Impact of a Stochastic Nonorographic Gravity Wave Parameterization on the Stratospheric Dynamics of a General Circulation Model

Abstract

The general circulation of the middle atmosphere, particularly of the mesosphere, is strongly dependent on the forcing arising from gravity wave processes. Their sources in the troposphere are both orographic and nonorographic, the latter being strongly intermittent. In climate models, the effects of gravity waves need to be parameterized, often assuming that their properties are constant. In this work we focus on the changes of the middle atmosphere due to the introduction of intermittency in a parameterization of nonorographic gravity waves, using a stochastic version of the Hines scheme. The stochastic approach is tailored to the diagnosed sensitivity of the model to this forcing, and peculiar changes emerge even if a relatively small amount of intermittency is prescribed. We analyze in detail the changes of the stratospheric dynamics in the tropical region and the global circulation of the middle atmosphere, when the stochastic parameterization is employed in place of the deterministic one. The mean state and variability of the model, realistic also in the default version, are preserved when stochasticity is added. Significant changes are observed in the mesosphere due to an enhanced poleward transport, leading to warming in the winter season. While there are some improvements of the mean state, the interannual variability is not significantly affected in the extratropics. The impacts on the simulated equatorial stratosphere are evident, as the stochasticity reduces the overall period of the quasi‐biennial oscillation but also leads to a net reduction of the variability between cycles.

Gravity Waves in Planetary Atmospheres: Their Effects and Parameterization in Global Circulation Models

“…The stochastic versions of their corresponding "full spectrum" prototype GW schemes produce much more sporadic instantaneous forcing; however, the average simulated fields turn out to be close in both cases. There were attempts to link the stochastic parameterizations to the intermittency of observed GWs e.g., [64]; however, the randomness did not eliminate the need for tunable "intermittency" parameters [65]. We note again that the latter had to be introduced to substitute for imperfections of the underlying theories of GW breaking/saturation.…”

Optimization of Gravity Wave Source Parameters for Improved Seasonal Prediction of the Quasi-Biennial Oscillation

“…The efficiency must be upscaled to account for the difference in total source momentum flux between the multiwave and single-wave schemes, but otherwise no other changes are necessary since, as shown by Eckermann (2011), the same time-mean flux and mean-flow acceleration are produced, leading to similar time-mean circulation responses in models. A range of subsequent studies have pursued similar stochastic approaches to parameterizing nonorographic GWD and have reported similarly acceptable performance in modeling both tropical and extratropical middle atmospheric circulations (e.g., Lott and Guez 2013;McCormack et al 2015;de la Cámara et al 2016;Garcia et al 2017;Serva et al 2018).…”

“…In contrast to curating climatological nonorographic gravity wave spectra through offline tuning, some studies have proposed and implemented schemes where source characteristics are derived directly from parameterized subgrid-scale convective activity within the model, which is a more internally consistent approach based on our physical understanding of gravity wave sources (e.g., Beres et al 2004;Richter et al 2010;Schirber et al 2014). However, varying implementations of nonorographic GWD parameterization in models and differences in modeled large-scale flow patterns mean that the parameterized wave spectrum that provides the highest NWP or climate skill may differ among models (e.g., Bushell et al 2015;Serva et al 2018).…”

Sensitivity of Gravity‐Wave Momentum Flux to Moisture in the Mei‐Yu Front Systems

“…Thus, parameterizing gravity waves in GCMs is necessary to represent their impact (see Alexander et al, , for a review). In recent years, many studies have tried to relate the parameterization of convectively generated gravity waves to wave sources (e.g., Bushell et al, ; Chun & Baik, ; Lott & Guez, ; Schirber et al, ; Serva et al, ), because the absence of sources in parameterizations limits their potential calibration with the growing number of observations and might be a cause of systematic errors (de la Cámara et al, ). Consequently, understanding the relation of convective gravity waves with their sources is not just of theoretical interest but also has practical implications.…”