Oxygen isotopic variation of falling snow particles with time during the lifetime of a convective cloud: observation and modelling

The δ¹⁸O (per mille deviation from SMOW) of falling snow particles was observed at two stations 8 km apart along the trajectory of a convective cloud. The comparison of the difference in δ¹⁸O between the two stations with radar observation reveals the following: (1) the δ18O 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; (2) the rate of decrease was large in cells in which precipitation intensity was large in the windward area. A one-dimensional time-dependent model is used to calculate the isotopic variation of falling snow 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 δ¹⁸O is large in cells with high precipitation efficiency, and in cells with large terminal velocity of precipitating particles.