The isotopic study on solid precipitation from a convective cloud : observation and modeling 対流雲がもたらす降雪粒子の安定同位体組成に関する研究 : 観測と数値計算
The isotopic study on solid precipitation from a convective cloud : observation and modeling
対流雲がもたらす降雪粒子の安定同位体組成に関する研究 : 観測と数値計算
The isotopic composition of solid precipitation depends on the following processes: evaporation from the sea surface, condensation and sublimation of cloud particles, fromation of these particles into precipitation and dynamic process of the cloud itself. To clarify the behavior of water isotopes in each process from the observation of solid precipitation at the surface, observations at two stations along a cloud trajectory and the numerical modeling of a precipitation process were made on the isotopic composition of solid precipitation generating from a convective cloud.The observations were made at two stations 8 km apart along a cloud trajectory at lshikari plain in 1985 and 1986, and at two stations 30 km apart in Hokuriku in 1983.The δD and δ^18o values of snow particles from a separate cloud or a radar echo which consists of several cells were linearly related. The values of deuterium excess of observed δD-δ^18o relations were much greater than 10 and differed with each case, while the slope was 8.5 - 9.6 in 7 of 9 cases.Several conclusions about the evaporation process can be drawn from these observations. First, a cloud which produced solid precipitation observed at the two sites consists of air parcels containing initial water vapor with the same isotopic composition. Second, the isotopic composition of initial water vapor which makes a cloud differs from case to case. Finally, the observed large deuterium excess is caused by kinetic evaporation from the sea surface, and is too large to be explained by Merlivat and Jouzel's (1979) model.Linearity betweenδD and δ^18o of precipitation made it possible to clarify fractionation during condensation and sublimation of cloud particles. The observed slopes of δD-δ^18o relations indicate that the value of (αD^-1)/(α180^-1) during condensation is close to that determined by Merlivat and Nief(1967) and Majoube (1971) under isotopic equilibrium, while that during sublimation is 8% larger than that determined by Merlivat and Nief (1967) and Majoube (1970) under isotopic equilibrium,due to kinetic effects. The model by Jouzel and Merlivat(1984) explains the observed enlargement of the value of (αD^-1)/(α180^-1) .The constancy of the isotopic composition of initial water vapor in a cloud also made it possible to discuss the precipitation process which includes condensation and sublimation of cloud particles, formation of these particles into precipitation and the dynamic process of the cloud itself, from the difference in the δ^18o of solid precipitation from a convective cloud observed between the two stations 8 km apart along the cloud trajectory. The δ^18o values of snow from a convective cloud decrease with time during its lifetime; the rate of decrease observed in 7 cases of convective radar echoes was 0.2 to 3.1%｡ during 15 min of movement of radar echoes between two stations, and was large during and after its maximum precipitation intensity. The rate of decrease was high incells in which precipitation was intense in the windward area, and a very small decrease (0.27%｡) was observed in an underdeveloped cell.A one-dimensional time-dependent model is used to calculate the isotopic variation of solid precipitation during the lifetime of a convective cloud cell. The calculated results explain the observed results well. In addition, calculated results show that the rate of decrease of δ^18o is large in cells with high precipitation efficiency, and in cells with large terminal velocity of precipitating particles. The model results also indicate that the more precipitation has fallen from a cell, the smaller δ^18o of solid precipitation from the cell.
名古屋大学博士学位論文 学位の種類 : 理学博士(課程) 学位授与年月日 : 平成1年3月25日