Energy flux diagnosis of secondary mountain wave generation

During the Terrain-Induced Rotor Experiment (T-REX) field campaign of March and April 2006, the new NCAR High-performance Airborne Platform for Environment Resarch (HIAPER) Gulfstream V aircraft was used to observe mountain waves in the upper troposphere and lower stratosphere. The introduction of GPS allowed for the direct measurement of energy fluxes. Such analysis has not previously been possible using aircraft data in gravity wave experiments.

Wavelet techniques were employed to document spatial and spectral properties of mountain waves using in-situ data. Wavelet cross-spectrum techniques allowed for the isolation of packets of up-going and down-going energy. Wavelet cospectra were used to examine the vertical and horizontal energy transport of propagating waves while the addition quadrature spectra were used to examine wave trapping. Energy flux signatures of vertically propagating waves, trapped waves, and down-going waves were shown have unique signatures that allow for classification of wave packets on the basis of their associated vertical and horizontal energy fluxes. Wavelet cross-spectra phase relations also allow for the determination of the direction of wave propagation.

Wave properties over the Sierra Nevada were analyzed for twelve cases with cross-barrier flow. Surprisingly, down-going gravity waves were found to be nearly as common as up-going waves. Additional investigation has revealed at least three cases of down-going waves produced from secondary wave generation collocated with the primary vertically propagating mountain waves. Large amplitude vertically propagating waves were commonly be superposed with a down-going wave in the same location. Down-going waves were sometimes stronger than up-going mountain waves leading to a net negative energy flux in at least two cases. A case from a T-REX ferry flight over the Wasatch Mountains is suggestive of in-situ sampling of gravity wave breaking and an associated vertical energy flux reversal.

The nearly ubiquitous presence of wave trapping and secondary generation in the twelve T-REX flights is suggestive that the paradigm of a pure vertically propagating wave may be an unrealistic oversimplification that rarely if ever occurs in the real world. Linear and non-linear mountain wave simulations were conducted using WRF and COAMPS to test the feasibility of wavelet studies of wave breaking and secondary wave generation using numerical models. Wave reflection off of the upper boundary was found to be the largest initial hurdle in such numerical simulations and reinforced the idea that upward going energy must be conserved. If a wave is unable to reach the upper stratosphere and mesosphere than its energy must go somewhere: seemingly always into trapped and downward propagating waves.