Optimization of a Diagnostic Cloud Fraction Scheme Based on Cloud Hydrometeor for the Korean Integrated Model

Abstract This study addressed the critical issue of consistency between cloud fractions and cloud hydrometeors (mass concentration) in atmospheric models by optimizing a diagnostic cloud fraction scheme for the Korean Integrated Model (KIM). The study improved the representation of subgrid-scale clouds by introducing a new method for calculating subgrid-scale cloud mass concentration considering the cloud lifetime. A diagnostic cloud fraction scheme was then adapted and optimized for the KIM. It incorporates a scaling factor that varies with altitude to account for the variations in vertical grid spacing and cloud overlapping effects. The optimized scheme generally reduced the cloud fraction, thereby aligning it closer to the fifth generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA5). This demonstrated an improved consistency between the cloud fraction and cloud hydrometeor distributions. The new approach yielded improvements in surface radiative fluxes (particularly shortwave radiation) compared with the Clouds and the Earth’s Radiant Energy System (CERES) observations. Consequently, the temperature biases reduced in many regions. In addition to successfully addressing the initial goal of improving the spatiotemporal consistency between the cloud fraction and cloud hydrometeor, the study identified areas for further refinement, particularly in high-altitude regions and in addressing the cold bias in the upper equatorial layer. This research represents a significant step forward in enhancing cloud representation in the KIM and provides potential applicability to other atmospheric models. Significance Statement This study addressed the critical issue of consistency between cloud fractions and cloud hydrometeors in atmospheric models by suggesting a new method for calculating subgrid-scale cloud mass concentration and an optimized diagnostic cloud fraction scheme for the Korean Integrated Model (KIM). The new approaches improved consistency between cloud fraction and cloud hydrometeor distributions compared to the original physical processes. The study successfully addressed the initial goal of improving spatiotemporal consistency between the cloud fraction and cloud hydrometeor, while also identifying areas for further refinement. This research represents a significant step forward in enhancing cloud representation in KIM and provides potential applicability to other atmospheric models, contributing to improved weather and climate predictions.