Secondary Gravity Waves From the Stratospheric Polar Vortex Over ALOMAR Observatory on 12-14 January 2016: Observations and Modeling

We analyze the gravity waves (GWs) observed by a Rayleigh lidar at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) (16.08 degrees E, 69.38 degrees N) in Norway at z similar to 20-85 km on 12-14 January 2016. These GWs propagate upward and downward away from z(knee) = 57 and 64 km at a horizontally-displaced location with periods tau(r) similar to 5-10 hr and vertical wavelengths lambda(z) similar to 9-20 km. Because the hodographs are distorted, we introduce an alternative method to determine the GW parameters. We find that these GWs are medium to large-scale, and propagate north/northwestward with intrinsic horizontal phase speeds of similar to 35-65 m/s. Since the GW parameters are similar above and below z(knee), these are secondary GWs created by local body forces (LBFs) south/southeast of ALOMAR. We use the nudged HIAMCM (HIgh Altitude Mechanistic general Circulation Model) to model these events. Remarkably, the model reproduces similar GW structures over ALOMAR, with z(knee) = 58 and 66 km. The event #1 GWs are created by a LBF at similar to 35 degrees E, similar to 60 degrees N, and z similar to 58 km. This LBF is created by the breaking and dissipation of primary GWs generated and amplified by the imbalance of the polar night jet below the wind maximum; the primary GWs for this event are created at z similar to 25-35 km at 49-53 degrees N. We also find that the HIAMCM GWs agree well with those observed by the Atmospheric InfraRed Sounder (AIRS) satellite, and that those AIRS GWs south and north of similar to 50 degrees N over Europe are mainly mountain waves and GWs from the polar vortex, respectively. Plain Language Summary Atmospheric gravity waves (GWs) are perturbations in the Earth's atmosphere which can be created by wind flow over mountains and breaking GWs. Here, a breaking GW is similar to the breaking of an ocean wave when it overturns. A breaking GW imparts momentum to the atmosphere, which creates secondary GWs. We report on the long-period inertia GWs seen over Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) in northern Norway during 12-14 January 2016. We find that the inertia GWs seen over ALOMAR were secondary GWs created by the breaking of primary GWs generated by the imbalance of the polar vortex. We did this via simulating this event with the HIAMCM model and directly comparing these results to lidar and Atmospheric InfraRed Sounder data. After we found that the HIAMCM results agreed very well with these data, we investigated the dynamics which led to the ALOMAR GWs using the HIAMCM model data. This is the first concrete model/data comparison study to show that GWs generated by the polar vortex are important for generating GWs observed in the Earth's mesosphere. This study also highlights the importance of the complicated process dubbed multi-step vertical coupling, for which secondary, not primary, GWs can explain the wintertime GWs seen in the mesosphere.

Automated summary

In this study, it was found that the long-period inertia gravity waves observed at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) were secondary gravity waves created by the breaking of primary gravity waves generated by the imbalance of the polar vortex. This research highlights the importance of the complicated process of multi-step vertical coupling, in which secondary gravity waves can explain the wintertime gravity waves observed in the mesosphere.