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  • Author or Editor: Marco Giorgetta x
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Haile Xue and Marco A. Giorgetta

Abstract

The diurnally evolving trapped lee wave over a small-scale two-dimensional steep mountain is investigated in large-eddy simulations based on a fully compressible and nonhydrostatic model [Icosahedral Nonhydrostatic (ICON)] with triangular grids of 50-m-edge length. An idealized atmospheric profile derived from a realistic case is designed to account for influences from the stagnant layer near the surface, the stability of the atmospheric boundary layer (ABL) and the upper-level jet. First, simulations were done to bridge from the linear regime to the nonlinear regime by increasing the mountain height, which showed that larger-amplitude lee waves with longer wavelength can be produced in the nonlinear regime than in the linear regime. Second, the effects of the stagnant layer near the surface and the ABL stability were explored, which showed that the stagnant layer or the stable ABL can play a similar wave-absorbing role in the nonlinear regime as in linear theories or simulations. Third, the role of the upper-level jet was explored, indicating that a stronger (weaker) upper-level jet can help to produce longer (shorter) lee waves. The stable ABL with a stagnant layer can more (less) efficiently absorb the longer (shorter) lee waves due to the stronger (weaker) jet, so that the wave response is more sensitive to the wave-absorption layer when an upper-level jet is present. Finally, the momentum budget was analyzed to explore the interaction between the upper and lower levels of the troposphere, which showed that the momentum flux due to the upward-propagating waves and trapped waves varies with the upper-level jet strength and low-level stagnancy and ABL stability.

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Thomas R. Krismer and Marco A. Giorgetta

Abstract

This study investigates the resolved wave forcing of the quasi-biennial oscillation (QBO) in the Max Planck Institute Earth System Model truncated at T63 with 95 vertical levels. The model, which parameterizes unresolved gravity waves, internally generates a QBO. The resolved waves contribute up to 50% and 30% to the total wave forcing (resolved plus parameterized) of the QBO westerly and easterly jet, respectively, mostly owing to waves with zonal wavenumbers lower than 20 and frequencies lower than 0.5 cpd. At higher frequencies and wavenumbers, the model underestimates the strength of the tropospheric wave sources when compared to Tropical Rainfall Measuring Mission (TRMM) observations and applies strong horizontal diffusion, which explains the shortage of wave momentum at these scales (relative to recent studies based on high-resolution models). The study further relates the vertical structure of equatorial Kelvin waves, which contribute most to the transport and deposition of westerly wave momentum, to their radiative dissipation and compares the role of longwave radiation and horizontal diffusion in the dissipation of the resolved waves in general. The Kelvin waves adjust their vertical wavelength according to their intrinsic phase speed and are efficiently damped by longwave radiation within westerly flow, where the vertical wavelength strongly decreases. Waves with zonal wavenumbers larger than 10, however, are mostly damped by horizontal diffusion. The latitudinal distribution of the resolved wave forcing reflects the latitudinal structure of the waves and is asymmetric with respect to the equator.

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