Summary
This project focused on high-resolution numerical simulations of gravity wave (GW) dynamics in the atmosphere, from the surface to the lower thermosphere.  Specific topics include self-acceleration dynamics associated with GW packets, GW/small scale interactions, GW/tidal wind interactions, and secondary wave generation.  All of these topics are non-linear in nature and thus cannot be analyzed fully with approximate techniques.  High-resolution numerical simulation properly accounts for non-linearities and thus provides clear insight into some of the more complex GW processes found in the atmosphere.  All of the simulations performed and analyzed are highly novel and provide the first numerical databases for many of the configurations studied.

Intellectual Merit

The self-acceleration studies focused on the differences between packetslocalized in either two or three spatial dimensions.  The two cases couldbe realized by time-varying winds blowing over either an extended mountainridge, or over an isolated mountain peak.  While both cases exhibitpronounced self-acceleration dynamics leading to turbulence, the tendency forinstability is less in the 3D case due to the lateral spreading of the packetas it propagates upward.  Likewise, the strength of both the emittedacoustic and secondary gravity waves is reduced in the 3D case.  Thesecondary waves are also highly 3D in the 3D case and appear as ifgenerated by a point source when observed on a plane well above theturbulent zone (see the bottom row in Figure 1).

The GW/fine scale interaction studies focused on small-scale (1/80th of theGW vertical wavelength) variations in the mean wind and mean temperatureprofiles.  Perhaps surprisingly, these seemingly insignificant structuresalways resulted in more rapid instability and turbulent breakdown ofthe underlying GW packet, even when the perturbations enhancedthe background stability! (See Figure 2).  Careful analysis of theresults indicated that baroclinic generation of additional vorticity wasresponsible for the increased tendency for instability, irrespective of thedetails of the temperature perturbation.  This finding is especiallyimportant since ubiquitous small-scale variations in the atmospheric windand temperature fields are likely to trigger turbulence in waves thatare predicted to be stable by linear theory.

The GW/tidal wind interaction studies focused on simulations of mountainwaves generated by strong wintertime winds blowing over the Southern Andesrange.  These waves propagate into the mesosphere where they overturnand produce a large turbulent zone.  Emitted secondary waves thenpropagate into the thermosphere.  Careful analysis of the results showsthat instability occurs initially at two distinct levels where the tidal windfields produce critical levels for the mountain waves.  As time goes on,however, the turbulence zone spreads vertically and ultimately coversa region at least 50 km deep.  The secondary waves generated in the lowerbreaking zone propagate freely through the upper critical level sincethey have non-zero ground-based frequency (see Figure 3). Strong acousticwaves are also generated in the turbulent zones and these propagate freelyinto the thermosphere.

All of our simulations that produce turbulence also produce secondary gravitywaves as well as acoustic waves.  While secondary waves appear to be anintrinsic byproduct of GW breaking, their characterization is quite complex.Figures 4 and 5 show a set of remarkable results where turbulence generatedin the lee of the Sierra-Nevada range generates secondary waves that thenpropagate and break at an altitude in excess of 100 km.  As Figure 5 shows,the location and orientation of the secondary waves is radically differentfrom the primary waves.  The secondary waves are also inherentlythree-dimensional since they are generated by 3D turbulence.  Thus even aquasi-2D wave packet will launch a fully-3D secondary wave field.  While mostof the secondary wave energy is at larger scale than the incident wave, thespectrum is rather full.  The wavelength of the most energetic secondarywaves does not appear to scale very well with the observed extent of theturbulent zone or with the extent of the zone where the mean wind is alteredby wave momentum deposition.  Additional research will be required toparameterize the secondary waves.

Broader Impacts
Gravity waves play a critical role in transporting momentum and energyfrom the troposphere to the upper levels of the atmosphere.  While theseeffects have significant impacts on the global circulation they arenotoriously difficult to understand and to model.  The high-resolutionnumerical simulations completed here provide novel insight into thesecomplex dynamics and provide a rich numerical database for future studyby the atmospheric community at large. This work also resulted in several published peer-reviewed journal articles.

Last Modified: 10/26/2021
Modified by: Ling Wang