Weather in mesospheric ice layers

Layers in the summer mesosphere are studied using an ice model which applies background conditions from a new model called LIMA (Leibniz Institute Middle Atmosphere Model). LIMA covers the height range 0-150 km with high resolution. At low altitudes LIMA assimilates ECMWF ERA 40 data which introduces variability in the upper atmosphere. LIMA adequately represents the conditions in the mesosphere/lower thermosphere. Ice formation is interactively coupled to background water vapour which leads to "freeze drying". Model ice layers vary in time and space and occasionally appear at mid latitudes. The geographical distribution of ice clouds generally agrees with observations. For example, the mean noctilucent cloud height at 69 degrees N is 83.8 km (observations: 83.3 km). The occurrence rates of (polar) mesosphere summer echoes from the model also agree with measurements. At high latitudes ice layers sometimes disappear ("ice holes"). From time to time wind fluctuations redistribute water vapor but in general freeze drying overwhelms.