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- Title
Dependence of Model Energy Spectra on Vertical Resolution.
- Authors
Waite, Michael L.
- Abstract
Many high-resolution atmospheric models can reproduce the qualitative shape of the atmospheric kinetic energy spectrum, which has a power-law slope of −3 at large horizontal scales that shallows to approximately −5/3 in the mesoscale. This paper investigates the possible dependence of model energy spectra on the vertical grid resolution. Idealized simulations forced by relaxation to a baroclinically unstable jet are performed for a wide range of vertical grid spacings Δ z. Energy spectra are converged for Δ z 200 m but are very sensitive to resolution with 500 m ≤ Δ z ≤ 2 km. The nature of this sensitivity depends on the vertical mixing scheme. With no vertical mixing or with weak, stability-dependent mixing, the mesoscale spectra are artificially amplified by low resolution: they are shallower and extend to larger scales than in the converged simulations. By contrast, vertical hyperviscosity with fixed grid-scale damping rate has the opposite effect: underresolved spectra are spuriously steepened. High-resolution spectra are converged except for the stability-dependent mixing case, which are damped by excessive mixing due to enhanced shear over a wide range of horizontal scales. It is shown that converged spectra require resolution of all vertical scales associated with the resolved horizontal structures: these include quasigeostrophic scales for large-scale motions with small Rossby number and the buoyancy scale for small-scale motions at large Rossby number. It is speculated that some model energy spectra may be contaminated by low vertical resolution, and it is recommended that vertical-resolution sensitivity tests always be performed.
- Subjects
SPECTR Audio (Company); SPECTRUM analysis; ATMOSPHERIC models; KINETIC energy; HIGH resolution spectroscopy; POWER law (Mathematics)
- Publication
Monthly Weather Review, 2016, Vol 144, Issue 4, p1407
- ISSN
0027-0644
- Publication type
Article
- DOI
10.1175/MWR-D-15-0316.1