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

• Published in: Journal of the atmospheric sciences Vol. 81; no. 10; pp. 1689 - 1710
• Main Authors: Seiki, Tatsuya, Nagao, Takashi M.
• Format: Journal Article
• Language: English
• Published: Boston American Meteorological Society 01.10.2024
• Subjects: Aggregation; Aircraft; Atmospheric temperature; Bias; Budgets; Cirrus clouds; Climate; Climate models; Cloud distribution; Cloud microphysics; Clouds; Crystal growth; Crystals; Efficiency; Freezing; Freezing level; Growth stage; Ice; Ice clouds; Ice cover; Ice crystals; Ice particles; Laboratories; Laboratory experimentation; Laboratory experiments; Microphysics; Modelling; Optical thickness; Parameter sensitivity; Parameterization; Particle size; Radar clutter; Radar echoes; Radiation budget; Radiative forcing; Satellite observation; Satellites; Sensitivity analysis; Simulation; Snow; Tropical environments; Troposphere; Upper troposphere; Vertical profiles
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/JAS-D-23-0208.1

Abstract Aggregation efficiency in the upper troposphere is highly uncertain because of the lack of laboratory experiments and aircraft measurements, especially at atmospheric temperatures below −30°C. Aggregation is physically broken down into collision and sticking. In this study, theory-based parameterizations for the collision efficiency and sticking efficiency 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 modeling. Sensitivity experiments of 9-day global simulations using a high-resolution climate model show that the use of collision efficiency 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 parameterization causes a significant 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 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 modeling reasonably functions even at atmospheric temperatures below −30°C; however, further improvements in the ice cloud modeling are needed. Significance Statement Long-standing biases in the radiative budget in climate models indicate the existence of a missing mechanism to realistically represent the ice cloud growth in the upper troposphere. This study focuses on aggregation efficiency, which has been assumed to be a tuning parameter to optimize the global radiative budget. Therefore, this study employs a theory-based parameterization to calculate the aggregation efficiency. According to the parameterization, aggregation efficiency in high clouds varies by the growth stage of the individual ice particles. As a result, small ice crystals are likely to grow more slowly, and the lifetime of cirrus clouds is prolonged to enhance cloud radiative forcing, particularly over the tropics. These results are promising for reducing the biases observed in climate models.