Atmospheric Aerosols Make Storm Systems Larger
The indirect effect of aerosols on clouds, radiation, and precipitation is one of the most uncertain factors in predicting future global climate change. Aerosol particles act as nuclei for cloud condensation, but their precise effects on clouds and precipitation are very complex for deep convective clouds. Little is known about how aerosols would affect the large storms called mesoscale convective systems. Researchers at the U.S. Department of Energy’s Pacific Northwest National Laboratory and Nanjing University of Information Science and Technology led a study to examine how aerosols affect different types of mesoscale convective systems, including squall lines and supercell systems. They found that aerosols enlarged the storm area and enhanced precipitation for all storms studied. Aerosols also led to more occurrences of deep clouds with suppression of shallower clouds. This work improves the fundamental understanding of aerosol impacts on large storms.
Mesoscale convective systems (MCSs) play a very important role in global precipitation, atmospheric circulation, and Earth’s radiative budget. Understanding how various types of large storms respond to aerosols is an important step in reducing the large uncertainty associated with aerosol indirect effects in current and future weather and climate predictions. This understanding also allows scientists to better advise policymakers considering the broader impacts of air pollution regulations in regions in which MCSs occur frequently.
The researchers investigated aerosol impacts on MCSs forming under different wind shear conditions. To do this, they used the Weather Research and Forecasting model coupled with spectral-bin microphysics and conducted sensitivity simulations by increasing the cloud condensation nuclei concentration. They found that increased aerosols enhanced precipitation enlarged the convective area and vertical mass fluxes, and induced stronger vertical motions in all MCSs. Increased updraft speed below 8 km in altitude is attributed to enhanced condensational heating by aerosols. For MCSs developed under weak wind shear over the profile as well as under strong low-level shear, however, enhanced low-level convergence is also a contributing factor. Interestingly, above 8 km in altitude, vertical speed reduces with increasing aerosols mainly because of reduced vertical pressure perturbation gradient force.
Increasing aerosols boosted accumulated rainfall and the mean rain rate, with a greater occurrence frequency of heavy rain. This larger rain rate is seen in both convective and stratiform regions. In general, storms developed in a larger aerosol condition had a higher frequency of deep clouds and more stratiform/anvil clouds but a lower frequency of shallow warm clouds; these effects are more significant when MCSs are more organized. Aerosols consistently invigorating MCSs under various wind shear conditions, as revealed by this study, have important implications for weather and climate in the regions in which MCSs occur frequently and are influenced by pollution.