Factors affecting isotopic composition of the rainwater in the Negev Desert, Israel

The study examines the δ¹⁸O of rain observed in the middle of the Negev Desert, Israel, based on seven rainstorms, associated with convective clouds resulting from midlatitude cyclones found over Trans-Jordan. The analysis is based on synoptic maps, vertical atmospheric cross sections, and air back trajectories. The rainwater sampling was done in temporal scale of hours. Four known factors were addressed here: the temperature effect, the amount effect, and two geographical effects: the marine and continental effects. The temperature effect is expressed by a positive relation between the lower level temperature and the isotopic composition. The amount effect, related to rainfall, is negative. The marine effect, expressed by a relation between the time the rain-producing air spends above the Mediterranean Sea, is positive and the continental effect, associated with the passage over land is negative. The linear relationships found in this study met the expectations, although only the amount effect was statistically significant. A fifth effect, entitled cloud top effect proposed here, relates to the height of the convective cloud layer top. It assumes that lower top prevents entrainment of upper cold air into the clouds, so the isotopic composition is enriched. The cloud top effect was found most statistically significant. Key Points Analysis of isotopic composition of rain in the Negev, Israel Air-trajectory analysis for assessment of continental and marine effects Cloud top is proposed and found as significant factor