Abstract-
Variability of flow regimes and turbulence scalings in
a model of an inhomogeneously stratified, tropopause jet is investigated through
high-resolution, forced, three-dimensional numerical simulations (with 512 or
1024 vertical levels). Multi-valued scaling branches, with respect to the local
gradient Richardson number, are shown to occur in several turbulent quantities
(such as variances, fluxes, mixing efficiency, outer scales and their ratios
etc.). We find two distinct scaling curves for the upper and lower flanks of the
jet, each showing a clear branch switching at a critical height. Distinct
scaling curves in the upper and lower flanks of the jet can be related to the
doubling of buoyancy frequency in the background profile (across the
tropopause). The vertical levels corresponding to the inflection points (maximal
shearing) in the quasi-equilibrium turbulent mean jet velocity profile is
identified as the best criterion for branch switching along each of the scaling
curves. A conceptual picture of regime transitions, for the jet-induced
inhomogeneously stratified turbulence near the tropopause, is identified based
on cross-overs in various turbulence outer scales. As one traverses away from
the center of the jet, the following sequence of regime transitions may be
identified: shear-dominated, buoyancy-affected, buoyancy-dominated. In the
innermost core, turbulence, relatively unaffected by shear and stratification,
is maintained through a balance between transport and dissipation terms. We also
find that several of the turbulence scaling relations undergo a significant
change at the transition from the buoyancy-affected to the buoyancy-dominated
regime. Another significant feature is the extension of the outer scaling curve
in the buoyancy-affected regime into the shear-dominated regime up to the
inflection point level. This indicates that a portion of the latter regime is
also influenced by stratification. Important scaling relations are deduced from
the simulation results, and are compared with existing observational/numerical
studies.
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