This study investigates the effects of aerosol vertical distribution on a deep convective cloud system. We intend to elucidate the mechanisms for aerosols entering the cloud from different heights, and how they affect cloud microphysics and precipitation. A thermal bubble is released at 1.5 km initially to run an idealized case using the Weather Research and Forecast (WRF) model. The aerosol layer with high concentration was initially put at different altitudes in the model to study the mechanisms and the number of aerosols entering the cloud. It was found that there are three mechanisms for aerosols from different heights to enter the cloud, depending on their relative height with the thermal bubble. Aerosols from lower altitudes (below 1 km) enter the cloud through pumping, while aerosols from higher altitudes (2–3 km, 3–5 km) enter the cloud through entrainment. Both mechanisms lead to low cloud condensation nuclei (CCN) concentration in the cloud. Only aerosols from intermediate altitudes (1–2 km), which is the same as the initial height of the thermal bubble, enter the cloud mainly by ascending with the bubble and lead to high CCN concentration in the cloud. The differences in activated CCN concentration affect the microphysical processes and precipitation remarkably. For the simulations with an initial aerosol layer at 1–2 km and 0–5 km, aerosols can enter the cloud more efficiently than the other four simulations. More activated CCNs in these two simulations lead to more graupels with smaller sizes at higher altitudes, which delays the precipitation but makes the precipitation last longer. However, the accumulated precipitation is similar in all six simulations, no matter what aerosol vertical distribution is like. The results in this study indicate that the altitude of aerosol layers determines the mechanisms for aerosols entering clouds, CCN concentration in the cloud, and to what extent the cloud microphysical processes and precipitation are affected.

This study uses idealized simulations to investigate the impact of cloud condensation nuclei (CCN) on a cumulus congestus. Thirteen cases with the initial CCN_C, which is the CCN concentration at 1% supersaturation with respect to water, from 10 to 10,000 cm⁻³ are simulated. The analysis focuses on the liquid phase due to the negligible ice phase in this study. A non-monotonic response of cloud properties and precipitation to CCN concentration is observed. When CCN_C is increased from 10 to 50 cm⁻³, the enhanced condensation due to the more numerous droplets invigorates the cumulus congestus. The delayed precipitation formation due to the smaller droplets also facilitates the cloud development. The two processes together lead to a higher liquid water path (LWP), higher cloud top, and heavier precipitation. The cumulus congestus has the highest cloud top, the strongest updraft, and the most accumulated precipitation and at CCN_C = 50 cm⁻³. When CCN_C is increased from 50 to 500 cm⁻³, the condensation near the cloud base is further enhanced and the precipitation is further delayed, both of which lead to more liquid water remaining in the cloud, and thus an even higher LWP and heavier precipitation rate in the later stage. However, the significantly enhanced evaporation near the cloud top limits the vertical development of the cumulus congestus, leading to a lower cloud top. When CCN_C is further increased to be higher than 1000 cm⁻³, the cumulus congestus is strongly suppressed, and no precipitation forms. The ratio of the precipitation production rate to vertical cloud water flux in the updraft is not a constant, as is generally assumed in cumulus parameterization schemes, but decreases significantly with increasing CCN concentration. It is also found that the CCN effect on the cumulus congestus relies on which parameters are used to describe the cloud strength. In this study, as CCN_C increases, the LWP and the maximum precipitation rate peak at CCN_C = 500 cm⁻³, while the cloud top height, maximum updraft, and accumulated precipitation amount peak at CCN_C = 50 cm⁻³.

Shallow-convective clouds (SCCs) play important roles in the Earth’s atmospheric system by affecting radiative balance, large-scale circulation, and transport of pollutants. It is common sense that topography exerts substantial impacts on SCCs. However, the underlying mechanisms are not well understood. Here, we performed large-eddy simulations (LESs) to investigate how three-dimensional (3D) topography affected SCCs. The 3D topography was constructed using two widely used two-dimensional (2D) topographies, a bell-shaped ridge varying in the $x$-direction and a series of sinusoidal ridges varying in the $y$-direction. The bell-shaped ridge was the major ridge. The upper parts and lower parts of the sinusoidal ridges were the minor ridges and minor valleys, respectively. The wavelength of the sinusoidal ridges was systematically varied. LESs were also performed separately using the 2D topographies. In the simulations with 3D topography, the upslope winds were mainly over the minor ridges and the return flows were mainly over the minor valleys, which was different from those in the simulations using 2D topographies. The upslope winds promoted the development of SCCs over the major ridges by producing large thermals and high humidity, similar to in the simulations using 2D topographies. Increasing the wavelength of minor ridges enlarged the region with convergence, and thereby increased the size of SCCs. Our results suggest that it is necessary to consider the 3D topography instead of the more conventional 2D topographies when investigating the topographic impacts on SCCs.