Boundary Layer, Urban Meteorology and Land-Surface Processes

Conditional analyses of the very stable boundary layer

Cedrick Ansorge

Max Planck Institute for Meteorology, Hamburg

26 August 2015, 4:00 PM

National Weather Center, Room 1313
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

An open boundary layer such as the planetary boundary layer and also its simplified counter-
part of turbulent Ekman flow is characterized by external intermittency. This means, turbulent
mixing may be absent in a significant volume fraction of the turbulent boundary layer's outer
region; the interface between the flow's turbulent and non-turbulent partitions may be seen as
the local fringe of the boundary layer, and it is commonly defined in terms of vorticity. When
exposed to strong stratification, this external intermittency may become global in the sense that
it extends down to the surface at some locations while the flow as a whole is still turbulent.

External and global intermittency challenge the conventional statistics approach when de-
scribing boundary-layer turbulence. We extend here the vorticity-based conditioning of a flow to
turbulent and non-turbulent sub-volumes by a high-pass filter operation. This modified method
consistently works in the outer and inner layers for stratifications ranging from the neutral limit
to extreme stability where the flow is close to a complete laminarization. Based on this con-
ditioning method, we find: First, external intermittency has a strong effect on the logarithmic
law for the mean velocity in Ekman flow under neutral stratification; if instead of the full field,
only turbulent sub-volumes are considered, the logarithmic layer extends over a three-times
broader range of heights. Second, order-of-one changes in turbulent quantities under strong
stratification can be explained by a modulation of the turbulent volume fraction rather than a
structural change of individual turbulence events; within the turbulent sub-volumes, the charac-
ter of individual turbulence events measured in terms of turbulence dissipation rate or averaged
fluctuation velocities is similar to that under neutral stratification.

This improved understanding of dynamics in the stably stratified boundary layers opens
new avenues to the parameterization of turbulent fluxes under strongly stratified conditions and
potentially constitutes a step towards an overarching modelling framework for stable boundary

26 August 2015 *** Note non-standard date/time for this particular BUL Seminar***

Boundary Layer, Urban Meteorology and Land-Surface Processes Seminar Series website