Sensitivity of WRF model simulations to parametrizations of depositional growth of ice crystal during the landfall of Typhoon Fitow (2013)

Abstract

AbstractUncertainty in the ice nucleation process is still large, yet the torrential precipitation associated with the landfall of a typhoon is strongly contributed by stratiform rainfall, mainly composed of ice‐phase clouds. This study investigates the ice growth parametrization proposed by Zeng et al. under the assumption that the ice crystal concentration is proportional to the mass of ice crystal. Sensitivity tests reveal that simulations with the Zeng scheme are especially sensitive to ice nuclei concentration. Zeng schemes with high ice crystal concentration simulate anomalously large rain rate, weak intensities and abnormal tracks, and this is directly related to the over‐production of ice nuclei concentration propagating into cloud ice and other microphysical fields mainly through the depositional growth of cloud ice from cloud water (PIDW). The Shen et al. scheme works better than the Zeng scheme by changing the radius of base ice crystal from 0 to 40 µm and reducing sensitivity of PIDW to the ice nuclei concentration. But Shen schemes simulate the number of ice crystals as anomalously large as those in Zeng schemes, since both Zeng and Shen schemes use the Fletcher equation in the parametrization of ice crystal concentration. When DeMott et al. ice crystal concentration parametrization is applied to the Zeng scheme, the anomalously large rainfall is significantly weakened, further indicating that Zeng schemes with high ice crystal concentration poorly simulate a typhoon, primarily due to the over‐nucleation of ice in the Fletcher parametrization.