Gravity-wave-induced cross-isentropic mixing: a DEEPWAVE case study

Abstract

Orographic gravity waves (i.e., mountain waves) can potentially lead to cross-isentropic fluxes of trace gases via the generation of turbulence. During the DEEPWAVE (Deep Propagating Gravity Wave Experiment) campaign in July 2014, we performed tracer measurements of carbon monoxide (CO) and nitrous oxide (N2O) above the Southern Alps during periods of gravity wave activity. The measurements were taken along two stacked levels at 7.9 km in the troposphere and 10.9 km in the stratosphere. A detailed analysis of the observed wind components shows that both flight legs were affected by vertically propagating gravity waves with momentum deposition and energy dissipation between the two legs. Corresponding tracer measurements indicate turbulent mixing in the region of gravity wave occurrence.

For the stratospheric data, we identified mixing leading to a change of the cross-isentropic tracer gradient of N2O from the upstream to the downstream region of the Southern Alps. Based on the quasi-inert tracer N2O, we identified two distinct layers in the stratosphere with different chemical composition on different isentropes as given by constant potential temperature Θ. The CO–N2O relationship clearly indicates that irreversible mixing between these two layers occurred. Further, we found a significant change of the vertical profiles of N2O with respect to Θ from the upstream to the downstream side above the Southern Alps just above the tropopause. A scale-dependent gradient analysis reveals that this cross-isentropic gradient change of N2O is triggered in the region of gravity wave occurrence.

The power spectra of the in situ measured vertical wind, Θ, and N2O indicate the occurrence of turbulence above the mountains associated with the gravity waves in the stratosphere. The estimated eddy dissipation rate (EDR) based on the measured three-dimensional wind indicates a weak intensity of turbulence in the stratosphere above the mountain ridge. The Θ–N2O relation downwind of the Alps modified by the gravity wave activity provides clear evidence that trace gas fluxes, which were deduced from wavelet co-spectra of vertical wind and N2O, are at least in part cross-isentropic.

Our findings thus indicate that orographic waves led to turbulent mixing on both flight legs in the troposphere and in the stratosphere. Despite only weak turbulence during the stratospheric leg, the cross-isentropic gradient and the related composition change on isentropic surfaces from upstream to downstream of the mountain unambiguously conserves the effect of turbulent mixing by gravity wave activity on the trace gas distribution prior to the measurements. This finally leads to irreversible trace gas fluxes across isentropes and thus has a persistent effect on the upper troposphere and lower stratosphere (UTLS) trace gas composition.