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Paul Pettre

Abstract

observational results of a one-month mesoscale experiment in a valley are used to emphasize the prominent part played by an inversion layer in air flow dynamics. A model based on the analogy between shallow water flow and air flow beneath an inversion is applied. Using the actual topography of the valley, theoretical arguments for the appearance of violent winds are presented and the model results are compared with the in-situ measurements. The qualitatively good agreement for the, wind speed and the height of the inversion layer allows for further confidence in the model predictions. For example, the existence of a sharp transition zone, or jump, with a strong horizontal gradient of wind speed and temperature present in the model is consistent with aircraft measurements.

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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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Paul Pettré
and
Jean-Claude André

Abstract

Preliminary results from the 1985 IAGO (Interaction Atmosphére-Glace-Océan) field programme in Adélie Land, Antarctica, allow for a detailed description of several katabatic events, especially of their vertical structure and time and space variations. Several Loewe's phenomena, where a sudden transition from shooting to tranquil flow takes place, have been observed. Two of them have been well documented and are shown here. For both cases, as observed at a downstream station close to the coast, the mean wind speed suddenly decreased from about 20 m s−1 to almost zero, while a large pressure increase was recorded (5.7 hPa on 3 December 1985 and 2.1 hPa on 18 December 1985). This paper aims at explaining such large surface-pressure changes, as more usual approaches cannot. For both cases it is found that the flow is stratified upwards with a surface well-mixed cold air layer, a very stable capping inversion layer, an overlying unstable layer thickening from upstream to downstream and a stable transition layer to the free atmosphere. A new approach is proposed, based on the application of the Boussinesq form of Bernoulli's theorem. This study supports the idea that the pressure change through the Loewe's phenomena is mainly connected with the strong acceleration of the downslope katabatic layer under the effect of gravity, rather than with the change in depth of the cold air layer. This new approach allows to fully explain the observed data and the surface-pressure change through the jump.

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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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