Numerical Simulation of Nonlinear Jet Streak Adjustment

Abstract

The geostrophic adjustment process in a propagating jet maximum is studied through numerical experiments performed with a two-layer, nonlinear primitive equation model. Gravity-inertia modes generated by the jet streak from an initial three-dimensional balanced flow are isolated with the aid of diagnostic quasi-geostrophic and balance models. Them modes appear to represent gravitational subsynoptic ?signal? rather than ?noise,? and their behavior is examined as a function of initial core strength. Persistent unbalanced patterns relative to the jet core are discovered, whose amplitude increases approximately as the square of the Rossby number. Their behavior is suggestive of forced, rather than free, ageostrophic modes, and their existence implies that the ?four-cell? vertical motion pattern characteristic of jet streaks in filtered models may be significantly altered in a real jet streak. The free response to initial data imbalances placed at varying locations in a jet streak environment is explored; localized perturbations in the nondivergent wind, geopotential and irrotational wind are individually inserted into a strong jet, and difference maps are then constructed with the aid of a control experiment. For the nondivergent wind anomaly, and to a lesser extent for the geopotential anomaly, results generally agree qualitatively with classical adjustment theory, once the effect of background vorticity on the deformation radius is considered. Departures from theory arise due to other nonlinearity, and the time-dependent nature of the mean flow. In the case of the irrotational wind anomaly, only gravitational modes am generated, as expected; their time evolution is influenced by the ambient vorticity, which alters the effective Coriolis frequency.