1Institute of Agricultural Sciences, ETH Zurich, Zurich, Switzerland
2Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
3US Geological Survey, Reston, VA, USA
4Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA
5Leibniz Institute for Tropospheric Research, Leipzig, Germany
Abstract. In this work, we present the first study resolving the temporal evolution of δ2H and δ18O values in cloud droplets during the course of 13 different cloud events. The cloud events were probed on a 937 m high mountain chain in Germany in the framework of the Hill Cap Cloud Thuringia 2010 campaign (HCCT-2010) in September and October 2010. δ values of cloud droplets ranged from −77‰ to −15‰ (δ2H ) and from −12.1‰ to −3.9‰ (δ18O) over the whole campaign. The cloud water line of the measured δ values was δ2H=7.8×δ18O+13×10−3 which is of similar slope but with higher deuterium excess (d-excess) than Central European Meteoric Water Lines. While seasonality was reflected in decreasing δ values towards the colder season, d-excess of cloud samples was an indicator of air mass origin: polar air masses had a higher d-excess than Mediterranean air masses. The variations in δ values during one cloud event could either result from changes in meteorological conditions during condensation or from variations in δ values of the water vapor feeding the cloud. To test which of both aspect dominated during the measured cloud events, we modeled the variation in δ values in cloud water using a closed box model. We could show that the variation in δ values of two cloud events was mainly due to changes in local temperature conditions. For the other eleven cloud events the variation was most likely caused by changes in the isotopic composition of the advected and entrained vapor. Frontal passages led to the highest gradients both in δ2H (≈6‰ per hour) and δ18O (≈0.6‰ per hour) during two of the latter cloud events. Moreover, a detailed trajectory analysis for the two longest cloud events revealed that variations in the entrained vapor were most likely related to rain out or changes in relative humidity and temperature at the moisture source region or both. This study illustrates the sensitivity of stable isotope composition of cloud water to changes in large scale air mass properties.