Abstract
Observations of polar mesospheric clouds have revealed the presence of solid ice particles in the upper mesosphere at high latitudes; however, their formation mechanism remains uncertain. In this study, we investigated the formation process of ice particles through nucleation from small amounts of water vapor at low temperatures. Previous studies that used classical nucleation theory have shown that amorphous solid water particles can nucleate homogeneously at conditions that are present in the mesosphere. However, the rate predictions for water in classical nucleation theory disagree with experimental measurements by several orders of magnitude. We adopted a semi-phenomenological model for the nucleation process, which corrects the evaluation of the molecular cluster formation energy using the second virial coefficient, which agrees with both experiments and molecular dynamics simulations. To calculate the nucleation process, we applied atmospheric conditions for the temperature, pressure, numerical density of dust grains, and cooling rate. The results indicate that homogeneous water nucleation is extremely unlikely to occur in the mesosphere, while heterogeneous nucleation occurs effectively. Dust grains generated by meteor ablation can serve as nuclei for heterogeneous nucleation. We also showed that the ice can form directly in a crystalline state, rather than an amorphous state.
Highlights
Observations of polar mesospheric clouds have revealed the presence of solid ice particles in the upper mesosphere at high latitudes; their formation mechanism remains uncertain
We adopted a semi-phenomenological model for the nucleation process, which corrects the evaluation of the molecular cluster formation energy using the second virial coefficient, which agrees with both experiments and molecular dynamics simulations
We considered how much of the water molecule was consumed by heterogeneous nucleation before tj, which is the time of the peak nucleation rate due to homogeneous nucleation
Summary
15 The summer polar mesosphere, located at altitudes of 80–90 km, is the coldest part of the Earth’s atmosphere. Clouds of ice particles can form at such heights, some of which are visible from the ground and are referred to as noctilucent clouds (Vaste, 1993). Noctilucent clouds are generally observed before sunrise and after sunset. Under similar conditions and at overlapping heights, strong radar echoes are observed, known as polar mesospheric summer echoes. Noctilucent clouds are related to the presence of water ice particles (Rapp and Lübken, 2004). Known as polar mesospheric clouds, have recently
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