Evaluation of Clouds in the Conformal Cubic Atmospheric Model Using the CFMIP Observation Simulator Package

Abstract

Cloud amounts and cloud phases reproduced from single-moment (SM) and double-moment (DM) microphysics schemes within Conformal Cubic Atmospheric Model (CCAM) at 105 km resolution are evaluated against CALIPSO-GOCCP and CloudSat observations using the CFMIP Observation Simulator Package (COSP). Both schemes demonstrate skill in reproducing liquid and ice clouds. However, a persistent deficiency in mid- and high-level liquid clouds remains, contributing to an underestimation of total cloud cover, particularly in the tropics. A comparison between the SM and DM schemes highlights two key improvements. First, DM-CCAM exhibits better agreement with observed low-level cloud fractions exceeding 60% in the eastern subtropical oceans. Second, it provides a more accurate representation of ice cloud amounts from 30° toward the poles. Further investigation into the vertical structure of clouds over the Tropical Warm Pool, Southeast Pacific Subtropical Stratocumulus region, and Southern Ocean reveals systematic biases in both CCAM simulations. Notably, the model tends to overproduce optically thick clouds while underrepresenting optically thin clouds in the upper troposphere. Additionally, the model simulates a higher occurrence of drizzle and light rain, along with a lower frequency of non-precipitating clouds in the lower troposphere. Future work will explore the impact of increased horizontal resolution (e.g., 4 km) on cloud characteristics within the CORDEX-Australasia domain, which could provide further insights into model performance and inform refinements.