Analysis of CCCma Radiative Transfer Calculations for Low-Level Overcast Liquid Clouds Over ARM SGP and ENA Sites

Abstract

This study uses the Canadian Centre for Climate Modeling and Analysis (CCCma) radiative transfer model to estimate shortwave flux for low-level overcast liquid clouds. Calculations are evaluated against measurements at the Atmospheric Radiation Measurement Southern Great Plains (SGP, land) and Eastern North Atlantic (ENA, ocean) sites, as well as top of atmosphere (TOA) fluxes inferred from Clouds and Earth's Radiant Energy System (CERES) from 2014 to 2023. Mean observed surface (TOA) SW fluxes for the selected cases are 235.7 W m⁻² (473.8 W m⁻²) at SGP and 348.7 W m⁻² (356.4 W m⁻²) at ENA. Cloud microphysical properties retrieved from CERES MODIS are input into the CCCma using three assumed profiles: (a) cloud droplet effective radius (r_e) and liquid water content (LWC) constant with height, (b) LWC and r_e increasing linearly with height, and (c) LWC and r_e increasing linearly from cloud base to ¾ height and then decreasing linearly up to cloud top. Overall, Method 3 produces the least error variance at both sites. At SGP, mean bias and root mean square error (RMSE) are −5.0 and 44.6 W m⁻² at the surface and −4.6 and 25.4 W m⁻² at TOA. At ENA, errors are +0.2 and 121.3 W m⁻² at the surface and −8.0 and 26.1 W m⁻² at TOA. Further screening cases with good agreement between satellite- and surface-based cloud properties, RMSEs for surface fluxes decrease to 24.3 and 25.8 W m⁻² at SGP and ENA. Comparisons with CERES Fu-Liou calculations showed overall better performance by the CCCma, especially at ENA.