Abstract
Abstract. The water vapor distribution in the upper troposphere–lower stratosphere (UTLS) region has a strong impact on the atmospheric radiation budget. Transport and mixing processes on different scales mainly determine the water vapor concentration in the UTLS. Here, we investigate the effect of mountain waves on the vertical transport and mixing of water vapor. For this purpose we analyze measurements of water vapor and meteorological parameters recorded by the DLR Falcon and NSF/NCAR Gulfstream V research aircraft taken during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) in New Zealand. By combining different methods, we develop a new approach to quantify location, direction and irreversibility of the water vapor transport during a strong mountain wave event on 4 July 2014. A large positive vertical water vapor flux is detected above the Southern Alps extending from the troposphere to the stratosphere in the altitude range between 7.7 and 13.0 km. Wavelet analysis for the 8.9 km altitude level shows that the enhanced upward water vapor transport above the mountains is caused by mountain waves with horizontal wavelengths between 22 and 60 km. A downward transport of water vapor with 22 km wavelength is observed in the lee-side of the mountain ridge. While it is a priori not clear whether the observed fluxes are irreversible, low Richardson numbers derived from dropsonde data indicate enhanced turbulence in the tropopause region related to the mountain wave event. Together with the analysis of the water vapor to ozone correlation, we find indications for vertical transport followed by irreversible mixing of water vapor. For our case study, we further estimate greater than 1 W m−2 radiative forcing by the increased water vapor concentrations in the UTLS above the Southern Alps of New Zealand, resulting from mountain waves relative to unperturbed conditions. Hence, mountain waves have a great potential to affect the water vapor distribution in the UTLS. Our regional study may motivate further investigations of the global effects of mountain waves on the UTLS water vapor distributions and its radiative effects.
Highlights
Water vapor is a major greenhouse gas in the upper troposphere–lower stratosphere (UTLS; Sherwood et al, 2010; Solomon et al, 2010)
Based on in situ aircraft measurements of water vapor and wind during the DEEPWAVE campaign, we combined selected methods to investigate the vertical transport of water vapor induced by mountain waves
While the meteorology of this day and the propagation of the observed mountain waves is discussed in Bramberger et al (2017) and Smith et al (2016), we concentrated on the effect of the mountain wave activity on the water vapor distribution in the UTLS
Summary
Water vapor is a major greenhouse gas in the upper troposphere–lower stratosphere (UTLS; Sherwood et al, 2010; Solomon et al, 2010). Changes in the water vapor distribution in the UTLS cause radiative forcing and may affect surface temperatures (Solomon et al, 2010; Riese et al, 2012). Gravity waves are known to play an important role in the circulation, structure and variability of the atmosphere (Fritts and Alexander, 2003). They distribute energy and momentum horizontally and vertically in the atmosphere (e.g., Smith et al, 2008; Geller et al, 2013; Wright et al, 2016)
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