Abstract
Abstract. Cloud microphysics schemes in global climate models have long suffered from a lack of reliable satellite observations of cloud ice. At the same time there is a broad consensus that the correct simulation of cloud phase is imperative for a reliable assessment of Earth's climate sensitivity. At the core of this problem is understanding the causes for the inter-model spread of the predicted cloud phase partitioning. This work introduces a new method to build a sound cause-and-effect relation between the microphysical parameterizations employed in our model and the resulting cloud field by analysing ice formation pathways. We find that freezing processes in supercooled liquid clouds only dominate ice formation in roughly 6 % of the simulated clouds, a small fraction compared to roughly 63 % of the clouds governed by freezing in the cirrus temperature regime below −35 ∘C. This pathway analysis further reveals that even in the mixed-phase temperature regime between −35 and 0 ∘C, the dominant source of ice is the sedimentation of ice crystals that originated in the cirrus regime. The simulated fraction of ice cloud to total cloud amount in our model is lower than that reported by the CALIPSO-GOCCP satellite product. This is most likely caused by structural differences of the cloud and aerosol fields in our model rather than the microphysical parametrizations employed.
Highlights
Clouds are an important modulator for Earth’s climate
With 2M as the starting configuration, we investigate the effect of the artificial sedimentation–cloud-cover feedback introduced by the new cloud cover scheme by limiting ice growth to liquid water saturation (LIM_ICE) and use an updated version of the cirrus cloud parametrization which includes heterogeneous ice nucleation on mineral dust below −35 ◦C (HET_CIR)
We presented the global performance of the new cloud microphysics scheme in the ECHAM6-HAM2 general circulation models (GCMs) introduced in D18
Summary
Clouds are an important modulator for Earth’s climate. They exert a net radiative effect of approximately −20 W m−2 and significantly cool the planet (Boucher et al, 2013). There, ice crystals can freeze homogeneously from pre-activated cloud droplets (Lohmann et al, 2016) and deliquesced aerosols (Koop et al, 2000) or nucleate directly on an ice nucleating particle (INP) The latter two processes do not require water saturation, leading to fundamentally different types of cirrus clouds (Krämer et al, 2016; Wernli et al, 2016; Gasparini et al, 2018) with significant differences in the respective microphysical and optical properties, highlighting the need for a correct representation of ice formation pathways of all clouds containing ice, not just those in the mixed-phase regime.
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