speed, and a decrease in Froude number from 3.5 to 0.3. This so-called katabatic jump manifests as a turbulent “wall” of blowing snow in which updrafts exceed 5 m s−1. The wall reaches heights of 1000 m and its horizontal extent along the coast is more than 400 km. By destabilizing the boundary layer downstream, the jump favors the trapping of a gravity wave train—with a horizontal wavelength of 10.5 km—that develops in a few hours. The trapped gravity waves exert a drag that considerably slows down the low-level outflow. Moreover, atmospheric rotors form below the first wave crests. The wind speed record measured at DDU in 2017 (58.5 m s−1) is due to the vertical advection of momentum by a rotor. A statistical analysis of observations at DDU reveals that katabatic jumps and low-level trapped gravity waves occur frequently over coastal Adélie Land. It emphasizes the important role of such phenomena in the coastal Antarctic dynamics.

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