Evaluation of autoconversion and accretion enhancement factors in general circulation model warm-rain parameterizations using ground-based measurements over the Azores

Wu, Peng; Xi, Baike; Dong, Xiquan; Zhang, Zhibo

A great challenge in climate modeling is how to parameterize subgrid 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, inline-formulaEauto and inline-formulaEaccr, which are often used in climate models to account for the influence of subgrid variance of cloud and precipitation water on the autoconversion and accretion processes. inline-formulaEauto and inline-formulaEaccr are computed for different equivalent model grid sizes. The calculated inline-formulaEauto values increase from 1.96 (30 km) to 3.2 (180 km), and the calculated inline-formulaEaccr values increase from 1.53 (30 km) to 1.76 (180 km). Comparing the prescribed enhancement factors in Morrison and Gettleman (2008, MG08) to the observed ones, we found that a higher inline-formulaEauto (3.2) at small grids and lower inline-formulaEaccr (1.07) are used in MG08, which might explain why most of the general circulation models (GCMs) produce too-frequent precipitation events but with too-light precipitation intensity. The ratios of the rain to cloud water mixing ratio (inline-formulaqr/inline-formulaqc) at inline-formulaEaccr=1.07 and inline-formulaEaccr=2.0 are 0.063 and 0.142, respectively, from observations, further suggesting that the prescribed value of inline-formulaEaccr=1.07 used in MG08 is too small to simulate precipitation intensity correctly. Both inline-formulaEauto and inline-formulaEaccr increase when the boundary layer becomes less stable, and the values are larger in precipitating clouds (inline-formulaCLWP>75 gminline-formula−2) than those in non-precipitating clouds (inline-formulaCLWP<75 gminline-formula−2). Therefore, the selection of inline-formulaEauto and inline-formulaEaccr values in GCMs should be regime- and resolution-dependent.

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Wu, Peng / Xi, Baike / Dong, Xiquan / et al: Evaluation of autoconversion and accretion enhancement factors in general circulation model warm-rain parameterizations using ground-based measurements over the Azores. 2018. Copernicus Publications.

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