Eigenstructure of eddy microfronts

Abstract

Aircraft measurements of strong turbulence in a bora flow and in weak intermittent turbulence near the top of a nocturnal surface inversion layer are examined. An important feature of both types of turbulence are narrow zones of sharp gradients of velocity and temperature. These concentrated gradients, which are referred to as fronts in laboratory turbulence, are studied here by applying conditional sampling criteria and analyzing the eigenvectors of the resulting lagged multivariate covariance matrix. In addition, a new technique is developed which specifies a weighting function instead of specific selection criteria with cutoff values. The mathematics derived for the new technique leads to the usual eigenvalue problem for principal components except that the covariance matrix becomes transformed. The above analyses indicate that the eddy microfronts seem to be related to the relative horizontal motion of the drafts generated by vertical motions in shear flows. The horizontal gradients are sharpest at the upstream edge of the drafts where convergence occurs between the horizontal motion of the draft and the horizontal motion of the ambient air. For strong turbulence, the eigenstructures identify several stages of eddy transport consistent with shear-driven overturning and transverse vorticity. For intermittent turbulence in strongly stratified flow, the eigen structures indicate the predominance of two-dimensional modes which coexist with overturning eddies and gravity wave modes. DOI: 10.1111/j.1600-0870.1988.tb00410.x