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Zhang, Yuwei
Li, Zhanqing
Aerosol-cloud interactions (ACI) remain the largest uncertainty in projections of its future changes in climate in response to the buildup of greenhouse gases, even though they have been extensively investigated. Convective clouds have complicated dynamics and microphysics, and aerosol effects on them are the least understood of any cloud type. This study aims to further our understanding of aerosol effects on convective clouds by tackling a few outstanding problems by means of observational data analysis and model simulations under a wide-range of environmental conditions. There are three primary objectives: (1) to investigate the impact of ultra-fine aerosol particles from the Manaus metropolis on convective clouds under the pristine environment of Amazon; (2) to explore and quantify the urbanization effect on convective storms over the Houston area where the anthropogenic effects of both land surface and aerosols are exceptionally strong; (3) to examine and compare the relative significances of fire-induced surface heating and aerosol effects on exceptionally deep convective clouds, or pyroCb. Ultrafine aerosol particles smaller than 50 nanometers (UAP<50) are abundant in the troposphere but have been conventionally considered too small to be activated as cloud condensation nuclei (CCN) to affect cloud formation. Observational evidence and numerical simulations of deep convective clouds (DCCs) over the Amazon show that DCCs forming in a low-aerosol environment can develop very large water vapor supersaturation. This is because fast droplet coalescence reduces integrated droplet surface area and subsequent condensation. UAP<50 from pollution plumes that are ingested into such clouds can be activated to form additional cloud droplets on which water condenses and forms additional cloud water and latent heating, thus intensifying convective strength. This “warm-phase invigoration” is demonstrated to have much stronger effects than the “cold-phase invigoration” previously proposed and does not affect the timing of precipitation because warm rain needs to form first to remove droplets and form high in-cloud supersaturation. Urbanization has local impacts on storms through changing urban land-cover and anthropogenic aerosols. The Chemistry version of Weather Research and Forecast model (WRF‐Chem) coupled with spectral‐bin microphysics (SBM) are first employed to examine how urban land and anthropogenic aerosols impact DCCs on 19-20 June 2013 over Houston. We find that urbanization in Houston drastically enhances convective intensity and precipitation, primarily due to the urban aerosol effects. Urban land effect does not change precipitation much but initiates mixed-phase clouds 20 min earlier due to urban heating. Urban aerosols accelerate the development of convective cells into ice phase clouds, resulting from larger latent heat release. With the Morrison bulk scheme, the model does not show significant aerosol impacts on convective intensity and precipitation, due to limitations in representation of aerosol-cloud interaction processes, particularly aerosol drop condensation. Wildfires can influence severe convective storms through releasing sensible heat and aerosols into the atmosphere. We developed a computationally efficient model capability based on WRF-Chem that can account for the impact of sensible heat fluxes from wildfires on thermodynamics. The model is used to investigate how the Texas Mallard Fire on 11-12 May 2018 led to the development of pyrocumulonimbus (pyroCb) clouds that are well simulated by accounting for both the effects of heat and aerosols emitted from the wildfire. Both heat and aerosol effects increase low-level temperatures and mid-level buoyancy and enhance convective intensity. Intensified convection along with more supercooled liquid condensate at high altitudes due to stronger vertical transport, results in larger hailstones and enhanced lightning. The effects of heat flux on the extreme convection are more significant than those of aerosol emissions. This is on the contrary to the effect of urbanization in Houston for which the effect of land surface change is smaller than that of aerosols, presumably because heat from fire is much more intensive than that from the urban heat island effect.