Dr. Erich Becker (NWRA) will deliver a talk on the topic, "Multi-step vertical coupling and secondary gravity waves after the Tonga eruption" in IAP colloquium on 2nd March 2023. 
He will discuss about the multi-step vertical coupling (MSVC) and how it describes a paradigm shift regarding the role of gravity waves (GWs) in the winter middle and upper atmosphere. It is well known that primary GWs propagate into the winter stratosphere and lower mesosphere, where they dissipate. However, since this process is localized in space and intermittent, secondary GWs are generated. These propagate into the lower thermosphere, dissipate, and generate tertiary GWs and so forth. Recent modeling and observational studies showed that secondary and tertiary GWs from MSVC are the predominant GWs in the upper mesosphere and in the thermosphere during wintertime. MSVC cannot be simulated with GW parameterizations as used in conventional whole atmosphere models. The HIgh Altitude Mechanistic general Circulation Model (HIAMCM) resolves medium-scale GWs from the surface up to z~450 km, including MSVC induced by primary GWs from jets, fronts, and orography. This is made possible by combining a sufficiently high spatial resolution with advanced methods for turbulent and molecular diffusion. For the simulation of observed events, the large-scales of the HIAMCM can be nudged to MERRA-2 reanalysis.

Even the highest resolutions feasible in GW-resolving whole-atmosphere models are by far not sufficient to simulate the primary GWs from deep convection or volcanic eruptions. If MSVC from such primary GWs needs to be taken into account, the localized and intermittent ambient-flow effects that generate secondary GWs at higher altitudes must be calculated by other means and then implemented into the global model. We perform these calculations using the Model for gravity wavE SOurce, Ray trAcing and reConstruction (MESORAC). Furthermore, the neutral dynamics from the HIAMCM can be coupled to the ionospheric model SAMI-3.  

He will present recent applications of the HIAMCM regarding the thermospheric response during the sudden stratospheric warming in 2017. Furthermore, he will present the results from the coupled MESORAC-HIAMCM-SAMI3 model system for the Tonga volcanic eruption that occurred on 15 January 2023. The presentation will show that the primary GWs from this event could not account directly for any of the far-field effects observed in the thermosphere/ionosphere around the globe.  Rather, the secondary GWs generated by the dissipation of the primary GWs from the Tonga event  can explain many of the spectacular global wave effects in the upper atmosphere. The SAMI-3 simulation shows the development  of ``super equatorial plasma bubbles'' over the Pacific in response to Tonga. Overall, their studies provide strong evidence that the upper atmospheric disturbances caused by the Tonga event were not due to Lamb waves (as has been speculated in many studies about this event), but were due to MSVC.