Mesoscale Energy Spectra of the Mei-Yu Front System. Part I: Kinetic Energy Spectra

Abstract

he mesoscale kinetic energy (KE) spectra of the mei-yu front system are investigated through idealized numerical simulations. In the mature stage, the upper-tropospheric KE spectrum resembles a ?3 power law for wavelengths between 1000 and 400 km and shallows to a slope of approximately ? at smaller wavelengths. A similar behavior can be observed in the lower stratosphere. At both levels, the rotational KE spectrum shallows nearly to the same extent as the divergent KE spectrum at smaller wavelengths, accounting for the transition in the total KE spectrum. About 12 h after the latent heating is turned off, the mesoscale KE spectra hardly show the distinct spectral transition, especially in the upper troposphere.The spectral KE budget for various height ranges is analyzed and compared. In the upper troposphere, the mesoscale KE is deposited through the buoyancy flux and removed by the advective nonlinearity and vertical pressure flux divergence. The buoyancy flux spectrum in the mature phase has a peak at scales of around 300 km and a plateau throughout the mesoscale, which suggests a significant injection of KE in the mesoscale. The negative contribution of the advective nonlinearity demonstrates that to some extent the mesoscale KE derives from a nonlinear upscale cascade, with the buoyancy-produced energy source located at the lower end of mesoscale spectrum. In the lower stratosphere, the mesoscale KE is deposited through the advective nonlinearity and vertical pressure flux divergence and removed by the buoyancy flux. This suggests that the lower-stratospheric KE spectrum is influenced by both the downscale energy cascade and vertically propagating IGWs.