A microphysics guide to cirrus – Part 3: Occurrence patterns of cloud particles

Abstract

Cloud particle size distributions (PSDs) are crucial in determining the clouds physical and optical properties and hence their radiative feedback to the climate. Here we present unprecedented occurrence patterns of cloud particles derived from 270 h of cloud measurements (≈ 975 000 PSDs). The focus of the analysis is on cirrus clouds, but liquid and mixed-phase clouds are also shown. In particular, cirrus PSDs for cold to warm cirrus temperatures and microphysically thin to thick cirrus clouds are provided in a novel presentation as heat maps. The observations are accompanied by simulations of ice crystal growth in in situ-origin cirrus, showing that the maximum size to which the cirrus ice crystals can grow increases from approx. 60 µm@T < 200 K to 230 µm@T > 220 K. Crystals larger than this size are most likely of liquid-origin. The combined evaluation of observations and simulations allows the attribution of processes shaping the PSDs. Important results are that, with increasing temperature and cirrus thickness, the most frequent ice particles change from smaller and fewer crystals of in situ-origin to larger and more crystals of both in situ and liquid-origin i.e. the cirrus type changes from in situ- to liquid-origin. In addition, three characteristic ice crystal size ranges are identified. The nucleation/evaporation size interval (∼ 3–20 µm), most frequent in the coldest, thinnest in situ-origin cirrus; the – most common – overlap size interval (∼ 20–230 µm), where both in situ-origin liquid-origin cirrus occur and the uplift/sedimenation size interval (>∼ 230 µm), which consists mostly of liquid-origin ice crystals.