ALBERT

Description: Stable water isotopes are powerful indicators of meteorological processes on a broad range of scales, reflecting evaporation, condensation, and airmass 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 high temporal resolution, enabling to tremendously extend the measurement data base in space and time. Here we present the first set of airborne spectroscopic stable water isotopes measurements over the western Mediterranean. Measurements have been acquired by a customised Picarro L2130-i cavity-ring down spectrometer deployed onboard of the Dornier 128 D-IBUF aircraft together with a meteorological flux measurement package during the HyMeX SOP1 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 in 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 of the stable isotope parameters δD, δ18O and d-excess during the study period. A time-averaged perspective of the vertical stable isotope composition reveals for the first time the mean vertical structure of stable water isotopes over the Mediterranean at high resolution. A d-excess minimum in the overall average profile is reached in the region of the boundary layer top due to precipitation evaporation, bracketed by higher d-excess values near the surface due to non-equilibrium fractionation and above the boundary layer due to the non-linearity of the d-excess definition. Repeated flights along the same pattern reveals 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.