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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
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Research article 19 Jun 2018

Research article | 19 Jun 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Evaluate autoconversion and accretion enhancement factors in GCM warm-rain parameterizations using ground-based measurements at the Azores

Peng Wu1, Baike Xi1, Xiquan Dong1, and Zhibo Zhang2 Peng Wu et al.
  • 1Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Arizona, USA
  • 2Physics Department, The University of Maryland, Baltimore County, Maryland, USA

Abstract. A great challenge in climate modelling is how to parametrize sub-grid cloud processes, such as autoconversion and accretion in warm rain formation. In this study, we use ground-based observations and retrievals over the Azores to investigate the so-called enhancement factors, Eauto and Eaccr, which are often used in climate models to account for the influences of sub-grid variances of cloud and precipitation water on the autoconversion and accretion processes. Eauto and Eaccr are computed at a variety of tempo-spatial scales corresponding to different model resolutions. The calculated Eauto increase from 1.79 (0.5-hr/36km) to 3.15 (3.5-hr/126km), and the calculated Eaccr increases from 1.25 (0.5-hr/36km) to 1.6 (5-hr/180km). Comparing the prescribed enhancement factors to the values from observations shows that GCMs are using a much higher Eauto (3.2) and lower Eaccr (1.07). This helps to explain why most of the GCMs produce too frequent precipitation events but with too light precipitation intensity. The ratios of rain to cloud liquid water at Eaccr=1.07 and Eaccr=2.0 are 0.048 and 0.119, respectively, further proving that the prescribed value of Eaccr=1.07 used in GCMs is too small to simulate correct precipitation intensity. Both Eauto and Eaccr increase when the boundary layer becomes less stable, and the values are larger in precipitating clouds (CLWP>75gm−2) than those in nonprecipiting clouds (CLWP<75gm−2). Therefore, the selection of Eauto and Eaccr values in GCMs should be regime-dependent.

Peng Wu et al.
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Peng Wu et al.
Peng Wu et al.
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Short summary
Prescribed autoconversion and accretion enhancement factors in GCM warm rain parameterizations contribute partially to the too frequent and too light problem in precipitation simulation. The two factors should be regime and resolution dependent. Decrease autoconversion enhancement factor and increase accretion enhancement factor in Morrison and Gettleman (2008) scheme can improve simulated precipitation frequency and intensity. Values for different model resolutions are also suggested.
Prescribed autoconversion and accretion enhancement factors in GCM warm rain parameterizations...