Principal Component Analysis of Vertical Profiles of Q1 and Q2 in the Tropics

Abstract

Rotated Principal component analysis (PCA) is applied to the combined vertical profiles of apparent heat source Q1 and apparent moisture sink Q2 from both disturbed and undisturbed periods of the Australian summer monsoon season. The data represent the heating and drying within two radiosonde arrays afforded by the Australian Monsoon Experiment (AMEX), The aim here is to identify dominant modes of variability in combined vertical profiles of Q1 and Q2. Rotation of the principal components (PCs)-done to assure stable, physically meaningful components-yields several PCs, deemed here to be statistically significant. The variation of individual Q1 and Q2 profiles from the mean profile can be expressed as linear combinations of the PCs; therefore, determination of the relative importance of each PC (through examination of its score) during differing convective conditions provides insight into their physical meaning. For instance, the contribution of PC 1 (that mode of variability that explains the maximum amount of variance between the profiles) is largest when mature cloud-cluster coverage is most expansive. Therefore, this PC is attributable to that combination of deep convection and associated stratiform anvil typical of mature cloud clusters. The remaining PCs WI into two categories: those whose contributions vary with the evolution of a convective system and those whose contributions vary diurnally. Principal components of the former group represent the effects of convection from shallow cumulus to stratiform anvil precipitation. Principal components of the latter group, those that show heating and drying patterns confined to the extremities of the troposphere, are attributable to diabatic boundary-layer fluxes and radiative processes at the top of the troposphere.