Evaluation of the Aggregation Efficiency Modeling at Colder Atmospheric Temperatures in Comparison to Satellite Observations

Aggregation efficiency fi ciency in the upper troposphere is highly uncertain because of the lack of laboratory experiments and aircraft measurements, especially at atmospheric temperatures below-30 degrees C. degrees C. Aggregation is physically broken down into collision and sticking. In this study, theory-based parameterizations for the collision efficiency fi ciency and sticking efficiency fi ciency are newly implemented into a double-moment bulk cloud microphysics scheme. Satellite observations of the global ice cloud distribution are used to evaluate the aggregation efficiency fi ciency modeling. Sensitivity experiments of 9-day global simulations using a high-resolution climate model show that the use of collision efficiency fi ciency parameterization causes a slight increase in the cloud ice amount above the freezing level over the tropics to midlatitudes and that the use of our new sticking efficiency fi ciency parameterization causes a significant fi cant increase in the cloud ice amount and a slight decrease in the snow amount particularly in the upper troposphere over the tropics. The increase/decrease in the cloud ice/snow amount in the upper troposphere over the tropics is consistent with the vertical profile fi le of radar echoes. Moreover, the ice fraction of the cloud optical thickness is still underestimated worldwide. Finally, the cloud radiative forcing increases over the tropics to reduce the bias in the radiation budget. These results indicate that our new aggregation efficiency fi ciency modeling reasonably functions even at atmospheric temperatures below-30 degrees C; degrees C; however, further improvements in the ice cloud modeling are needed.