13.4A Evaluation of Autoconversion and Accretion Enhancement Factors in GCM Warm-Rain Parameterizations Using Ground-Based Measurements at the Azores

Thursday, 10 January 2019: 2:15 PM
North 127ABC (Phoenix Convention Center - West and North Buildings)
Peng Wu, The Univ. of Arizona, Tucson, AZ; and B. Xi, X. Dong, and Z. Zhang

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 parameterizations. Using retrieved vertical profiles of cloud and rain water mixing ratios (qc and qr), Eauto and Eaccr are computed at a variety of tempo-spatial scales corresponding to different model resolutions. The calculated Eauto and Eaccr both increase with model grid spacing. Comparing with observations, the prescribed Eauto and Eaccr in Morrison and Gettleman (2008, MG08) are higher for Eauto (3.2) and lower for Eaccr (1.07) when model grid <90 km but both are close to observations for greater grids. This helps to explain the too frequent yet too light problem in GCM precipitation simulations using MG08. Both Eauto and Eaccr increase when the boundary layer becomes less stable, and the values are larger in precipitating clouds (CLWP>75 gm-2) than those in nonprecipiting clouds (CLWP<75 gm-2). Therefore, the selection of Eauto and Eaccr values in GCMs (MG08 scheme) should be regime- and grid-spacing- dependent.
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