Three‐dimensional modeling of the trajectories of visible noctilucent cloud particles: An indication of particle nucleation well below the mesopause

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Abstract
Noctilucent clouds (NLC) form in summer at high latitudes at altitudes close to 83 km. Visually they can be observed best in the latitude band 50° to 65°. The cloud particles are most likely transported by the upper atmosphere winds over substantial distances from their birth place (site of nucleation) to the site of their observation and subsequently to warmer regions where they sublimate again. Here we use a 3‐D Lagrangian ice transport model to identify the nucleation sites of NLC particles which are observed vertically above a latitude of 69°N (e.g., by ground‐based lidars at the ALOMAR observatory). We limit the study to particles which have radii larger than either 50 nm or 20 nm when being observed. We find that these particles were nucleated on average only 3 km higher than observed at 69°N. The latter statement implies that nucleation of NLC particles occurs on average more than a scale height below the local mesopause. Furthermore, NLC particles observed overhead at 69°N were nucleated on average about 9° in latitude north of the observation site. Their transport time between nucleation and reaching the observation site is clearly and positively correlated with the latitude difference between the nucleation and observation sites (a similar strong correlation does not exist between the radius and the transport time of the observed particle). The median value of transport times of NLC particles observed above 69°N is 36 hours.

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