MIMAS (Mesospheric Ice Microphysics And tranSport model) describes the formation of noctilucent clouds by tracing an ensemble of 40 million condensation nuclei. Atmospheric models such as LIMA and KMCM generate time-dependent simulations of atmospheric temperature, winds and air density. These so-called dynamic fields are used by MIMAS to transport water vapor and condensation nuclei. At altitudes around 88 km and temperatures below -140°C, ice particles form around the condensation nuclei. These grow quickly within the layer around 83 km, where low temperatures (about -130°C) are combined with sufficient water vapor. After growing to the typical NLC particle size of ~50 nm, the ice particles sediment to lower, warmer altitudes increasingly rapidly, and sublimate only a few hours after they become visible.
The following animation shows a noctilucent cloud simulated with MIMAS, using dynamic fields from LIMA. NLC occur in an area around the pole extending roughly to 50°N, shown by the colored contours. The frequency and brightness of the ice clouds decreases with distance from the pole, NLC south of 60°N are uncommon. Only the part of the ice cloud along the terminator (twilight zone) is visible by the naked eye: The mesosphere in 83 km altitude is still illuminated by the sun at a solar elevation of up to 9° below the horizon, which enables NLC particles to scatter sunlight while it is already night for an observer on the ground. The noctilucent cloud itself moves slowly eastward, while the constitutent ice particles (black dots) are transported by the westward directed wind through the cloud.
MIMAS is also used to trace the development of NLC particles: At 69°N, less than 1 percent of formed ice particles grow to a radius of 50 nm, those particles reach an age of 36 hours on average. The main growth phase of visible ice particles has a duration of less than 6 hours, particle sedimentation is compensated by upwelling during this period. The size of NLC particles can be validated by lidars. These measurements also indicate that the particles are not spherical, but rather a mix of elongated and flattened cylindrical shapes.
- J. Kiliani, G. Baumgarten, F.-J. Lübken, U. Berger, and P. Hoffmann, Temporal and spatial characteristics of the formation of strong noctilucent clouds, J. Atmos. Solar-Terr. Phys., 104, 151-166, doi:10.1016/j.jastp.2013.01.005, 2013.
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