Abstract
Aerosol particles can affect cloud microphysical properties by serving as ice nuclei (IN). Large uncertainties exist in the ice nucleation parameterizations (INPs) used in current climate models. In this Part II paper, to examine the sensitivity of the model predictions to different heterogeneous INPs, WRF-CAM5 simulation using the INP of Niemand et al. (N12) [1] is conducted over East Asia for two full years, 2006 and 2011, and compared with simulation using the INP of Meyers et al. (M92) [2], which is the original INP used in CAM5. M92 calculates the nucleated ice particle concentration as a function of ice supersaturation, while N12 represents the nucleated ice particle concentration as a function of temperature and the number concentrations and surface areas of dust particles. Compared to M92, the WRF-CAM5 simulation with N12 produces significantly higher nucleated ice crystal number concentrations (ICNCs) in the northern domain where dust sources are located, leading to significantly higher cloud ice number and mass concentrations and ice water path, but the opposite is true in the southern domain where temperatures and moistures play a more important role in ice formation. Overall, the simulation with N12 gives lower downward shortwave radiation but higher downward longwave radiation, cloud liquid water path, cloud droplet number concentrations, and cloud optical depth. The increase in cloud optical depth and the decrease in downward solar flux result in a stronger shortwave and longwave cloud forcing, and decreases temperature at 2-m and precipitation. Changes in temperature and radiation lower surface concentrations of OH, O3, SO42−, and PM2.5, but increase surface concentrations of CO, NO2, and SO2 over most of the domain. By acting as cloud condensation nuclei (CCN) and IN, dust particles have different impacts on cloud water and ice number concentrations, radiation, and temperature at 2-m and precipitation depending on whether the dominant role of dust is CCN or IN. These results indicate the importance of the heterogeneous ice nucleation treatments and dust emissions in accurately simulating regional climate and air quality.
Highlights
Ice microphysical processes in both ice-containing clouds such as cirrus clouds and mixed-phase clouds play an important role in the climate system
For mixed-phase clouds, immersion freezing is represented based on the formulation of Bigg [23]; contact freezing follows the parameterization of Young [28] and Cotton et al [25], and heterogeneous deposition/condensation nucleation is based on M92 in which ice formation is described as a function of ice supersaturation [4]
Compared to the simulation with M92, the nucleated ice crystal number concentrations (ICNCs) due to heterogeneous immersion freezing nucleation simulated by N12 are significantly larger in the northern domain, which includes the dust source regions and areas downwind, but significantly lower in the southern domain
Summary
Ice microphysical processes in both ice-containing clouds such as cirrus clouds and mixed-phase clouds play an important role in the climate system. The enhanced ice formation in the mixed-phase clouds usually leads to enhanced transformation of liquid to ice by the Wegener-Bergeron-Findeisen (WBF) mechanism due to the higher saturation vapor pressure over liquid than ice at the same temperatures [13,14,15] Such influence is sometimes referred to as “glaciation indirect effect” of aerosols on clouds. For mixed-phase clouds, immersion freezing is represented based on the formulation of Bigg [23]; contact freezing follows the parameterization of Young [28] and Cotton et al [25], and heterogeneous deposition/condensation nucleation is based on M92 in which ice formation is described as a function of ice supersaturation [4]. Following the comprehensive evaluation of WRF-CAM5 that uses the default INP of M92 in Part I of our two-part paper, in this part II paper, we conduct sensitivity simulations with a new INP, N12, which connects ice formation with dust particle properties, to examine the sensitivity of WRF-CAM5 to heterogeneous ice nucleation parameterizations and the role of mineral dust in radiation and cloud formation via affecting ice nucleation processes only and affecting both cloud droplet and ice nucleation processes, as well as the radiation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
