On the Temperature Dependence of the Cloud Ice Particle Effective Radius-A Satellite Perspective

Cloud ice particle effective radius in atmospheric models is usually parametrized. A widely-used parametrization comprises a strong dependence on the temperature. Utilizing available satellite-based estimates of both cloud ice particle effective radius and cloud-top temperature we evaluate if a similar temperature-dependence exists in these observations. We find that for very low cloud-top temperatures the modeled cloud ice particle effective radius generally agrees on average with satellite observations. For high sub-zero temperatures however, the modeled cloud ice particle effective radius becomes very large, which is not seen in the satellite observations. We conclude that the investigated parametrization for the cloud ice particle effective radius, and parametrizations with a similar temperature dependence, likely produce systematic biases at the cloud top. Supporting previous studies, our findings suggest that the vertical structure of clouds should be taken into account as factor in potential future updates of the parametrizations for cloud ice particle effective radius. Plain Language Summary Atmospheric models are often used to diagnose and predict the atmospheric state including clouds. One very important property of clouds that consist of ice particles is the cloud ice particle effective radius. This ice effective radius is based on assumptions about the size and shapes of the ice particles in clouds, and thus parametrized, and is one of the important variables needed for calculating the effect of clouds on electromagnetic radiation, in particular on the solar radiation that enters the Earth's atmosphere. In our study we found that the parametrized ice effective radius agrees well on average and global scale with the ice effective radius inferred from satellite observations for cold clouds. However, we also found that for warmer ice clouds the parametrized ice effective radius is much higher than in satellite observations. Our study suggests that parametrizations of the ice effective radius used in atmospheric models show potential for improvements.