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

[摘要] Orographic gravity waves (i.e., mountain waves) can potentially lead to cross-isentropic fluxes of trace gases via the generation ofturbulence. During the DEEPWAVE (Deep Propagating Gravity Wave Experiment) campaign in July 2014, we performed tracer measurements of carbon monoxide( CO ) and nitrous oxide ( N 2 O ) above the Southern Alps during periods of gravity wave activity. The measurements were taken along twostacked levels at 7.9  km in the troposphere and 10.9  km in the stratosphere. A detailed analysis of the observed wind componentsshows that both flight legs were affected by vertically propagating gravity waves with momentum deposition and energy dissipation between the twolegs. 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 N 2 O from the upstream to thedownstream region of the Southern Alps. Based on the quasi-inert tracer N 2 O , we identified two distinct layers in the stratosphere withdifferent chemical composition on different isentropes as given by constant potential temperature  Θ . The CO – N 2 O relationshipclearly indicates that irreversible mixing between these two layers occurred. Further, we found a significant change of the vertical profiles of N 2 O with respect to  Θ from the upstream to the downstream side above the Southern Alps just above the tropopause. A scale-dependentgradient analysis reveals that this cross-isentropic gradient change of N 2 O is triggered in the region of gravity wave occurrence. The power spectra of the in situ measured vertical wind,  Θ , and N 2 O indicate the occurrence of turbulence above the mountainsassociated with the gravity waves in the stratosphere. The estimated eddy dissipation rate (EDR) based on the measured three-dimensional wind indicates aweak intensity of turbulence in the stratosphere above the mountain ridge. The Θ – N 2 O relation downwind of the Alps modified by thegravity wave activity provides clear evidence that trace gas fluxes, which were deduced from wavelet co-spectra of vertical wind and N 2 O , areat 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. Despiteonly weak turbulence during the stratospheric leg, the cross-isentropic gradient and the related composition change on isentropic surfaces fromupstream to downstream of the mountain unambiguously conserves the effect of turbulent mixing by gravity wave activity on the trace gas distributionprior to the measurements. This finally leads to irreversible trace gas fluxes across isentropes and thus has a persistent effect on the uppertroposphere and lower stratosphere (UTLS) trace gas composition.

[学科分类] 医学（综合）