Impacts of Limited Model Resolution on the Representation of Mountain Wave and Secondary Wave Dynamics in Local and Global Models: 2. Mountain Wave and Secondary Wave Evolutions in the Thermosphere. Issue 9 (2nd May 2022)

• Record Type: Journal Article
• Authors: Fritts, David C.; Lund, Adam C.; Lund, Thomas S.; Yudin, Valery
• Is Part Of: Journal of geophysical research. Volume 127:Issue 9(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-05-02
• Subjects: thermospheric mountain waves -- secondary gravity waves -- acoustic waves -- wave breaking -- instabilities and turbulence -- resolution influences on resolved fields -- resolution needs for modeling of gravity wave dynamics; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2021JD036035
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: A companion paper by Fritts et al. (2022), https://doi.org/10.1002/2021JD035990 describes the consequences of decreasing horizontal resolution in the description of mountain wave (MW) propagation, breaking, and large‐scale responses over the Southern Andes reaching into the mesosphere. This paper extends that analysis into the thermosphere, where MWs are confined below a critical level, but secondary gravity waves and acoustic waves become prominent and dominate the wave fields at higher altitudes. Like MWs at lower altitudes, the character and responses of secondary waves are strongly dependent on model resolution. MWs readily penetrate above a zonal wind minimum in the upper mesosphere and exhibit responses for varying resolution similar to those at lower altitudes. Both the MW and local mean responses weaken somewhat for resolution varying from 0.5 to 2 km, weaken more significantly for 4‐km resolution, and fail to approximate the high‐resolution results for 8‐km resolution. MW momentum fluxes and induced local mean responses are very different than those in the mesosphere, but exhibit similar variability with coarsening resolution. Secondary gravity waves at larger scales arise due to MW‐induced mean wind decelerations, hence are not highly sensitive to model resolutions of ∼0.5–2 km approximating MW breaking. However, acoustic waves are forced primarily by MW breaking, thus are poorly described at 2‐km resolution and absent at 4‐km resolution. These results reveal the dynamics that can and cannot be explicitly modeled as resolution is coarsened, and may aid in assessing, parameterizing, and/or compensating for the unresolved dynamics and their consequences. Plain Language Summary: This paper addresses the influences of decreasing resolution on the dynamics of secondary gravity waves (SGWs) arising in the thermosphere due to generation accompanying mountain wave (MW) breaking dynamics in the upper stratosphere and lower mesosphere. For high spatial resolution of 0.5 km able to describe the instability dynamics that account for MW breaking, SGW generation is strong and is driven by both (a) MW self‐acceleration dynamics at larger spatial scales and (b) MW breaking dynamics at smaller spatial scales. Reductions in model resolution degrade the SGW responses in two ways: (a) decreasing resolution to 2 km removes the ability to describe MW breaking dynamics and weakens the smaller‐scale SGW responses; and (b) decreasing resolution to 4 and 8 km significantly weakens or eliminates the larger‐scale SGW responses in the thermosphere due to inadequate spatial resolution and/or numerical impacts on their phase and vertical group velocities. Key Points: High spatial resolution of gravity wave dynamics at lower altitudes is critical in achieving realistic responses in the thermosphere Failure to describe gravity wave breaking dynamics significantly decreases secondary wave generation at smaller spatial scales Thermospheric secondary gravity waves excited by mountain waves in the winter hemisphere have dominant horizontal scales of ∼100–200 km