An Isotopic Study on Meteoric Precipitation

Abstract

Abstract18O content in cloud droplets is higher in a closed system than in an open system. It is also higher under a non-equilibrium condition than under an equilibrium condition.In case of the frontal type of precipitation, the observed variation with time in 18O content agrees fairly well with the calculation using an open model. The change in 18O content in convective shower which gives snow pellets can be explained by an open model.The isotopic exchange between falling rain drops and vapor is more effective in enriching heavier isotopes than evaporation.The calculated rate constant of isotopic exchange reaction between falling rain drops and vapor is about half of that given by FRIEDMAN et al. (1962) . A smaller amount of isotopic exchange than calculation was observed in rain water.The relationship between D and 18O contents in precipitation under an equilibrium condition can be expressed as an approximate straight line starting from the original point with a slope less than (aD-1)/(a18O-1) in the region of slight isotopic fractionation.The deviations from the straight line are explained by the effect of kinetic processes. The downward shift below the line is due to kinetic evaporation, while the upward shift is due to either the supply of source vapor under a kinetic condition or isotopic exchange between rain drops and vapor which has evaporated under a kinetic condition.A constant term in Craig's equation for the relation between D and 18O contents is derived by the fact that 18O content in source vapor is lower by 1.2% on an average as compared with D content.

Hydrogen and oxygen isotopic characteristics of precipitation in coastal areas of Japan determined by observations for 23 years at Akita and for 1-2 years at other several localities

“…In the case of precipitation from stratiform clouds, vapor condensation progresses continuously from low to high altitudes according with the ascending of air mass on the discontinuous plain, and if cloud droplets fall as precipitation soon after condensation, the precipitation at a lower position of the stratiform cloud has higher isotopic ratios, as expressed by the Rayleigh-type fractionation. Therefore, the precipitation near the front has higher δD, while it has lower δD far from the front (Dansgaard, 1964;Miyake et al, 1968). The difference between June and July is thought to be due to the difference in average distance from the front, which means June has more contribution of precipitation far from the front, while July has that near the front.…”

“…Regarding the difference in the progress of Rayleightype fractionation, even in the case of vertical progress of isotopic fractionation in a convective cloud, large variation in δD as well as δ 18 O is observed at the ground, for instance, several ‰ change in δ 18 O of precipitation from the beginning to the end of a shower (Miyake et al, 1968;Sugimoto and Higuchi, 1989) or significant decreases in δD (10-30‰) and δ 18 O (2-3‰) of precipitation from a single air mass during movement inland of about 8 km (Sugimoto et al, 1988). As another example, Fig.…”

“…The linear relationship with a slope of 8 shown by Eq. (1) can be approximately explained by a simplified model of the isotopic fractionation in the precipitation forming process (e.g., Dansgaard, 1964;Miyake et al, 1968). When an air mass containing water vapor is cooled and the vapor is condensed (cloud droplet, liquid or solid), isotopic fractionation occurs between the condensed water and the residual vapor.…”

Stable isotopic composition of water vapor in the tropics

“…Finally, diffusive exchange of isotopes causes rain to scavenge heavy isotopes from the ambient vapor [Miyake et al, 1968], thereby reducing isotope ratios both in and downwind from regions of organized precipitation [Gedzelman and Lawrence, 1990].…”

A new interpretative framework for below-cloud effects on stable water isotopes in vapour and rain

“…Previous studies often investigated only one part of the two-phase system (e.g., Miyake et al, 1968;Celle-Jeanton et al, 2004;Barras and Simmonds, 2009;Risi et al, 2010;Muller et al, 2015;Managave et al, 2016). They studied rain in high temporal resolution and came up with partially contrasting explanations for the observed short-term isotopic variations.…”