The Momentum Budget in the Stratosphere, Mesosphere, and Lower Thermosphere. Part I: Contributions of Different Wave Types and In Situ Generation of Rossby Waves

Abstract

A momentum budget is examined in the stratosphere, mesosphere, and lower thermosphere using simulation data over ~11 years from a whole-atmosphere model in terms of the respective contributions of gravity waves (GWs), Rossby waves (RWs), and tides. The GW forcing is dominant in the mesosphere and lower thermosphere (MLT), as indicated in previous studies. However, RWs also cause strong westward forcing, described by Eliassen–Palm flux divergence (EPFD), in all seasons in the MLT and in the winter stratosphere. Despite the relatively coarse model resolution, resolved GWs with large amplitudes appear in the MLT. The EPFD associated with the resolved GWs is eastward (westward) in the summer (winter) hemisphere, similar to the parameterized GW forcing. A pair of positive and negative EPFDs are associated with the RWs and GWs in the MLT. These results suggest that the RWs and resolved GWs are generated in situ in the MLT. Previous studies suggested that a possible mechanism of RW generation in the MLT is the barotropic/baroclinic instability. This study revisits this possibility and examines causes of the instability from a potential vorticity (PV) viewpoint. The instability condition is characterized as the PV maximum at middle latitudes on an isentropic surface. Positive EPFD for RWs is distributed slightly poleward of the PV maximum. Because the EPFD equals the PV flux, this feature indicates that the RW radiation acts to reduce the PV maximum. The PV maximum is climatologically maintained in both the winter and summer mesospheres, which is caused by parameterized GW forcing.

The climatology of the Brewer–Dobson circulation and the contribution of gravity waves

“…It is interesting that the magnitude of dU/dt (φ, z) in the summer low-latitude region is comparable to that of GW (φ, z) but confined in the lower stratosphere. The direction and latitudinal location of this circulation are consistent with the middle atmosphere Hadley circulation, although dominant altitude region may be slightly lower than the theoretical expectation (i.e., upper stratosphere) (Semeniuk and Shepherd, 2001). It is also worth noting that there is also a weak equatorward circulation in dU/dt (φ, z) in the winter hemisphere located in the midlatitude region in the NH (DJF) and at relatively low latitudes in the SH (JJA).…”

“…The observed temperature in tropical regions is almost uniform latitudinally even in solstitial seasons where and when the latitudinal gradient of radiative heating by ozone is not negligible. This suggests the presence of thermally driven circulation called the middle atmosphere Hadley circulation, which was first indicated and examined by Dunkerton (1989) and revisited by Semeniuk and Shepherd (2001). The middle atmosphere Hadley circulation is confined at latitudes lower than 30 • and composed of a summer-to-winter hemisphere cell with an upward (downward) branch in the summer (winter) hemisphere.…”

Interhemispheric differences of mesosphere/lower thermosphere winds and tides investigated from three whole atmosphere models and meteor radar observations

“…The mean circulation at the MLT is substantially controlled by tides and gravity waves, carrying energy and momentum from their source regions to the altitude of their dissipation (e.g., Lindzen, 1981a, and references therein). Gravity wave dissipation drives the hemispheric summer mesopause temperatures up to 100 K away from the radiative equilibrium (McLandress et al, 2006;Becker, 2012;Sato et al, 2018). However, GCMs often do not have the spatial and temporal resolution to resolve GWs and, thus, depend on GW parameterizations of the different primary GW sources such as orography (mountain waves), frontal systems, jet stream imbalances, deep convection and shear instabilities (Fritts and Alexander, 2003;Plougonven and Zhang, 2014, also see for a review).…”

“…However, GCMs often do not have the spatial and temporal resolution to resolve GWs and, thus, depend on GW parameterizations of the different primary GW sources such as orography (mountain waves), frontal systems, jet stream imbalances, deep convection and shear instabilities (Fritts and Alexander, 2003;Plougonven and Zhang, 2014, also see for a review). Recently, there were studies suggesting that non-primary GWs also contribute to the momentum budget at the MLT Becker and Vadas, 2018;Sato et al, 2018) resulting in an even more complex vertical coupling posing new challenges not yet considered in available GW parameterizations.…”

The semiannual oscillation (SAO) in the tropical middle atmosphere and its gravity wave driving in reanalyses and satellite observations

“…Following the approach used in previous studies (e.g., Oberheide et al, 2002;Ern et al, 2013;Smith et al, 2017;Sato et al, 2018), quasi-geostrophic winds can be calculated from the geopotential fields derived from satellite soundings. For stationary conditions, and neglecting the drag exerted by atmospheric waves, the zonal and meridional momentum equations can be written as follows…”