Impacts of Ice Nucleation Parameterizations and Dust on Deep Convective C louds and Precipitation
Tuesday, 16 December 2014
Ice nucleation plays a critical role in the formation of ice clouds and converting liquid to ice in mixed-phase clouds. Many past heterogeneous ice nucleation parameterizations (INPs) were developed based on field measurements with the artifacts of shattering, and also not connected with aerosol particles. Recent laboratory and field measurements have led to a few new ice nucleation parameterizations connecting with aerosols, which should be implemented to models and evaluated by observations. In this study, we evaluated three very recently developed heterogeneous INPs linked with dust particles: Niemand et al. (2012), DeMott et al. (2013), and Phillips et al. (2013), using the Weather Research and Forecasting (WRF) model coupled with the physics and aerosol packages from the Community Atmospheric Model version 5 (CAM5). Results from the three new schemes and the default scheme by Meyers et al. (1992) are compared with available observations including aircraft measurements of cloud anvil properties of a storm case from the Deep Convective Clouds and Chemistry (DC3) field campaign. The Meyers scheme simulates higher ice number concentrations (Ni), while the Phillips scheme simulates lower Ni and shows the best agreement with aircraft measurements in the anvil area. Although the total precipitation amount and evolution of convection are not sensitive to different INPs, the probability density functions of both precipitation and reflectivity are considerably affected by the selected INPs. Simulation with the Phillips scheme shows higher ice nucleation rate at the warmer temperatures of the mixed-phase clouds regime, leading to much smaller ice formation in the upper levels. Reduced ice formation in the upper levels is caused by changes in droplet freezing, which is affected by convection and cloud morphology. Given relatively high dust concentration in the simulated case, sensitivity experiment is performed to examine dust impact with different INPs by reducing dust number concentration to 20 % of its current value. The Phillips scheme exhibits the largest sensitivity in terms of simulated cloud properties and precipitation with decreasing dust concentrations.