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VERITAS

Verification of the turbulence parameterization and the description of the vertical structure of the marine atmospheric boundary layer in mesoscale numerical simulation models for wind analysis and forecasts (e.g., WRF) on the basis of FINO1 data.
The project started on July 1, 2008 and was finished on November 30, 2011. The project work was mainly done by Richard Foreman. All results of the project are available from the PhD thesis of Richard Foreman .

This project is part of the wind energy meteorology  efforts of the institute which is closely related to the efforts made to analyse and describe the vertical structure of the atmospheric boundary layer over different surfaces (cities, rural areas, complex terrain, water) as a prerequisite for trace gas and aerosol emission, transformation and dispersion studies.

Highlights from this project:

Drag coefficient in the maine boundary layer

A new formulation for the dependence of the friction velocity and the drag coefficient in the marine boundary layer as function of wind speed. The formulation gives a linear dependence of friction velocity on wind speed for the fully turbulent regime (above about 8 m/s wind speed):

u* = a u10 - b

with the two universal constants a = 0.051 and b = 0.14 m/s. This formulation leads to a drag coefficient which approaches a constant maximum value at high wind speeds. The limit value of the drag coefficient is equal to a2 = 0.0026. Details are given in: Foreman, R., S. Emeis, 2010: Revisiting the Definition of the Drag Coefficient in the Marine Atmospheric Boundary Layer. J. Phys. Oceanogr., 40, 2325-2332.

 

Updating the "constants" in the Mellor-Yamada-Janjic type of turbulence parameterization

 

Fokussing on the constant B1 which relates to the turbulent kinetic energy near the surface, a new set of closure constants has been derived using new laboratory data and data from the offshore platform FINO1. The new set of closure constants leads to a much more ralistic description of turbulent kinetic energy (and thus turbulence intensity) in the lower half of the atmospheric boundary layer (i.e., higher turbulence intensity is simulated there). Details are given in:  Foreman, R., S. Emeis, 2012a: Method for increasing Turbulent Kinetic Energy in the Mellor-Yamada-Janjić boundary layer parametrization. Bound.-Lay. Meteorol .

 

 

Drag coefficient in the marine boundary layer as function of wave steepness

 

Following a strict dimensional analysis, the drag coefficient must primarily be a function of the steepness of the waves. Data analysis shows that the drag coefficient scales with the square of the wave steepness. Details are given in: Foreman, R., S. Emeis, 2012b: Correlation equation for the marine drag cofficient and wave steepness. Accepted 22 July 2012 for publication in Ocean Dynamics.

 

 

 

Contact: Stefan Emeis

 

Acknowledgements

 

 

This work has been funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety via the PTJ under grant number FKZ 0325060Simultaneously the project has been work package 5 of the RAVE project OWEA