Raindrop Size Distributions Simulated Using a Bin Microphysics Scheme: Different Biases in Stratiform and Convective Rain From an Extratropical Cyclone.

Bin microphysics schemes prognose the raindrop size distribution (RSD), which can be directly evaluated through comparison with disdrometer observations. This evaluation will provide implications on the reliability of simulated cloud microphysics by bin microphysics schemes. In this study, the RSDs of a precipitation event associated with an extratropical cyclone passing South Korea are simulated using a bin microphysics scheme and compared with those observed by a ground‐based disdrometer. The simulated mean RSD overall agrees with the observation. However, notable overestimations appear in the large‐ (3.3–4.3 mm) and small‐ (0.56–1.88 mm) diameter ranges, which respectively stem from the biases in two different time periods, one dominated by stratiform rain and the other largely involved with convective rain. In the stratiform‐rain‐dominated period, the melting of snow is the largest contributor to RSDs. The overestimation in the large‐diameter range in this period can be associated with overly active ice–ice collection at upper levels, which generates a local maximum in RSD at the diameter of 3.3 mm that is not seen in the observed RSDs. In the convective‐rain‐involved period, the warm‐rain collision–coalescence is the largest contributor to RSDs. The overestimation in the small‐diameter range and underestimation in the large‐diameter range imply that the collisional growth of raindrops is represented to be weaker than that in reality. The findings in this study suggest that the RSDs simulated using a bin microphysics scheme can have some systematic biases associated with misrepresentation of some microphysical processes. Plain Language Summary: Bin microphysics schemes are a type of cloud microphysics schemes that allow for the particle size distributions of hydrometeors to take any form at the expense of huge computational resources, aiming to represent the cloud microphysics as realistically as possible. In this study, the raindrop size distributions predicted using a bin microphysics scheme are evaluated through comparison with ground‐based observations obtained using a disdrometer. The mean raindrop size distribution is overall well reproduced, but the raindrop number concentration is overestimated in the small‐ and large‐diameter ranges. The overestimation in the large‐diameter range occurs in the period when stratiform rain is dominant, attributable to the overgrowth of ice particles via ice–ice collection and their melting into large raindrops. The overestimation in the small‐diameter range occurs in the period when convective rain is largely involved, which is attributed to the insufficient growth of raindrops via coalescence with other drops. Key Points: Raindrop size distributions simulated using a bin microphysics scheme are evaluated in comparison with disdrometer observationsFor stratiform rain, the number of large raindrops is overestimated due to overgrowth of snow particles by aggregation and their meltingWhen convective rain is largely involved, the number of small raindrops is overestimated due to insufficient warm‐rain collision–coalescence [ABSTRACT FROM AUTHOR]