USE OF OBLIQUELY ROTATED PRINCIPAL COMPONENT ANALYSIS TO IDENTIFY COHERENT STRUCTURES

Principal component analysis (PCA) with oblique relation is applied to Large Eddy Simulation (LES) results to discern and quantify coherent structures within the convective boundary layer (CBL). Sensitivity tes ts are first conducted on a moderately convective LES run. Once the ab ility of PCA to generate robust results is verified, the method is app lied to LES runs spanning a range of stability regimes. Interregime si milarities and differences in the coherent structures are discussed. F or the moderately convective LES run, three-dimensional convective cel ls are arrayed in two-dimensional bands aligned with the geostrophic w ind. The resulting gravity waves in the free atmosphere and convective inflow and outflow in the boundary layer are also captured by the PCA . Convective modes are more sensitive to the ratio of w to u* than ar e the dynamic modes. PCA has demonstrated advantages over previous ana lysis methods. PCA score maps provide information on the spatial distr ibution of phenomena that has not been available from traditional cond itional sampling studies. Principal components provide information on the vertical structures of phenomena that would be obscured by life-cy cle effects or erratic tilts from the vertical in the conventional app roaches to either conditional sampling or composite analysis. Future w ork includes application of this technique to multi-level observationa l time series from a surface-layer tower for the Riser Air/Sea Experim ent (RASEX).