Abstract
Abstract. Stable isotopes of water vapour are powerful indicators of meteorological processes on a broad range of scales, reflecting evaporation, condensation, and air mass mixing processes. With the recent advent of fast laser-based spectroscopic methods, it has become possible to measure the stable isotopic composition of atmospheric water vapour in situ at a high temporal resolution. Here we present results from such comprehensive airborne spectroscopic isotope measurements in water vapour over the western Mediterranean at a high spatial and temporal resolution. Measurements have been acquired by a customized Picarro L2130-i cavity-ring down spectrometer deployed onboard the Dornier 128 D-IBUF aircraft together with a meteorological flux measurement package during the HyMeX SOP1 (Hydrological cycle in Mediterranean Experiment special observation period 1) field campaign in Corsica, France, during September and October 2012. Taking into account memory effects of the air inlet pipe, the typical time resolution of the measurements was about 15–30 s, resulting in an average horizontal resolution of about 1–2 km. Cross-calibration of the water vapour measurements from all humidity sensors showed good agreement under most flight conditions but the most turbulent ones. In total 21 successful stable isotope flights with 59 flight hours have been performed. Our data provide quasi-climatological autumn average conditions and vertical profiles of the stable isotope parameters δD, δ18O, and d-excess during the study period. A d-excess minimum in the overall average profile is reached in the region of the boundary-layer top, possibly caused by precipitation evaporation. This minimum is bracketed by higher d-excess values near the surface caused by non-equilibrium fractionation, and a maximum above the boundary layer related to the increasing d-excess in very depleted and dry high-altitude air masses. Repeated flights along the same pattern reveal pronounced day-to-day variability due to changes in the large-scale circulation. During a period marked by a strong inversion at the top of the marine boundary layer, vertical gradients in stable isotopes reached up to 25.4 ‰ 100 m−1 for δD and 24.0 ‰ 100 m−1 for the d-excess.
Highlights
The vertical distribution of water vapour in the atmosphere is highly variable
The isotopic composition in the lower troposphere and boundary layer themselves has so far not been interpreted with respect to the unprecedented information it can provide about the atmospheric transport history of water vapour
We report on airborne measurements of δD, δ18O, and the d-excess obtained from a custom-modified commercial cavity-ring down spectrometer (CRDS) with an enhanced laser setup
Summary
The vertical distribution of water vapour in the atmosphere is highly variable. It is shaped by source, transport, and sink processes, such as evaporation and condensation, horizontal and vertical advection, and mixing as determined by the atmospheric stability. Thereafter, Taylor (1972) published a key data set of the isotopic composition in atmospheric water vapour from 20 flights made during 1967/68 over southern Germany These profiles showed substantial vertical and temporal (daily to seasonal) variation. Since the first airborne deployment of a laser spectrometer by Webster and Heymsfield (2003) in the tropical UT/LS, a number of studies have investigated the isotopic composition in stratospheric water vapour and the tropical tropopause layer (Hanisco et al, 2007; Iannone et al, 2009; Sayres et al, 2009; Dyroff et al, 2010). The isotopic composition in the lower troposphere and boundary layer themselves has so far not been interpreted with respect to the unprecedented information it can provide about the atmospheric transport history of water vapour.
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