Surface-pressure change through Loewe's phenomena and katabatic flow jumps : study of two cases in Adélie Land, Antarctica

Preliminary results from the 1985 IAGO (Interaction Atmosphere-Glace-Ocean) field programme in Adelie 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 super(-) super(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 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.