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Volume 15, issue 15
Atmos. Chem. Phys., 15, 8717-8738, 2015
https://doi.org/10.5194/acp-15-8717-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 15, 8717-8738, 2015
https://doi.org/10.5194/acp-15-8717-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 07 Aug 2015

Research article | 07 Aug 2015

Prognostic precipitation with three liquid water classes in the ECHAM5–HAM GCM

V. Sant1,a, R. Posselt2, and U. Lohmann1 V. Sant et al.
  • 1Institute of Atmospheric and Climate Science, ETH Zürich, Zürich, Switzerland
  • 2Federal Institute for Meteorology and Climatology, MeteoSchweiz, Zürich, Switzerland
  • anow at: Max Planck Institute for Meteorology, Hamburg, Germany

Abstract. A new parameterization with three prognostic liquid water classes was implemented into the general circulation model (GCM) ECHAM5 with the aerosol module HAM in order to improve the global representation of rain formation in marine stratiform clouds. The additionally introduced drizzle class improves the physical representation of the droplet spectrum and, more importantly, improves the microphysical processes relevant for precipitation formation compared to the standard parameterization. In order to avoid a mismatch of the liquid and ice phase, a prognostic treatment of snow has been introduced too. This has a significant effect on the amount and altitude of ice clouds, which in turn affects not only the in- and outgoing radiation but also the parameterized collection rates. With the introduction of a prognostic precipitation scheme, a more realistic representation of both liquid and ice phase large-scale precipitation is achieved compared to a diagnostic treatment. An encouraging finding is that with the prognostic treatment the increase of the liquid water path in response to anthropogenic aerosols is reduced by about 25 %. Although the total net radiative forcing is decreased from −1.3±0.3 to −1.6±0.3 W m−2 from the control to the prognostic model version, the difference is within the interannual variability. Altogether the results suggest that the treatment of precipitation in global circulation models has not only a significant influence on the phase of clouds and their conversion rates, but also hints towards uncertainties related to a prognostic precipitation scheme.

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We have introduced prognostic precipitation for both liquid (drizzle and rain) and solid (snow) phase precipitation into the global circulation model ECHAM5-HAM. This has a significant effect on the clouds and the parameterized collection rates, also reducing the sensitivity of the liquid water path to the anthropogenic aerosol forcing. Altogether the results suggest that the treatment of precipitation in global circulation models has a significant influence on the phase and lifetime of clouds.
We have introduced prognostic precipitation for both liquid (drizzle and rain) and solid (snow)...
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