Authors:

F. Lott, R. Rani, C. McLandress, A. Podglajen, A. Bushell, M. Bramberger, H.-K. Lee, J. Alexander, J. Anstey, H.-Y. Chun, A. Hertzog, N. Butchart, Y.-H. Kim, Y. Kawatani, B. Legras, E. Manzini, H. Naoe, S. Osprey, R. Plougonven, H. Pohlmann, J. H. Richter, J. Scinocca, J. García-Serrano, F. Serva, T. Stockdale, S. Versick, S. Watanabe, K. Yoshida

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Abstract:

Gravity-wave (GW) parameterizations from 12 general circulation models (GCMs) participating in the Quasi-Biennial Oscillation initiative (QBOi) are compared with Strateole 2 balloon observations made in the tropical lower stratosphere from November 2019–February 2020 (phase 1) and from October 2021–January 2022 (phase 2). The parameterizations employ the three standard techniques used in GCMs to represent subgrid-scale non-orographic GWs, namely the two globally spectral techniques developed by Warner and McIntyre (1999) and Hines (1997), as well as the “multiwaves” approaches following the work of Lindzen (1981).

The input meteorological fields necessary to run the parameterizations offline are extracted from the ERA5 reanalysis and correspond to the meteorological conditions found underneath the balloons. In general, there is fair agreement between amplitudes derived from measurements for waves with periods less than 1 h and parameterizations. The correlation between the daily observations and the corresponding results of the parameterization can be around 0.4, which is 99% significant, since 1200 days of observations are used. Given that the parameterizations have only been tuned to produce a quasi-biennial oscillation (QBO) in the models, the 0.4 correlation coefficient of the GW momentum fluxes is surprisingly good. These correlations nevertheless vary between schemes and depend little on their formulation (globally spectral versus multiwaves for instance). We therefore attribute these correlations to dynamical filtering, which all schemes take into account, whereas only a few relate the gravity waves to their sources. Statistically significant correlations are mostly found for eastward-propagating waves, which may be due to the fact that during both Strateole 2 phases the QBO is easterly at the altitude of the balloon flights. We also found that the probability density functions (pdfs) of the momentum fluxes are represented better in spectral schemes with constant sources than in schemes (“spectral” or “multiwaves”) that relate GWs only to their convective sources.

Plain Language Summary:

Gravity waves are fluctuations in the atmosphere (seen in the temperature, wind velocity, and pressure fields) that transport energy and momentum from their sources in the troposphere and middle atmosphere to their sinks in the middle atmosphere. This way they exert a profound influence on the global circulation. Due to their relative small spatial scales, atmospheric general circulation models do not explicitly resolve these waves, and their effects on the circulation resolved by the model need to be parameterized. Pa36 rameterizations of gravity waves generated by fronts and flow imbalances typically as37 sume that wave sources are at a certain vertical level in the troposphere, which is easy to implement but neglects the fact that these processes can occur at all altitudes in the atmosphere. In this study, we explore to which extent parameterizations of gravity wave due to fronts and flow imbalances can be improved by allowing waves to be emitted from all model levels. Our results show evidence of modest corrections of some model biases, and a clear improvement in the parameterized gravity waves energy spectra.

Key Points:

1. Non-orographic gravity wave parameterization is improved by including inertial waves and waves sources at all model levels.
2. Parameterized energy spectrum becomes much closer to observations.
3. Global model with the new parameterization performs well, some model biases are modestly alleviated.