The Role of Initial Cloud Condensation Nuclei Concentration in Hail Using the WRF NSSL 2-moment microphysics scheme

The effects of the initial cloud condensation nuclei (CCN) concentrations (from 100 mg^-1 to 3000 mg^-1) on hail have been investigated in an idealized supercell experiment of WRF model with NSSL 2-moment microphysics scheme. The increasing CCN concentration leads to delays in hail formation in cloud as well as in hail precipitation at the surface. The reason for the delays is mainly due to the reduced warm rain process and the slowed hail embryos initiation by the riming of smaller ice crystals and snow particles. At the same time, the initial CCN concentration has significant non-monotonic effects on the mixing ratio and number concentrations of hail both in cloud and at the surface with a threshold CCN concentration between 300 mg^-1 and 500 mg^-1. The increasing CCN concentration is conductive (suppressive) to the amounts of surface hail precipitation below (above) the threshold. The reason for the non-monotonic effects is from both the thermodynamics and microphysics. Below the threshold, the mixing ratios of cloud droplets and ice crystals increase dramatically with CCN, resulting in more latent heat released from vapor condensation and intensified updrafts in cloud. The abundant supercooled water is collected by both hail and graupel, which is favorable for hail production. Above the threshold, the mixing ratio of cloud droplets and ice crystals increase continuously but updraft is weakened when CCN concentration continues to increase; the even smaller ice crystals reduce the formation of hail and the even smaller cloud droplets weaken the riming efficiency so that graupel and hail also decrease with CCN, which is unfavorable for hail growth.