The Coriolis effect on coherent structures in planetary boundary layers

Abstract

Coherent structures are often visible in atmospheric boundary layers as convective clouds and irregular fog. Large eddy simulations (LES) provide data to study the coherent structures by means of multivariate methods of statistical analysis. One of such methods is a proper orthogonal decomposition (POD). A POD can isolate most energetic three-dimensional structures in turbulent boundary layers. Coherent structures in planetary boundary layers (PBLs) were found to be inherently different from those in laboratory boundary layers without background rotation. This study attributes the differences to an interaction between the vorticity of coherent structures and the constant background vorticity of the planet rotation. LES of neutrally stratified PBLs clearly show the Coriolis force effect. The Coriolis effect supports vortices with vorticity parallel to the planetary vorticity. Simultaneously, the Coriolis effect destroys vortices with vorticity counter-parallel to the planetary vorticity. Convective PBLs show an interplay of vorticities of shear- and buoyancy-induced coherent structures. The Coriolis effect is less important in convective PBLs in comparison with neutral PBLs. The horizontal and vertical components of the Coriolis force act in essentially different ways. The vertical component mainly damps the coherent structures. It also turns the structures relative to the geostrophic wind. The horizontal component mainly makes the structures asymmetrical.