The Troposphere-to-Stratosphere Transition in Kinetic Energy Spectra and Nonlinear Spectral Fluxes as Seen in ECMWF Analyses

Abstract

Global horizontal wavenumber kinetic energy spectra and spectral fluxes of rotational kinetic energy and enstrophy are computed for a range of vertical levels using a T799 ECMWF operational analysis. Above 250 hPa, the kinetic energy spectra exhibit a distinct break between steep and shallow spectral ranges, reminiscent of dual power-law spectra seen in aircraft data and high-resolution general circulation models. The break separates a large-scale ''balanced'' regime in which rotational flow strongly dominates divergent flow and a mesoscale ''unbalanced'' regime where divergent energy is comparable to or larger than rotational energy. Between 230 and 100 hPa, the spectral break shifts to larger scales (from n 5 60 to n 5 20, where n is spherical harmonic index) as the balanced component of the flow preferentially decays. The location of the break remains fairly stable throughout the stratosphere. The spectral break in the analysis occurs at somewhat larger scales than the break seen in aircraft data. Nonlinear spectral fluxes defined for the rotational component of the flow maximize between about 300 and 200 hPa. Large-scale turbulence thus centers on the extratropical tropopause region, within which there are two distinct mechanisms of upscale energy transfer: eddy-eddy interactions sourcing the transient energy peak in synoptic scales, and zonal mean-eddy interactions forcing the zonal flow. A well-defined downscale enstrophy flux is clearly evident at these altitudes. In the stratosphere, the transient energy peak moves to planetary scales and zonal mean-eddy interactions become dominant.

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“…6a which represents the vertically integrated total energy is not simple. Blažica et al (2013) showed that on mesoscale in midlatitudes the divergent component of kinetic energy is at least equally as large as the rotational component and that the slopes of the corresponding spectra depend on the altitude (see also Burgess et al, 2013). In our global case with the tropics dominated by divergent circulations, we find that the unbalanced component makes up 80 % or more of the total energy beyond zonal wave number 100 (about 150 km in midlatitudes) (Fig.…”

“…Furthermore, energy spectra derived from models are usually based on single level data (e.g. Burgess et al, 2013;Blažica et al, 2013) and they show only kinetic energy; correspondingly, their comparison with the spectrum in Fig. 6a which represents the vertically integrated total energy is not simple.…”

Rotational and divergent kinetic energy in the mesoscale model ALADIN

“…Below 150 km, the slope of E DIV spectrum is close to (1.8 while the E VOR spectrum has a slope about (2. For larger scales, the difference between the slopes of divergent and rotational spectra is clearly increasing as the horizontal scale becomes larger as found in the global model data studied by Burgess et al (2013) and in the analysis of global rotational and inertio-gravity energy by Zagar et al (2009Zagar et al ( , 2010.…”

“…Koshyk et al (1999) showed that in several general circulation models divergent and rotational kinetic energy are about equally important in the stratosphere and the mesosphere while in the upper troposphere rotational energy is dominant. A recent evaluation of the European Centre for MediumRange Weather Forecasts (ECMWF) model spectra at T799 resolution by Burgess et al (2013) shows that at 200 hPa global divergent energy becomes prevalent over global rotational energy at 400 km whereas above 100 hPa divergent component is dominant over rotational component across all scales smaller than Â1200 km (global wavenumber Â30).…”

“…While Koshyk et al (1999) and Burgess et al (2013) discussed this question for the upper troposphere and the stratosphere in the global models of lower resolution, we focus on the middle and the lower troposphere around 100 km and smaller. The main result is the ratio between divergent kinetic energy and the total kinetic energy in a typical mid-latitude mesoscale model of resolution around 4 km.…”