A case study of aerosol effects on stratiform clouds with embedded convection in North China Plain using WRF

Abstract:

Stratiform clouds with embedded convection occur frequently in North China Plain (Zhu et al., 2015). However, less attention has been paid to aerosol effects on this type of cloud and precipitation system, especially for aerosol mixture of dust from long-range transport and local pollution (Jia et al., 2019). In this study, WRF (Weather Research & Forecast) model simulates a case of stratiform clouds with embedded convection in Hebei province in May 2017, by including aerosol effects on cloud droplet activation (Abdul-Razzak and Ghan, 1998) and ice nucleation (DeMott et al., 2010) in sbu_ylin 2-moment microphysics scheme (Zhao et al., 2018). We utilize one-way nesting approach for WRF simulations. Grid nudging to reanalysis data is enabled in the outer domain, and cloud microphysics is resolved in the inner domain with a resolution of 2.4 km. Aircraft penetrations in stratiform and convection parts of cloud provide vertical profiles of aerosol and cloud measurements. Simulation results well capture the observed patterns of precipitation and Doppler radar composite reflectivity. The microphysics scheme coupled with aerosol reproduces the observed vertical distributions of liquid/ice water content and cloud droplet number concentration, in both stratiform and convective clouds. However, model simulates noticeably lower ice particle number concentration and higher particle size than measurements. It is mainly because the ice nucleation in WRF only considers immersion freezing. After adding depositional nucleation on pollution aerosols (Meyers et al., 1993), model generates more and smaller ice particles in the levels of 5000 to 7000 meter, alleviating the biases in ice particle number and size. In addition, we conduct a series of sensitivity experiments to understand aerosol effects on the cloud and precipitation system by varying the number of aerosols acting as cloud condensation nuclei and as ice nuclei, respectively and concurrently.