Abstract
Mixed-phase clouds consist of both supercooled liquid water droplets and solid ice crystals. Despite having a significant impact on Earth‘s climate, mixed-phase clouds are poorly understood and not well represented in climate prediction models. One piece of the puzzle is understanding and parameterizing riming of mixed-phase cloud ice crystals, which is one of the main growth mechanisms of ice crystals via the accretion of small, supercooled droplets. Especially the extent of riming on ice crystals smaller than 500 μm is often overlooked in studies – mainly because observations are scarce. Here, we investigated riming in mixed-phase clouds during three airborne campaigns in the Arctic, the Southern Ocean and US east coast. Riming was observed from stereo-microscopic cloud particle images recorded with the Particle Habit Imaging and Polar Scattering (PHIPS) probe. We show that riming is most prevalent at temperatures around −7 °C, where, on average, 43 % of the investigated particles in a size range from 100 ≤ D ≤ 700 μm showed evidence of riming. We discuss the occurrence and properties of rimed ice particles and show correlation of the occurrence and the amount of riming with ambient meteorological parameters. We show that riming fraction increases with ice particle size (< 20 % for D ≤ 200 μm, 35–40 % for D ≥ 400 μm) and liquid water content (25 % for LWC ≤ 0.05 g m−3, up to 60 % for LWC = 0.5 g m−3). We investigate the ageing of rimed particles and the difference between "normal" and "epitaxial" riming based on a case study.
Highlights
Mixed-phase clouds (MPCs), consisting of both supercooled liquid droplets and ice particles, play a major role in the life cycle of clouds and the radiative balance of the Earth (e.g. Korolev et al.2017)
We show that riming fraction increases with ice particle size
ACLOUD and SOCRATES as well as the riming fraction in correlation with different ambient parameters: Temperature (a), area-eq diameter of the underlying ice particle measured by Particle Habit Imaging and Polar Scattering (PHIPS) (b), CDP liquid water content (c) and vertical HCR Doppler velocity (d)
Summary
Mixed-phase clouds (MPCs), consisting of both supercooled liquid droplets and ice particles, play a major role in the life cycle of clouds and the radiative balance of the Earth (e.g. Korolev et al.2017). The ice particle can accrete even more droplets and grow further until it reaches the ground as graupel Ice particle growth, both in size and mass, can change cloud lifetime and radiative properties. Multiple studies have used radar measurements to retrieve information about snow and riming density based on their vertical Doppler velocity (Mosimann et al 1993; Leinonen and Szyrmer 2015; Leinonen et al 2018; Mason et al 2018; Kneifel and Moisseev 2020) Those methods proved to be fit to determine the riming state of large, precipitating snow and graupel particles.
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