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The parameterization scheme that represents gravity waves due to convection in LMDz-6A, the atmospheric components of the IPSL coupled climate model (IPSLCM6), is directly compared to Strateole-2 balloon observations made in the lower tropical stratosphere from November 2019 to February 2020. The input meteorological fields necessary to run the parameterization offline are extracted from the ERA5 reanalysis and correspond to the instantaneous meteorological conditions found underneath the balloons. In general, we find a fair agreement between measurements of the momentum fluxes due to waves with periods less than 1hr and the parameterization. The correlation of the daily values between the observations and the results of the parameterization is around 0.4, which is statistically elevated considering that we analyse around 600 days of data and surprisingly good considering that the parameterization has not been tuned: the scheme is just the standard one that helps producing a Quasi-Biennial Oscillation in the IPSLCM6 model. Online simulations also show that the measured values of momentum fluxes are well representative of the zonally and averaged values of momentum fluxes needed in LMDz-6A to simulate a QBO. The observations also show that longer waves with periods smaller than a day carry about twice as much flux as waves with periods smaller than an hour, which is a challenge since the low period waves that make the difference are potentially in the “grey zone” of most climate models

A uniform approximation of flow over gentle hills with a turbulent closure based on mixing length theory is derived. It permits to describe the transition from neutral to stratified flow in the production of mountain drag. Our results corroborate previous studies showing that the transition from the form drag associated to the mountain induced changes in boundary layer friction to the mountain gravity waves drag can be captured by theory. We also confirm that the first is associated with downstream sheltering with relative acceleration at the hill top, the second with upstream blocking with strong downslope winds. We also show that the downslope winds penetrate well into the inner layer. The theory show that the altitude at which the incident flow need to be taken to calculate the drag is related to the inner layer depth at which dissipative effects equilibrate disturbance advection. We also show that the parameter that capture the transition, which in our case is a Richardson number, is directly related to the altitude of the turning levels of the gravity waves with respect to the mountain length. Our uniform solutions are also used to describe the wave field aloft and the distribution of the Reynolds stress in the vertical. Some directions to combine neutral and stratified effects in the parameterization of subgrid scale orographies in large scale models are given.

Circumglobal Rossby wave patterns (CRWPs) are large-scale configuration characterized by amplified Rossby waves stretching in the zonal direction across a broad portion of a latitude circle. Previous research considered CRWPs mainly from two points of view: either as particular type of planetary wave, undergoing quasi-resonant amplification when trapped between two turning latitudes, or as a single Rossby wave packet (RWP) being ducted zonally in presence of particularly strong meridional potential vorticity gradient, that acts as a waveguide. Although with different characteristics, a structured waveguide for Rossby waves is needed in both cases.Space/time spectral analysis allows to study CRWPs from their spectral projection over a range of wavenumber/phase speed harmonics, with the implicit assumption that a CRWP would project only over few zonal wavenumbers and propagation speeds characterizing it over a broad portion of the hemisphere. Such spectra are calculated from meridional wind at 250hPa over a sliding time window of 61d, in order to consider the intraseasonal variability in spectral power. The application of principal component analysis to this data set allows for few leading variability modes in the spectral domain during boreal winter, and these modes appear to be related to CRWPs.They correspond to the baroclinic propagation of amplified RWPs from the Pacific to the Atlantic storm track in a hemispheric flow configuration displaying enhanced meridional gradients of geopotential height over midlatitudes. The first CRWP is forced by tropical convection anomalies over the Indian Ocean and features the propagation of amplified Rossby wave packets over northern midlatitudes, while the second one propagates rapidly over latitudes between 35◦N and 55◦N and appears to have extratropical origin. An anomalous equatorward propagation of Rossby waves from the Atlantic eddy-driven jet to the North African subtropical jet is observed for both CRWPs These results support the perspective that CRWPs are substantially related with the circumglobal propagation of RWPs.

National audience; Le présent bilan de mandature 2016-2021 de la CPCN (Conférence des présidentes et présidents de sections et de commissions interdisciplinaires du Comité national de la recherche scientifique) a une triple finalité. Il constitue d’abord une forme d’autoévaluation de l’action de la CPCN et, à travers elle, de l’action coordonnée des sections et commissions interdisciplinaires (CID), au cours de cette mandature, en rappelant autant que de besoin le contexte dans lequel cette action s’est inscrite. Il vise également à rendre compte de cette action auprès de la communauté scientifique française à travers une diffusion la plus large possible. Il doit enfin permettre un passage de relais avec les sections et CID et avec la CPCN qui seront mises en place à l’automne 2021.

Describing the statistics of gravity wave (GW) fields represents a major motivation for both current research on atmospheric GWs and long-range infrasound propagation. In practice, the probability density functions (PDF) of the momentum fluxes are estimated combining observations, numerical modelling, and theory. Numerical models (such as WRF) show that the PDFs vary in a robust way relative to the background local wind speed. For non-orographic GWs, these PDFs are approximated as lognormal distributions, with characteristics found to depend on the background wind speed. Studies show that some trends are not observed using a state-of-the-art stochastic parameterization of GWs, unless the phase velocities of GW sources (essentially tropospheric) are dramatically changed. As the vertical wavelength and the phase velocity are related to each other, such changes also affect the interaction between infrasound and lower-stratospheric GWs. Consequently, significant efforts have been made to use ground-based acoustic sensors for characterizing the GW sources, including the use of neural networks. This approach provides a promising way to describe the statistics of GW sources from ground-truth infrasound events and an additional constraint to tune stochastic parameterizations of GWs.

We analyse the stratospheric Kelvin and Rossby-gravity wave packets with periods of a few days in nine high-top (i.e. with stratosphere) models of the fifth Coupled Model Intercomparison Project (CMIP5). These models simulate realistic aspects of these waves, and represent them better than the tropospheric convectively coupled waves analyzed in previous studies. There is nevertheless a large spread among the models, and those with a Quasi-Biennial Oscillation (QBO) produce larger amplitude waves than the models without a QBO. For the Rossby-gravity waves this is explained by the fact that models without a QBO never have positive zonal mean zonal winds in the lower stratosphere, a situation that is favorable to the propagation of Rossby-gravity waves. For the Kelvin waves, larger amplitudes in the presence of a QBO is counter intuitive because Kelvin waves are expected to have larger amplitude when the zonal mean zonal wind is negative, and this is always satisfied in models without a QBO. We attribute the larger amplitude to the fact that models tuned to have a QBO require finer vertical resolution in the stratosphere. We also find that models with large precipitation variability tend to produce larger amplitude waves. However, the effect is not as pronounced as was found in previous studies. In fact, even models with weak precipitation variability still have quite realistic stratospheric waves, indicating either that (i) other sources can be significant or that (ii) the dynamical filtering mitigates the differences in the sources between models.