EMPIRICAL ORTHOGONAL FUNCTION-ANALYSIS OF THE WEAKLY CONVECTIVE ATMOSPHERIC BOUNDARY-LAYER .1. EDDY STRUCTURES

Three-dimensional empirical orthogonal functions (EOFs), representing atmospheric turbulence structures, are determined from a large-eddy si mulation of a weakly convective, planetary boundary layer. The method of analysis is based on Lumley's proper orthogonal decomposition (POD) but has been extended to include temperature as well as velocity stru cture. The horizontal domain of the simulation is a square with side l ength equal to 30 times the inversion height in order to allow for the formation of multiple large-scale structures. The geostrophic wind is 22.5 m s(-1) and the surface temperature flux is 0.03 K m s(-1). Amon g the structures revealed by the analysis are longitudinal roll vortic es, apparently in both inflection-point and thermal modes, and gravity waves in the capping temperature inversion. Modes associated with wav e breaking and entrainment at the inversion may also be present. Spati ally compact characteristic structures (thermal plumes) are constructe d from the EOFs using a maximum-spatial-compactness method, and the re sulting structures are compared with the simpler spatial compositing ( conditional sampling) method. The two methods are found to give very s imilar results in this case, hence leading to the conclusion that the 3D EOF analysis is best suited to studying quasiperiodic structures su ch as rolls and gravity waves, whereas compositing is best suited to s tudying spatially compact structures such as the plumes.