Blocking and Deflection of Airflow by the Alps

Abstract

A method of computing low-level trajectories from observed sea level pressure is shown to be capable of distinguishing cases of blocked and non-blocked flow around the Alps. A flow-splitting parameter (S), derived from the trajectories, is found to be a reasonable criterion for distinguishing blocked from nonblocked cases, with a value of S = 1.5 serving as a useful threshold for classification. In considering eight cases of postfrontal cold flow against the Alps, only a few can be clearly identified as blocking or noblocking, and even these identifications may not pertain to all levels and all segments of the Alpine chain. The trajectory-based classification scheme compares well with sounding and flight level data from the NOAA P-3 research aircraft. The degree of blocking is shown to be related to the upstream Froude number and the strength of the upstream pressure nose. A remarkable result is that the observed surface wind patterns appear blocked in all cases considered here. We refer to this as ?boundary layer blocking? Unsteady effects tend to stretch out streaklines east to west along the northern foothhills of the Alps.