Empirical Models of the Eddy Heat Flux and Vertical Shear on Short Time Scales

Abstract

The relationship between the eddy heat flux and vertical shear in the extratropical atmosphere is studied by developing various linear stochastic models fitted to the observed January and July Northern Hemispheric data. Models are univariate or bivariate, continuous or discrete. An objective procedure selects the second-order bivariate model as most appropriate in midlatitudes. The first-order continuous bivariate model indicates that feedback within the flux-shear system is comparable to damping on short time scales (days), but is somewhat weaker on intermediate time scales (weeks). Observational errors are found to influence several results. When these errors are not accounted for, dissipation is found to be quite strong, with a damping time for the shear of 1 to 3 days and, in apparent contradiction to the results of viscid finite amplitude models of baroclinic instability, damping of the eddy heat flux is somewhat stronger in July than in January. When observational errors are considered, the damping time for the shear is about five days and damping of the flux in midlatitudes is nearly equal for January and July.