Search

International audience; Stratospheric balloons from the Vorcore campaign have provided a unique description of the gravity-wave field in the lower stratosphere above Antarctica and the Southern Ocean, during the austral spring of 2005. Mesoscale simulations are carried out to analyze further the gravity-wave field. First, the realism of the simulated waves is assessed by comparison to the observations. A satisfactory overall agreement is found, but different behaviour is noted for orographic waves (overestimation in the simulations relative to the observations) and non-orographic waves (underestimation). Second, the gravity-wave field is analyzed in more detail than was possible from observations alone. It is necessary to distinguish and quantify orographic and non-orographic waves separately. Orographic waves are larger and more intermittent, yet affect only a limited geographical region. Hence, although orographic sources stand out as 'hot spots' for gravity waves, their contribution to momentum fluxes entering the stratosphere is comparable to or smaller than the contribution of non-orographic sources. A diagnostic for intermittency, the Gini coefficient, is proposed. It clearly marks the distinction between orographic and non-orographic sources (intermittencies of ~ 0.8 and ~ 0.5 respectively). Sensitivity to resolution is quite small regarding the spatial structure of the gravity-wave field, but is significant for the amplitudes. The momentum flux values increase by ~ 2 when the horizontal resolution is doubled, and possible biases of both simulations and observations are discussed. Nonetheless, the good agreement between observations and simulations and the complementary information on the biases of each dataset promises that in the future these different estimates of gravity-wave momentum fluxes may converge. © 2012 Royal Meteorological Society.

International audience; A formalism is proposed to parameterize the gravity waves due to convection in general circulation models with a stratosphere. It is based on a stochastic approach, where a large ensemble of monochromatic gravity waves is built up by launching a few waves at each time step, and by adding the effect of these waves, to that of the waves launched before, during the same day. The frequency and horizontal wave numbers of each wave are chosen randomly with fixed probability distribution, but the wave amplitude is directly related to precipitation, which is converted into heating rate. Linear theory is then used to predict the gravity wave generated by the heating rate. Off-line tests are carried out using reanalysis and global precipitation data. These tests demonstrate that the scheme launches gravity wave momentum fluxes that are much more erratic in amplitude than when uniform sources are considered. Consequently, the scheme tends to produce momentum flux deposition at lower levels than for the case when uniform sources are considered. We verify that the parameterization, when included in a general circulation model with vertical resolution in the stratosphere dz ~500m, is able to produce a quasi-biennial oscillation, without being detrimental to other aspects of the model climatology, like the semiannual oscillation and the behavior of the extratropics. © 2013. Her Majesty the Queen in Right of Canada. American Geophysical Union.

[1] A formalism is proposed to represent a broadband spectrum of Gravity Waves (GWs) via the superposition of a large ensemble of statistically independent monochromatic ones. To produce this large ensemble at a reasonable numerical cost, we use the fact that the life cycles of the waves needed to be parameterized in General Circulation Models (GCMs) have time scales that largely exceed the time step of the model. We can therefore launch few waves with characteristics chosen randomly at each time step, and make them having an effect on a longer time scale by applying an AR1 relation between the gravity waves drag at a given time and that at the next time step. The stochastic GW parameterization is applied to a GCM in the tropics, and its additional drag causes a realistic Quasi-Biennial Oscillation (QBO). The more realistic wind structure also results in a better representation of the large scale equatorial waves, like the Rossby Gravity Waves (RGWs) with periods around 4–5 day.