Publications

1987

• The pressure drag and momentum flux during a cold frontal passage over the Alps
  
  • Carissimo Bertrand
  , 1987. The strong pressure drag, the momentum flux and the air flow changes occurring during the interaction of a cold front with a meso-scale mountain are investigated using a combination of observations from the Alpine Experiment and theoretical models. The surface pressure observations are analysed every three hours to derive the pressure drag and reveal that peak values are reached when tight pressure gradients are located on steep orography. These values would be underestimated by conventional synoptic analyses or coarse resolution numerical models. The momentum flux above the topography is estimated from aircraft measurements taken during the strong drag events. Compared with the surface pressure drag, the observed momentum fluxes are small. They are in line however with the values computed in this situation by a "wave drag" parametrization. The accuracy of these fluxes is also discussed. In order to understand these observations, the dynamics of the interaction between a cold front and the mountain is further investigated. Additional observations show that the pressure fall and ri se accompanying the front in flat terrain is insufficient to fully account for the pressure drop across the mountain and for the drag magnitudes observed. A dynamical explanation for the large observed drag during frontal passage is therefore sought using a simple layer model. Although this model is somewhat deficient in simulating the prefrontal stage in the interaction, it is capable of reproducing the observed frontal retardation and the drag evolution. This model also shows that a substantial increase in the post-frontal drag can be created by the blocking of cold air if it is strong and remains localized on the upstream slope of the mountain. An additional series of numerical experiments with a continuously stratified model is performed to compare the air flow over the mountain with and without the presence of a front and the evolution of the front with and without the mountain. The vertical shear in the wind across the front, by its ability to create a secondary frontal circulation interacting with the mountain, is shown to be capable of producing an increase in the prefrontal drag.