The impact of aerosols on cloud microphysics and dynamics in deep convective clouds over the UK

Flash flooding arise from deep convective storms developing along continuous convergence lines. The understanding of aerosol-cloud interactions is fundamental to improve the forecasting of deep convective storms. However, aerosols and clouds are still the largest uncertainty source. Most of modelling studies have focused on the role of ice particles on deep convective clouds, however, a number of heavy rain events occur in warm rain clouds. Recent COnvective Precipitation Experiment observation have shown radar reflectivity exceeding 50 dBZ in convective clouds wherein large drops are induced by freezing of drizzle or raindrops. Atmospheric aerosols serve as cloud condensation nuclei, wherein water vapour con­denses onto the cloud condensation nuclei when the supersaturation of ambient air is reached at critical value to activate droplets. Stronger vertical velocity induces higher supersaturation when a volume of moist air is ascending adiabatically resulting in more activated cloud condensation nuclei which grow to cloud droplets. An increase in cloud drop number concentration could have knock-on effects on ice processes and can feedback on the cloud dynamics. Vertical velocity in numerical models is not suitable for the peak supersaturation of aerosol activation because it represents a mean value of a grid box in the model and the velocity tends to be less resolved in kilometre-scale modelling. The investigation of the unresolved motion using a coarse-graining method, applying the improved vertical velocity to model simulation, and the sensitivity of cloud microphysics and dynamics to cloud drop number concentration were conducted in this study. The standard deviation of vertical velocity tended to be less v'6 in 200 m simulation than in 50 m reference simulation. The improved vertical velocity presented over cloud boundary regions and resulted in an increase in cloud drop number concentration. The aerosol activation scheme as a function of vertical velocity produced higher cloud droplets at cloud base, the higher droplets induced higher ice particles, and then deeper clouds and stronger precipitation presented.