Turbulence, Waves and Rotational Wind Shears in the Upper Troposphere and Lower Stratosphere (UTLS): Measurements, High Resolution Simulations and Numerical Prediction Challenges

Turbulence, waves and rotational wind shears in the upper troposphere and lower stratosphere (UTLS) are investigated for two Intensive Observational Periods (IOPs) of the Terrain-induced Rotor Experiment (T-REX) conducted in Owens Valley, CA. The simulations use a UTLS micro-scale model that is driven by the finest mesoscale nest. During IOP8, the simulation results reveal presence of perturbations with short wavelengths in zones of strong vertical wind shear in the UTLS that cause a reversal of momentum fluxes. The spectral properties of these perturbations and the attendant vertical profiles of heat and momentum fluxes show strong divergence near the tropopause indicating that they are generated by shear instability along shear lines locally induced by the primary mountain wave originating from the lower troposphere. This is further confirmed by results of an idealized simulation initialized with the temperature and wind profiles obtained from the micro-scale model. The polarization relation between the horizontal wind components is evidenced by the hodograph of the horizontal wind vector. The polarized Richardson number is introduced to characterize the instabilities of non-parallel shear flows induced by rotating winds and polarized inertia-gravity waves in UTLS. For IOP6, we analyze distributions of trace gases in UTLS observed in aircraft measurements. They show small scale fluctuations with amplitudes and phases that vary along the path of the flight. Detailed comparisons between these fluctuations and the observed vertical velocity show that the behavior of these short fluctuations is due not only to the vertical motion, but also to the local mean vertical gradients where the waves evolve, which are modulated by larger variations. The micro-scale model simulation results show favorable agreement with in situ radiosonde and aircraft observations of vertical velocity, horizontal wind and temperature. The high vertical resolution offered by the micro-scale model is found to be critical for resolution of smaller scale processes such as formation of inversion layers associated with trapped lee waves in the troposphere, and propagating mountain waves in the lower stratosphere. Localized sharp shear layers, wave breaking and stiff gradients of potential temperature and vertical velocity are resolved in the UTLS within the embedded micro-scale nest.

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