Séminaire

Vertically propagating gravity waves (GWs) transfer their momentum and energy to the large-scale flow in the middle atmosphere, where they are dissipated through the wave breaking, critical-level filtering, and radiative damping processes. The current resolution of general circulation models (GCMs), even for high-resolution ones with horizontal grid spacing of ~0.25o, do not fully resolve GWs, and thus their effects have to be parameterized in GCMs. Among the various sources of GWs, convection can generate high-frequency GWs, which have a broad phase speed spectrum and can propagate to high altitudes without seasonal restrictions. In this seminar, observational characteristics of convective GWs (CGWs), parameterization of CGWs for use in GCMs, and impacts of CGWs in the large-scale circulations in the middle atmosphere are presented. The observational evident of CGWs will be shown, based on satellites, super-pressure balloons, meteor radar, and high-resolution radiosonde measurements. Regarding the CGW parameterization, the basis of GW parameterization and the development history of CGWs will be provided. The impacts of the parameterized CGWs in the middle atmosphere circulations are given, based on recent works from my research group: (i) the quasi-biennial oscillation (QBO), (ii) polar-night jet in the southern hemisphere (SH) wintertime, and (iii) Madden-Julian oscillation (MJO). In the tropical stratosphere, CGWs can significantly contribute to the momentum budget of the QBO. The positive momentum forcing by parameterized CGWs is comparable to that by Kelvin wave during the easterly-to-westerly transition, while the negative momentum forcing by parameterized CGWs during the westerly-to-easterly transition is significantly larger than any other equatorial planetary waves (Kang et al. 2018, JAS). Regarding the polar-night jet, Choi and Chun (2013, JAS) demonstrated that excessive jet and cold-pole biases in the SH stratosphere during the wintertime, which have been a long-lasting problem in GCMs, can be alleviated significantly by including a CGW parameterization into a GCM. Recently, Kalish et al. (2018, JGR) showed that cloud-top momentum flux of CGWs evolve following the MJO phases, and the propagation speed of convective cloud associated with the MJO is similar to the dominant eastward-propagating speed of CGWs in the cloud top. Some important issues on the CGW parameterization will be discussed at the end of the seminar.