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. A great challenge in climate modeling is how to parameterizesubgrid cloud processes, such as autoconversion and accretion in warm-rainformation. In this study, we use ground-based observations and retrievalsover the Azores to investigate the so-called enhancement factors, Eauto and Eaccr , which are often used in climate modelsto account for the influence of subgrid variance of cloud and precipitationwater on the autoconversion and accretion processes. Eauto and Eaccr are computed for different equivalent model grid sizes. Thecalculated Eauto values increase from 1.96 (30 km) to 3.2(180 km), and the calculated Eaccr values increase from 1.53(30 km) to 1.76 (180 km). Comparing the prescribed enhancement factors inMorrison and Gettleman (2008, MG08) to the observed ones, we found that ahigher Eauto (3.2) at small grids and lower Eaccr (1.07)are used in MG08, which might explain why most of the general circulation models (GCMs) producetoo-frequent precipitation events but with too-light precipitation intensity. Theratios of the rain to cloud water mixing ratio ( qr / qc ) at Eaccr=1.07 and Eaccr=2.0 are 0.063 and 0.142, respectively, from observations,further suggesting that the prescribed value of Eaccr=1.07 used inMG08 is too small to simulate precipitation intensity correctly. Both Eauto and Eaccr increase when the boundary layer becomesless stable, and the values are larger in precipitating clouds ( CLWP>75  gm −2) than those in non-precipitating clouds ( CLWP  gm −2 ). Therefore, the selection of Eauto and Eaccr values in GCMs should be regime- and resolution-dependent.

atmospheric chemistry and physics

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