Michael Waite, NCAR (waite@ucar.edu) Peter Bartello, McGill University
The atmospheric energy spectrum at midlatitudes has a slope of -3 at scales larger than around 500 km, and -5/3 at smaller scales (i.e. the mesoscale). The -3 part of the spectrum is generally attributed to quasi-geostrophic (QG) turbulence (i.e. balanced motion), while the -5/3 part is not fully understood. Because the mesoscale is characterized by strong stratification and weak rotation, nonrotating stratified turbulence (not necessarily balanced) is often studied as a first order approximation. Indeed, the mystery of the mesoscale spectrum has motivated much of the work on stratified turbulence over the last three decades.
We will present simulations of strongly stratified turbulence over a wide range of Rossby numbers (Ro) which illustrate the transition from QG turbulence at small Ro to stratified turbulence at large Ro. We will place this work in the context of recent results on stratified turbulence. The energy spectrum, the partition of energy between linear vortical and wave modes, and the characteristic vertical length scale will be discussed. With sufficient numerical resolution, the vertical scale is shown to be a universal function of Ro which transitions from (f/N)L at small Ro to U/N at large Ro around Ro ~ 0.1 (here f is the Coriolis parameter, N is the Brunt-Vaisala frequency, L is the energy-containing horizontal scale, U is the r.m.s. energy, and Ro = U/fL). Implications for more realistic atmosphere and ocean models will also be discussed.