The Applicability of Hydraulic Theory to Gap Winds Observed in the Wipp Valley

A well documented shallow foehn and gap flow case of 20 Oct 1999 (MAP IOP08) in the Wipp valley in the Austrian Alps was examined in this study. A synthesized data set was constructed for detailed analysis by combining ground-based Doppler lidar and the P3 aircraft in situ measurements, along with other available data.

The shallow foehn flow in the valley, in the lee of the Brenner pass, was capped by a relatively strong elevated temperature inversion (5 K across) and nominally exhibited a vertical structure favorable for the application of hydraulic theory. Upstream of the pass, the flow was partially blocked and a weaker inversion capped a much deeper layer. A significant potential temperature increase of approximately 3 K relative to the upstream conditions was observed within the Wipp valley in a well mixed layer extending from the ground up to the elevated inversion. (We note that this warming cannot be explained by a pseudo-adiabatic process of condensation and fallout of precipitation, but is likely a result of the net subsidence in the lee of the main Alpine crest.)

In spite of the upstream flow blocking and strong subsidence, the potential temperature profile in the Wipp valley preserves a mixed-layer structure near the surface capped by a strong elevated inversion. One possible mechanism for maintaining the thermodynamic structure in the Wipp valley is through a combination of significant net subsidence and shear-induced turbulent mixing. The mixing maintains the neutrally stratified boundary layer and produces a strong capping inversion.

In support of this hypothesis, we found evidence of relatively strong turbulent mixing throughout the Wipp-valley boundary layer. The variance of the vertical velocity (calculated from 1 Hz in situ P3 aircraft measurements), with maximum values exceeding 5 m^2 s^(-2), was used as a proxy for the TKE. Furthermore, the data also showed that air parcels sampled on the P3 flight tracks through the elevated inversion were descending through the inversion layer at an average vertical speed w = -0.57 m/s. A quantitative estimate of mass transport along the valley under the inversion, showed an increase in mass transport toward the valley exit. Since the contribution from tributary valleys was negligible, this increase in mass flux can only be balanced by downward mass transport through the inversion layer.

Thus, several lines of evidence suggest the strong inversion observed near the top of the gap flow in the Wipp valley was not transported over the Brenner pass as a material surface, but was continually created in the lee through a process of subsidence and turbulent mixing. Despite superficial similarities, on 20 Oct 1999, there was no fundamental dynamical equivalence between the elevated inversion in the Wipp valley and the free surface in hydraulic theory.