A High‐Resolution Whole‐Atmosphere Model With Resolved Gravity Waves and Specified Large‐Scale Dynamics in the Troposphere and Stratosphere.

We present a new version of the HIgh Altitude Mechanistic general Circulation Model (HIAMCM) with specified dynamics. We utilize a spectral method that nudges only the large‐scale flow to MERRA‐2 reanalysis. The nudged HIAMCM simulates gravity waves (GWs) down to horizontal wavelengths of about 200 km from the troposphere to the thermosphere like the free‐running model, including the generation of secondary and tertiary GWs. Case studies show that the simulated large‐scale GWs are consistent with those in the reanalysis, while the medium‐scale GWs compare well with observations in the northern winter 2016 stratosphere from the Atmospheric InfraRed Sounder (AIRS). GWs having wavelengths larger than about 1,350 km can be described with the nonlinear balance equation. The GWs relevant in the stratosphere, however, have smaller scales and require a different approach. We propose that the GW amplification due to kinetic energy transfer from the large‐scale flow combined with GW potential energy flux convergence helps to identify the mesoscale GW sources due to spontaneous emission. The GW amplification is strongest in the region of maximum large‐scale vertical wind shear in the mid‐stratosphere. Maps of the time‐averaged stratospheric GW activity simulated by the HIAMCM and computed from AIRS satellite data show a persistent hot spot over Europe during January 2016. At about 40 km, the average GW amplitudes are maximum in the region of fastest large‐scale flow. We argue that refraction of GWs originating in the troposphere, as well as GWs from spontaneous emission in the stratosphere contribute to this effect. Plain Language Summary: We present a new version of the HIgh Altitude Mechanistic general Circulation Model (HIAMCM) with specified dynamics in the troposphere, stratosphere, and lower mesosphere. The HIAMCM is a spectral model with high spatial resolution (the shortest horizontal wavelength is about 156 km) and a model top at about 450 km. The nudging of the model to reanalysis is formulated in spectral space and restricted to the large‐scale flow. This ensures that gravity waves (GWs) are self‐consistently simulated over the whole altitude range like in the free‐running model. The simulated large‐scale GWs are quantitatively consistent with those in the reanalysis, while the medium‐scale GWs compare well with satellite observations in the northern winter 2016 stratosphere. We analyze a case of GW generation in the stratosphere from imbalance of the polar vortex. We propose that this process can be identified by the transfer of kinetic energy transfer from the large‐scale flow to the GWs combined with the GW potential energy flux convergence. The in‐situ generation of GWs in the stratosphere likely contributes to the averaged distribution of GW activity in the upper stratosphere, which is maximum around the edge of the polar vortex. Key Points: Nudging in spectral space allows for self‐consistent simulation of gravity waves up to the thermosphereSimulated gravity waves in the winter stratosphere agree well with satellite observationsSpontaneous emission of GWs in the winter stratosphere depends critically on vertical wind shear [ABSTRACT FROM AUTHOR]