The future of nuctilucent clouds

Noctilucent clouds (NLC) consist of water ice particles which appear in the summer mesopause region at middle and polar latitudes. They owe there existence to extremely low temperatures present in this part of the atmosphere. We have applied the background model LIMA (Leibniz Institute Model
of the Atmosphere) and a microphysical model MIMAS (Mesospheric Ice Microphysics And tranSport model) to study the long term historical development of NLC. More recently, we extended these studies including future climate change predictions by modifying the concentration of carbon dioxide and methane. Carbon dioxide leads to a cooling of nearly the entire middle atmosphere (fostering the conditions for the presence of NLC), whereas methane is nearly completely converted to water vapor in the mesosphere leading to larger and more abundent ice particles, i. e., to brighter and more frequent NLC. In this study we present model simulations of the future development of NLC. We investigate typical NLC parameters, such as mean particle radius, ice number densities, and backscatter coefficients, and their relationship to background conditions (temperature, water vapor). It turns out that ice particle parameters (size, backscatter) are nearly entirely determined by the amount of water vapor, whereas the (geometric) altitude of NLC is mainly given by a shrinking of the atmosphere (due to cooling) below NLC altitudes. The effective transport of water vapor known as `freeze drying' leads to a significant enhancement (nearly doubling) of water vapor at NLC heights within this century.