The Impact of Aerosol Vertical Distribution on a Deep Convective Cloud

Minzhong Zhang,Huiwen Xue,Ruihao Zhu,Yangze Ren,Xin Deng

doi:10.3390/atmos12060675

Minzhong Zhang, Huiwen Xue + Show 3 more

Open Access

https://doi.org/10.3390/atmos12060675

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Abstract

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.

Highlights

The concentration of aerosol particles has great variability in space and time
We investigate the effects of aerosol vertical distribution on the properties of a deep convective cloud using the idealized simulations in the Weather Research and Forecast (WRF) model
A series of idealized simulations in the WRF model with an aerosol layer at different heights are performed to understand the response of a deep convective cloud to the vertical distribution of aerosol

Summary

Introduction

The concentration of aerosol particles has great variability in space and time. The vertical distribution of aerosol particles varies greatly from case to case according to airborne measurement results. Lelieveld et al (2002) [9] revealed that pollution layers may distribute from the surface to the lower stratosphere of 15 km high as a result of long-range transport. Precipitation increases with CCN aerosols when the aerosol concentration is not too high, while precipitation decreases in highly polluted cases. For the anvil regions of the deep convective clouds, it is found that increased CCN aerosols can lead to higher cloud top heights and higher anvil ice mixing ratios in a polluted environment (Morrison and Grabowski, 2011 [22]). Whether the vertical distribution of CCN aerosols has considerable impacts on the mixed-phase microphysics, and precipitation of deep convective systems has been investigated by only a few studies.

Model and Case

The Mechanisms for Aerosols Entering the Deep Convective Cloud

The of Precipitation

Findings

Conclusions and Discussions