Observations from the German Front Experiment are presented here that show the existence—in conditions with a dominant flow component parallel to the main Alpine chain—of a mesoscale region to the north of the Alps where the absolute and potential vorticity (PV) are substantially negative. These structures exist before the front arrives to the Alps and appear to be affected little by the passage of the front. A dynamical explanation for these and other mesoscale structures is sought by considering a simple unsheared airflow impinging on the Alps from the west. A linear frictionless model for the steady-state response is used as well as a full nonlinear numerical model with and without friction. A vastly simplified Alpine orography is considered as well as one that adequately describes its mesoscale detail.
The results show that the frictionless linear dynamics lead to a zone north of the Alps with anticyclonic vorticity but with uniform (positive) potential vorticity. With boundary-layer processes included in a nonlinear simulation substantial PV anomalies are produced. This leads to negative PV, and absolute vorticity, north of the Alps and positive PV south of the Alps. The region of PV anomalies in the model bears a suggestive similarity to that in the observations. The PV structures are attributed to frictional processes acting in a boundary layer that acquires a slope due to the sloping mountain sides. This mechanism only operates in this situation.
Other mesoscale aspects of the flow are discussed in regions around the Alps for which we have as yet no detailed observational evidence; for example, there is strong flow retardation immediately downstream of the orography. An important conclusion is that the Alps, in conditions of parallel flow, are a significant source of potential vorticity anomalies in the lower troposphere. These are advected away from the orography and must be an important part of the tropospheric PV budget.