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MI
Meteorological Institute

Photo: NARVAL Mission 2013/14, UHH and MPI-M

Latest Publications

Holube, K. M., Lunkeit, F., Vasylkevych, S. and Žagar, N., 2024: Resonant excitation of Kelvin waves by interactions of subtropical Rossby waves and the zonal mean flow, J. Atmos. Sci., https://doi.org/10.1175/JAS-D-24-0033.1

Lembo, V. , S. Bordoni, E. Bevacqua, D. I. V. Domeisen, C. L. E. Franzke, V. M. Galfi, C. Garfinkel, C. I. Grams, A. Hochman, R. Jha, K. Kornhuber, F. Kwasniok, V. Lucarini, G. Messori, D. Pappert, I. Perez-Fernandez, J. Riboldi, E. Russo, T.A. Shaw, I. Strigunova, F. Strnad, P. Yiou, and N. Žagar, 2024: Dynamics, statistics and predictability of Rossby waves, heatwaves and spatially compounded extreme events. Bull. Amer. Meteor. Soc., https://doi.org/10.1175/BAMS-D-24-0145.1, in press.

Zolghadrshojaee, M., Tegtmeier, S., Davis, S. M., and Pilch Kedzierski, R., 2024: Variability and long-term changes in tropical cold-point temperature and water vapor, Atmos. Chem. Phys., 24, 7405–7419, https://doi.org/10.5194/acp-24-7405-2024.

Mahó, S. I., S. Vasylkevych, and N. Žagar, 2024: Excitation of mixed Rossby-gravity waves by wave - mean flow interactions on the sphere. Quart. J. Roy. Met. Soc., 1-17. DOI: 10.1002/qj.4742.

Abstract
The equatorial mixed Rossby-gravity wave (MRGW) is an important contributor to tropical variability. Its excitation mechanism capable of explaining the observed MRGW variance peak at synoptic scales in the troposphere remains elusive. This study investigates wave-mean flow interactions as a generation process for the MRGWs using the TIGAR model, which employs Hough harmonics as the basis of spectral expansion on the sphere, thereby representing MRGWs as prognostic variables. Idealised numerical simulations reveal the interactions between waves emanating from a symmetric tropical heat source and an asymmetric subtropical zonal jet as an excitation mechanism for the MRGWs. The excited MRG waves have variance spectra resembling the observed MRGWs in the tropical troposphere. The MRG energy spectrum has a maximum at zonal wavenumbers k = 4 − 5 also in the case of an asymmetric forcing that generates MRGWs across large scales. Effects of wave-wave interactions appear of little importance for the MRGW growth compared to wave-mean flow interactions. Application of the zonal-mean zonal wind profiles from ERA5 reaffirms the importance of the asymmetry of the zonal mean flow.

Neduhal, V., N. Žagar, F. Lunkeit, I. Polichtchouk, and Ž. Zaplotnik, 2024: Decomposition of the horizontal wind divergence associated with the Rossby, inertia-gravity, mixed Rossby-gravity and Kelvin waves on the sphere. Journal of Geophysical Research: Atmospheres, 129, e2023JD040427. https://doi.org/10.1029/2023JD040427

Abstract
The paper presents a new method for the decomposition of the horizontal wind divergence among the linear wave solutions on the sphere: inertia-gravity (IG), mixed Rossby-gravity (MRG), Kelvin and Rossby waves. The work is motivated by the need to quantify the vertical velocity and momentum fluxes in the tropics where the distinction between the Rossby and gravity regime, present in the extratropics, becomes obliterated. The new method decomposes divergence and its power spectra as a function of latitude and pressure level. Its application on ERA5 data in August 2018 reveals that the Kelvin and MRG waves made about 6% of the total divergence power in the upper troposphere within 10S-10N, that is about 25% of divergence. Their contribution at individual zonal wavenumbers k can be much larger; for example, Kelvin waves made up to 24% of divergence power at synoptic k in August 2018. The relatively small roles of the Kelvin and MRG waves in tropical divergence power are explained by decomposing their kinetic energies into rotational and divergent parts. The Rossby wave divergence power is 0.3-0.4% at most, implying up to 6% of global divergence due to the beta effect. The remaining divergence is about equipartitioned between the eastward- and westward-propagating IG modes in the upper troposphere, whereas the stratospheric partitioning depends on the background zonal flow. This work is a step towards a unified decomposition of the momentum fluxes that supports the coexistence of different wave species in the tropics in the same frequency and wavenumber bands.