Abstract
The stable isotopes (2H, 18O) of precipitation change due to the sub-cloud secondary evaporation during raindrop fall. The study of the temporal and spatial variation of sub-cloud secondary evaporation and its causes by using hydrogen and oxygen stable isotopes is of great significance to the study of the regional water cycle process. Based on the hourly meteorological data of 648 meteorological stations in 17 provinces (cities) of the Yangtze River Basin from March 2018 to February 2019, we analyzed the temporal and spatial characteristics of precipitation excess deuterium variation (Δd) in the region, based on the improved Stewart model. We discuss the various influence factors under different magnitude Δd value change and the impact factor of each partition sub-cloud secondary evaporation influence of the difference. The results show the following: (1) In terms of hourly variation, the sub-cloud secondary evaporation in the daytime is stronger than that at night. In terms of monthly variation, different regions of the study area have different characteristics; that is, the effect of sub-cloud secondary evaporation is more significant in summer and autumn in the northern subtropics and south temperate zones, and in spring and summer in the mid-subtropics and plateau climate zones. (2) There were significant spatial differences in the study area in different seasons, and the effect of sub-cloud secondary evaporation was the most significant in the plateau climate area throughout the year. (3) When the rainfall is 0–5 mm, the temperature is >30 °C, the vapor pressure is <3 hPa, the relative humidity is 50–60%, and the raindrop diameter is 0.5–1 mm; the sub-cloud secondary evaporation effect is the most obvious.
Highlights
Hydrogen and oxygen stable isotopes (2 H, 18 O) are important components in water molecules [1,2], and deuterium excess (d-excess) is a second-order isotopic parameter connecting stable hydrogen and oxygen isotopes [3,4]
The evaporation of raindrops falling from the bottom of the cloud layer to the ground and passing through unsaturated air, a process called sub-cloud secondary evaporation, enriches the heavy isotopes (18 O, 2 H, etc.) in the precipitation and decreases deuterium excess, thereby changing the isotope composition of precipitation [10,11,12]
Further studies on the sub-cloud secondary evaporation effect of precipitation isotope in these areas can provide a scientific basis for the study of the regional water-cycle process by using stable isotopes of precipitation
Summary
Hydrogen and oxygen stable isotopes (2 H, 18 O) are important components in water molecules [1,2], and deuterium excess (d-excess) is a second-order isotopic parameter connecting stable hydrogen and oxygen isotopes [3,4]. The stable isotopes of hydrogen and oxygen (2 H, 18 O), as well as the deuterium excess, can provide a deeper understanding of the existence and influencing factors of the sub-cloud secondary evaporation effect. They are of great significance for further study of the regional water cycle. The temperature and relative humidity during the fall of raindrops are the main factors This model provides theoretical support for the subsequent sub-cloud secondary evaporation isotope fractionation. Froehlich et al [10] found that when the remaining fraction of raindrop mass after evaporation (f ) in the study area is large, the f and the deuterium excess variation (∆d) have a linear relationship of 1‰/1%
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