Authors:

B Ribstein, C Millet, F Lott, A de la Cámara

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Abstract:

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. Parameterizations of gravity waves generated by fronts and flow imbalances typically assume 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.

A multiwave non-orographic gravity waves (GWs) scheme is adapted to represent waves of small intrinsic phase speed, inertial waves, and wave emission from all altitudes. This last change removes the launching altitude parameter, an arbitrary parameter systematically used in GW schemes. In offline calculations using reanalysis fields, these changes impose larger amplitude saturated waves everywhere in the middle atmosphere, which produces more realistic GW vertical spectra than in previous configurations. The same scheme, tested online in the Laboratoire de Meteorologie Dynamique Zoom (LMDz) general circulation model, performs at least as well as the operational non-orographic GW scheme. Some modest benefits are seen, for instance, in the equatorial tilt with altitude of the winter jets in the middle atmosphere. Although the scheme includes the effects of inertial waves, which are detected in the mesosphere by different observational platforms, the configuration that gives a reasonable climatology in LMDz hinders their vertical propagation and limits their presence at mesospheric altitudes.

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.