Abstract
Abstract. In this work, we present the first study resolving the temporal evolution of δ2H and δ18O values in cloud droplets during 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. The 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 than other Central European Meteoric Water Lines. Decreasing δ values in the course of the campaign agree with seasonal trends observed in rain in central Europe. The deuterium excess was higher in clouds developing after recent precipitation revealing episodes of regional moisture recycling. The variations in δ values during one cloud event could either result from changes in meteorological conditions during condensation or from variations in the δ values of the water vapor feeding the cloud. To test which of both aspects dominated during the investigated 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 during two of the latter cloud events led to the strongest temporal changes in both δ2H (≈ 6‰ per hour) and δ18O (≈ 0.6‰ per hour). 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 and regional recycling of moisture.
Highlights
Stable water isotopologues (1H218O and 1H2H16O) are naturally available indicators of atmospheric processes involved in water vapor transport and phase changes on different time scales
We present the first water isotopic data set resolving in time the δ value variation of individual cloud events during the Hill Cap Cloud Thuringia 2010 (HCCT2010) campaign in Thuringia in September and October 2010
We presented the first study resolving the temporal variation of δ2H and δ18O values of cloud droplets during single cloud events
Summary
Stable water isotopologues (1H218O and 1H2H16O) are naturally available indicators of atmospheric processes involved in water vapor transport and phase changes on different time scales. On the time scale of days to years, measurements of stable water isotopologues in precipitation and analyzing their spatial and temporal distribution all over the globe improved our understanding of the hydrological cycle Dawson, 1998; Yakir and Sternberg, 2000; Farquhar et al, 2007). For these fields of application, a detailed mechanistic understanding of the isotopic signal in atmospheric water vapor and its changes during transport and precipitation formation is essential
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