The decay of the ALPEX low-level jet: Fine structure observation by aircraft data

Summary This case study describes the decay of a low-level jet in the alpine foreland as a planetary boundary layer phenomenon. Few measurements are known, which document this transition period of the boundary layer from night to day. Analysis of 1 and 20 Hz data of temperature and the three wind components of the ELECTRA aircraft mission on April 16, 1982 between 5 and 11 GMT allow an assessment of the temporal and spatial fine structure during the decay phase of the jet in the morning. Using the flight technique of horizontal and vertical zigzagging, the coupling of thermal stability, turbulence intensity, topography and behavior of the jet is shown.Vertical profiles and cross-sectional analysis of mean and turbulent parameters document three separate jet decay phases: decoupling, transition and erosion. During the first phase, the jet maximum is situated between surface and synoptic inversions, decoupling from surface friction and prohibiting momentum transport towards the free atmosphere. During the transition phase, the jet maximum increases in altitude. In the erosion phase, the jet covers the top of a developing well mixed layer. Turbulent mixing and entrainment on top of the layer are responsible for the decay of the jet. This is confirmed by calculating the decrease of the wind speed maximum from the turbulent momentum flux and the growth rate of the mixing layer by means of a mixed layer model in comparison to the measured wind speed jump and to other observations.With 7 Figures