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

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, E_auto and E_accr, 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 (q_c and q_r), E_auto and E_accr are computed at a variety of tempo-spatial scales corresponding to different model resolutions. The calculated E_auto and E_accr both increase with model grid spacing. Comparing with observations, the prescribed E_auto and E_accr in Morrison and Gettleman (2008, MG08) are higher for E_auto (3.2) and lower for E_accr (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 E_auto and E_accr 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 E_auto and E_accr values in GCMs (MG08 scheme) should be regime- and grid-spacing- dependent.