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

Abstract

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, Eₐᵤtₒ and Eₐ𝒸𝒸ᵣ, 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. Eₐᵤtₒ and Eₐ𝒸𝒸ᵣ are computed for different equivalent model grid sizes. The calculated 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 in Morrison and Gettleman (2008, MG08) to the observed ones, we found that a higher 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) produce too-frequent precipitation events but with too-light precipitation intensity. The ratios 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 in MG08 is too small to simulate precipitation intensity correctly. Both Eₐᵤtₒand Eₐ𝒸𝒸ᵣ increase when the boundary layer becomes less stable, and the values are larger in precipitating clouds (CLWP>75 gm⁻²) than those in non-precipitating clouds (CLWP<75 gm⁻²). Therefore, the selection of Eₐᵤtₒand Eₐ𝒸𝒸ᵣ values in GCMs should be regime- and resolution-dependent.