The Origin of High Ice Crystal Number Densities in Cirrus Clouds

Abstract

Recent measurements with four independent particle instruments in cirrus clouds, which formed without convective or orographic influence, report high number densities of ice particles (as high as nice = 50 cm?3) embedded in broad density distributions (nice = 0.1?50 cm?3). It is shown here that small-scale temperature fluctuations related to gravity waves, mechanical turbulence, or other small-scale air motions are required to explain these observations. These waves have typical peak-to-peak amplitudes of 1?2 K and frequencies of up to 10 h?1, corresponding to instantaneous cooling rates of up to 60 K h?1. Such waves remain unresolved in even the most advanced state-of-the-art global atmospheric models. Given the ubiquitous nature of these fluctuations, it is suggested that the character of young in situ forming cirrus clouds is mostly determined by homogeneous freezing of ice in solution droplets, driven by a broad range of small-scale fluctuations (period ?a few minutes) with moderate to high cooling rates (1?100 K h?1).