Mass exchange in the stable boundary layer by coherent structures

Observations of multi-dimensional water vapor structures in the first 75 m of the stable boundary layer (SBL) were made using a high resolution scanning Raman lidar in October 2000 during the Vertical Transport and Mixing Experiment (VTMX). Lidar images reveal the intermittent presence of plumes and low-frequency structures contributing to much of the nocturnal surface–atmosphere mass exchange. Furthermore, periodic wave–turbulence interactions between solitary waves aloft and coherent structures at the surface were observed. Results show that when low-level waveguides are present in the SBL, downward transport by pressure fluctuations arising from Kelvin–Helmholz instabilities hundreds of meters above the surface appears to pump energy near the surface thereby supporting the development of coherent structures. These structures have a vertical extent determined by the depth of the low-level waveguide inversion. The present results suggest that, in certain nocturnal conditions, coherent structures can transport more than a third of the mass exchanged between the surface and the lowest region of the stable atmospheric boundary layer.