Impacts of Limited Model Resolution on the Representation of Mountain Wave and Secondary Gravity Wave Dynamics in Local and Global Models. 1: Mountain Waves in the Stratosphere and Mesosphere. 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: mountain waves -- instabilities and turbulence -- resolution influences on resolved fields and momentum fluxes -- resolution needs for modeling of gravity wave dynamics; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2021JD035990
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Long‐term efforts have sought to extend global model resolution to smaller scales enabling more accurate descriptions of gravity wave (GW) sources and responses, given their major roles in coupling and variability throughout the atmosphere. Such studies reveal significant improvements accompanying increasing resolution, but no guidance on what is sufficient to approximate reality. We take the opposite approach, using a finite‐volume model solving the Navier‐Stokes equations exactly. The reference simulation addresses mountain wave (MW) generation and responses over the Southern Andes described using isotropic 500 m, central resolution by Fritts et al. (2021), https://doi.org/10.1175/JAS-D-20-0207.1 and Lund et al. (2020), https://doi.org/10.1175/JAS-D-19-0356.1 . Reductions of horizontal resolution to 1 and 2 km result in (a) systematic increases in initial MW breaking altitudes, (b) weaker, larger‐scale generation of secondary GWs and acoustic waves accompanying these dynamics, and (c) significantly weaker and less extended responses in the mesosphere in latitude and longitude. Horizontal resolution of 4 km largely suppresses instabilities, but allows weak, sustained mean‐flow interactions. Responses for 8 km resolution are very weak and fail to capture any aspects of the high‐resolution responses. The chosen mean winds allow efficient MW penetration into the mesosphere and lower thermosphere, hence only exhibit strong pseudo‐momentum deposition and mean wind decelerations at higher altitudes. A companion paper by Fritts et al. (2022), https://doi.org/10.1029/2021JD036035 explores the impacts of decreasing resolution on responses in the thermosphere. Plain Language Summary: Mountain waves play major roles in the large‐scale circulation and structure of the atmosphere extending well into the thermosphere. Their primary influences at large scales depend on energy and momentum transports from sources at lower altitudes and deposition at higher altitudes accompanying wave‐breaking dynamics and instabilities that cannot currently be described by global models. Responses to these small‐scale dynamics in the stratosphere and lower mesosphere include local mean flow decelerations that extend large distances downstream, upstream, and laterally. The ability to describe these dynamics degrades rapidly with decreasing resolution, causing current global models to exhibit systematic circulation biases that significantly limit and degrade weather and climate prediction. We assess these impacts with a suite of simulations of wintertime flow over the S. Andes at horizontal resolutions from 0.5 to 8 km. Our results reveal that spatial resolution of ∼2 km or better is required to adequately address these deficiencies extending into the mesosphere. Key Points: Simulations of mountain wave (MW) dynamics reveal decreasing fidelity in MW and large‐scale fields at degraded resolution Two‐km resolution or better yields reasonable fidelity to MW fields, momentum fluxes, and large‐scale fields into the mesosphere Four‐km resolution fails to capture realistic MWs, fluxes, and responses; 8 km resolution yields no resemblance to well‐resolved MW fields