On the application of a numerical model to simulate the coastal boundary layer – Niels Bohr Institute - University of Copenhagen

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On the application of a numerical model to simulate the coastal boundary layer

Jesper Nielsen Nissen, PhD defence, November 13th 2pm
This work aims to study the seasonal difference in normalized wind speed above the surface layer as it is observed at the 160 m high mast at the coastal site Høvsøre at winds from the sea (westerly).

Normalized wind speeds above the surface layer are observed to be 20 to 50 % larger in the winter/spring seasons compared to the summer/autumn seasons at winds from west within the same atmospheric stability class.

A method combining the mesoscale model, COAMPS, and observations of the surface stability of the marine boundary layer is presented. The objective of the method is to reconstruct the seasonal signal in wind speed and identify the physical process behind. The method proved reasonably successful in capturing the relative difference in wind speed between seasons, indicating that the simulated physical processes are likely candidates to the observed seasonal signal in normalized wind speed.

The lower part of the seasonal normalized wind speed profiles were also captured reasonably well. However did the method consistently over-predict the absolute values of the normalized wind speeds at the upper part of the profile and suggestions to improve the skills of the method in this region are discussed.

The winds from west at Høvsøre also showed an increased in upper level variance of the wind speed during spring and winter when compared to summer and autumn.

It is shown, that excess in temperature over England relative to the North Sea is a player because it triggers wind speed oscillations in the boundary layer over the North Sea. The oscillations were found to introduce up to 20 % departure in the simulated normalized wind speed at 100 m height, compared to simulations where no upstream land was accounted for or situations where the upstream land was colder than the North Sea.

The signal was found to be stronger during spring and winter as compared to summer and autumn and serves as indicator for the more complex nature of boundary layer processes during winter and spring compared to summer and autumn.