Substructure of a MAP streamer

A study is undertaken of the substructure of an upper‐level potential‐vorticity streamer that approached the European Alps during the field phase of the Mesoscale Alpine Programme (MAP). The diagnosis is based upon a Lagrangian Forward Projection (LFP) technique which can provide a spatial refinement of the operational European Centre for Medium‐Range Weather Forecasts analysis fields. The reconstructed fields capture several notable fine‐scale features of the streamer's structure that include spiral arms of a vortex‐like feature and a richly structured western flank to the streamer comprising a deep primary fold surmounted by a significant secondary fold with an intermediate distinctive striation that extends back into the stratosphere. These features are not at variance with independent high‐resolution satellite water‐vapour images and measurements gathered during a flight mission using an airborne water‐vapour DIfferential Absorption Lidar (DIAL) instrument, and hence lend a measure of credence to both the LFP approach and the DIAL measurements. Moreover, the analysis sheds light on the evolution and dynamics of the streamer's vortices, the origin of the folds, the mismatch between the satellite observations and fields from other sources, and also has implications for stratosphere–troposphere exchange and the accompanying chemical mixing.