Abstract

Abstract. The importance of Arctic mixed-phase clouds on radiation and the Arctic climate is well known. However, the development of mixed-phase cloud parameterization for use in large scale models is limited by lack of both related observations and numerical studies using multidimensional models with advanced microphysics that provide the basis for understanding the relative importance of different microphysical processes that take place in mixed-phase clouds. To improve the representation of mixed-phase cloud processes in the GISS GCM we use the GISS single-column model coupled to a bin resolved microphysics (BRM) scheme that was specially designed to simulate mixed-phase clouds and aerosol-cloud interactions. Using this model with the microphysical measurements obtained from the DOE ARM Mixed-Phase Arctic Cloud Experiment (MPACE) campaign in October 2004 at the North Slope of Alaska, we investigate the effect of ice initiation processes and Bergeron-Findeisen process (BFP) on glaciation time and longevity of single-layer stratiform mixed-phase clouds. We focus on observations taken during 9–10 October, which indicated the presence of a single-layer mixed-phase clouds. We performed several sets of 12-h simulations to examine model sensitivity to different ice initiation mechanisms and evaluate model output (hydrometeors' concentrations, contents, effective radii, precipitation fluxes, and radar reflectivity) against measurements from the MPACE Intensive Observing Period. Overall, the model qualitatively simulates ice crystal concentration and hydrometeors content, but it fails to predict quantitatively the effective radii of ice particles and their vertical profiles. In particular, the ice effective radii are overestimated by at least 50%. However, using the same definition as used for observations, the effective radii simulated and that observed were more comparable. We find that for the single-layer stratiform mixed-phase clouds simulated, process of ice phase initiation due to freezing of supercooled water in both saturated and subsaturated (w.r.t. water) environments is as important as primary ice crystal origination from water vapor. We also find that the BFP is a process mainly responsible for the rates of glaciation of simulated clouds. These glaciation rates cannot be adequately represented by a water-ice saturation adjustment scheme that only depends on temperature and liquid and solid hydrometeors' contents as is widely used in bulk microphysics schemes and are better represented by processes that also account for supersaturation changes as the hydrometeors grow.

Highlights

  • The surface energy budget over the Arctic ice pack is determined to a large extent by radiative fluxes that in turn are strongly dependent on the presence of clouds

  • We perform sensitivity simulations with and without ice microphysics to evaluate the impact of the CCN spectrum shape, process of warm rain formation, different ice initiation mechanisms, and the Bergeron-Findeisen process on glaciation time and longevity of mixed-phase clouds observed during the ARM Mixed-Phase Arctic Cloud Experiment (MPACE) Intensive Observing Period (IOP)

  • Based on differences between our sensitivity simulations that do not include ice microphysics, we find that the process of water-water interaction may be relatively minor compared to that of the CCN spectrum shape for droplet activation for the MPACE single-layer mixed-phase clouds

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Summary

Introduction

The surface energy budget over the Arctic ice pack is determined to a large extent by radiative fluxes that in turn are strongly dependent on the presence of clouds. A few BRM models use designated distribution functions for different types of ice hydrometeors and calculate growth rates of microphysical processes due to several transformations of liquid and solid hydrometeors in mixed-phase clouds (Cotton, 1972b; Young, 1974; Scott and Hobbs, 1977; Chen and Lamb, 1994; Khain and Sednev, 1996; Reisin et al, 1996a; Takahashi and Shimura, 2004). The model is able to reproduce persistent mixedphase stratocumulus cloud decks as well as cloud microphysical properties (liquid and ice water content, droplet, and ice nuclei concentration profiles) within the observed ranges for particular combinations of ice formation mechanisms mentioned above They found that glaciation time and longevity of mixed-phase MPACE clouds are determined by formation of ice nuclei due to water drop evaporation and drop freezing during evaporation, whereas processes of ice multiplication were less important. In Appendix A we outline some details of the BRM scheme that are relevant to this study

Model description
Equations for size distribution functions
Initiation of liquid phase
Initiation of solid phase
Treatment of Bergeron-Findeisen process
Simulation setup
Results
Sensitivity runs with warm microphysics
Sensitivity runs with ice microphysics
Comparison with observations
Discussion

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