Upper‐tropospheric flow features and the Alps: An overview

One focus of the Mesoscale Alpine Programme (MAP) was the study of upper‐tropospheric potential vorticity (PV) anomalies that take the form of narrow meridionally elongated troughs termed ‘PV streamers’. A systematic effort was undertaken within the MAP framework to: establish a streamer climatology, develop appropriate instrumentation and undertake a measurement programme to better ascertain their structure, study both their intrinsic dynamics and their modification by the Alps, and perform model experiments to examine their significance for numerical weather prediction. Here an overview is given of the progress made toward achieving these objectives. In particular it is shown that PV streamers translating toward the Alpine region are dynamically distinctive and constitute an identifiable precursor of and contribute seminally to heavy precipitation events on the south side of the Alps and possess rich mesoscale sub‐structures that can be examined with novel water vapour absorption lidar instrumentation. It is also shown that the accurate representation of a streamer might well be a critical prerequisite for accurate quantitative prediction of mesoscale precipitation.