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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  22 Oct 2020

22 Oct 2020

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This preprint is currently under review for the journal ACP.

An Estimate of Global, Regional and Seasonal Cirrus Cloud Radiative Effects Contributed by Homogeneous Ice Nucleation

David L. Mitchell1, John Mejia1, Anne Garnier2, Yuta Tomii1, Martina Krämer3,4, and Farnaz Hosseinpour1 David L. Mitchell et al.
  • 1Desert Research Institute, Reno, 89512-1095, USA
  • 2Science Systems and Applications, Inc., Hampton, Virginia, USA
  • 3Institute for Energy and Climate Research (IEK-7), Research Center Jülich, Jülich, Germany
  • 4Institute for Atmospheric Physics (IPA), Johannes Gutenberg University, Mainz, Germany

Abstract. There are two fundamental mechanisms through which cirrus clouds form; homo- and heterogeneous ice nucleation (henceforth hom and het). The relative contribution of each mechanism to ice crystal production often determines the microphysical and radiative properties of a cirrus cloud. This study attempts to estimate the radiative contribution of hom relative to het by constraining the cloud microphysics in a climate model to conform with satellite retrievals of cirrus cloud effective diameter De, where the sampled cirrus cloud base had a temperature T < 235 K (−38 °C). The CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite retrievals for cirrus clouds are compared against an updated in situ cirrus cloud property climatology to evaluate similarities and differences. In this climate modeling study, we ask how the cloud radiative effect (CRE) based on retrieved cirrus cloud properties compares with the CRE predicted by a model configuration representing cirrus clouds formed only through het and also with the CRE predicted by the standard configuration of the model. To answer this question, we constrained version 2 of the Morrison-Gettelman cloud microphysics scheme (MG2), which is used in several climate models, using effective diameter (De) retrievals from the CALIPSO satellite. A new subroutine within the MG2 scheme provides retrieved De as a function of temperature (T), latitude, season and land fraction, while ice particle mass and area relationships are used to relate De to the ice particle size distribution (PSD) slope and to produce new relationships for the number- and mass-weighted ice fall speeds. These and other modifications rendered the MG2 microphysics consistent with the De constraint. Using 40-year simulations of the Whole Atmosphere Community Climate Model version 6 (WACCM6), the CRE outside the tropics from the De-constrained WACCM6 was greater than standard WACCM6 by 1.63 W m−2 in the Northern Hemisphere (NH) and 2.59 W m−2 in the Southern Hemisphere (SH). Using the version of WACCM6 designed to represent cirrus clouds formed only by het (instead of using standard WACCM6), this difference was 2.37 W m−2 in the NH and 2.55 W m−2 in the SH. These differences are larger when only non-summer months are considered.

David L. Mitchell et al.

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David L. Mitchell et al.

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Publications Copernicus
Short summary
This may be the first estimate of the radiative contribution of homogeneous ice nucleation in cirrus clouds on a global, regional and seasonal scale. This is achieved by constraining an atmospheric global climate model with measured cirrus cloud properties via satellite remote sensing. The results show that the overall radiative warming contributed by homogeneous ice nucleation at the top of the atmosphere is 2.4 W m-2 outside the ± 30° latitude zone during non-summer months (JJA).
This may be the first estimate of the radiative contribution of homogeneous ice nucleation in...