Boundary Layer, Urban Meteorology and Land-Surface Processes

STUDY OF STABLY STRATIFIED BOUNDARY LAYERS USING
TURBULENT EKMAN FLOW

Cedrick Ansorge
Max Planck Institute for Meteorology
Hamburg, Germany

21 November 2014, 3:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

We study turbulence in a simplified configuration of the stably stratified planetary boundary layer using direct numerical simulation. As a simplified physical model, we use Ekman flow under stable stratification; this problem can be studied in terms of two parameters, a Reynolds number measuring shear effect and a Richardson number measuring the relative importance of stratification effects with respect to shear. The Reynolds number is varied in the range 500 < Re = δ+ < 1500 where δ+ = δu*/ν is the boundary layer thickness δ expressed in wall units ν/u*. We vary the stratification, expressed in terms of a bulk Richardson number from very weak stability, where turbulence acts as a passive scalar, to very strong stability, where the flow relaminarizes partly. When the aspect ratio is sufficiently large,i.e. at a horizontal extent of about 20δ × 20δ, large-scale modes are present in the flow. These large-scale modes govern the spatio-temporal structure of external and global intermittency in the flow.

Backed by our DNS data, we present work on an analytical theory towards a drag law for Ekman flow which is out of equilibrium. Vertical integration of momentum equations shows that drag is directly related to inertial oscillations present in Ekman flow. Our results suggest a modification of conceptual models of inertial oscillations consisting in a modification of the damping term and a frequency shift.

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