Small-Scale Convective
Gravity Waves: Contributions to the Large-Scale Circulations in the
Middle Atmosphere
Hye-Yeong Chun
Yonsei University, Seoul
4 Sept, Noon, in 2155
Abstract:
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, and meteor
radar 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.