The applicability of hydraulic theory to gap winds observed in the Wipp Valley
A well documented shallow foehn and gap flow case in the Wipp valley in Austrian Alps from 20 Oct 1999 provided an excellent opportunity to examine the applicability of the Reduced Gravity Shallow Water (RGSW) model to such a flow. For that purpose a synthesized data set was constructed from ground-based Doppler lidar and the P3 aircraft in situ measurements. The validity of the P3 data was crucial for further analysis so a detailed comparison between the radial wind component observed by the Doppler lidar and the same component of the horizontal winds measured in situ by the P3 was performed. It was found that the two data sets can be combined to give accurate composite analyses of the flow in the Wipp valley.Comparison of available radio soundings have shown a significant increase of potential temperature increase (approximately 3 K) inside the valley, in the lee of the Brenner pass, in a deep layer from the ground to above the elevated inversion. It was found that the warming cannot be explained by a pseudo-adiabatic process of condensation and fallout of precipitation, but can be attributed to the net subsidence over the main Alpine crest.Examining the along-gap wind and potential temperature fields, some qualitative similarities but also discrepancies between the observations and the RGSW model were found. By far the most significant discrepancy is the failure of the elevated inversion to behave like a material surface. The evidence found in the P3 data suggested that the strong inversion observed near the top of the gap flow in the valley was not transported over the Brenner pass, but was created in the lee by the combined effects of subsidence and turbulent mixing.Further evidence that the elevated inversion was not dominating the flow dynamics was obtained by comparing two semi-idealized numerical simulations. The first simulation, which included an elevated inversion in the initial conditions, resulted in a flow qualitatively similar to the observed. However, the second simulation, in which the elevated inversion was replaced by a uniform potential temperature profile, also produced a very similar flow pattern.
- Atmospheric sciences