Blocking, descent and gravity waves: Observations and modelling of a MAP northerly föhn event

Abstract

AbstractA northerly föhn event observed during the special observational period of the Mesoscale Alpine Programme is investigated based on observational analysis and numerical modelling. The focus of this study includes three dynamical processes associated with mountain perturbations and their interactions, namely, windward flow blocking, descent and warming on the lee side, and mountain waves. Observations indicate the presence of a deep weak‐flow layer underneath a stable layer, associated with Alpine‐scale blocking. Satellite imagery reveals a föhninduced cloud‐free area to the south of the Alps, which is consistent with flow descent diagnosed from radiosondes and constant‐volume balloons. Moderate‐amplitude stationary waves were observed by research aircraft over the major Alpine peaks. Satellite images and balloon data indicate the presence of stationary trapped‐wave patterns located to the north of the Alpine massif.Satisfactory agreement is found between observations and a real‐data COAMPS simulation nested to 1 km resolution. COAMPS indicates the presence of trapped waves associated with a sharp decrease of Scorer parameter above a stable layer in the mid‐troposphere. Underneath the stable layer, moist low‐level flow is blocked to the north of the Alps. The warm air in the stable layer descends in the lee and recovers its altitude over a relatively short horizontal distance through a hydraulic jump.Blocking reduces the effective mountain and hence significantly reduces mountain drag. A simple empirical formula for estimation of the effective mountain height, he, is derived based on numerical simulations. The formula states he/hc=√h/hc, where h is the real mountain height and hc is the critical mountain height to have flow stagnation. Copyright © 2005 Royal Meteorological Society.