Aerosol and land surface impacts on tropical convective processes

Abstract

In this three-part dissertation, we investigate the dynamical and microphysical processes that determine how tropical convective clouds respond to changes in aerosols and land surface properties. We focus on the variability in such processes across thermodynamic environments and cloud types. Using a combination of large eddy simulations (LES), long-term satellite observations, and Lagrangian object-tracking, we explore the physical mechanisms underlying these interactions. First, we investigate how aerosol–cloud–precipitation interactions influence convective transport and aerosol removal. We run a suite of sixteen LES with varying aerosol loadings and chemical compositions using the Regional Atmospheric Modeling System (RAMS). We find that increasing aerosol loading leads to increased convective transport of aerosol to the mid-troposphere and decreased aerosol removal through rainout. This means that in more polluted environments, not only is the aerosol loading larger than in pristine environments, but clouds are less able to regulate aerosol loadings via rainout. We further use tobac (tracking and object-based analysis of clouds), a cloud object-tracking algorithm, to explore shifts in the cloud population as a function of aerosol loading and type. We describe contrasting aerosol effects on rainfall from shallow cumulus and congestus clouds, leading to non-monotonic trends in domain rainfall. Decomposing these trends into cloud type-specific effects highlights the utility of Lagrangian approaches in elucidating processes driving varied aerosol–cloud interactions. Second, we explore the impact of widespread anthropogenically driven deforestation on cloud properties in the tropics. We use two decades of satellite data and statistical attribution methods to demonstrate that long-term deforestation in Southeast Asia robustly alters cloud properties. We also provide the first observational evidence that the magnitude of the cloud response to deforestation depends on the atmospheric environment, specifically on moisture and aerosol loading. These results emphasize that regional differences in climatology must be considered when assessing deforestation impacts on clouds and the climate system. Finally, we investigate the mechanisms driving land surface–cloud interactions using LES and cloud object-tracking. We conduct two sets of simulations over Borneo with identical atmospheric initial and boundary conditions but differing land cover to explore how land surface changes impact convection. We discuss how conversion of tropical forests to palm oil plantations influences the surface energy budget, driving robust decreases in sensible heat flux but enhanced evapotranspiration. We identify and track tens of thousands of clouds and show deforestation decreases region-wide shallow cloud cover but enhances cloudiness along deforestation boundaries via mesoscale vegetation breezes. We also discuss deforestation-driven changes to the sea breeze, deep convection, and precipitation. Our results demonstrate that shallow and deep convection are coupled to the surface through processes acting on different spatiotemporal scales. These findings emphasize that deforestation impacts vary spatially as well as diurnally. The research in this dissertation has advanced our understanding of the physical processes driving land–aerosol–cloud interactions and quantified how cloud populations shift in response to aerosol and land cover changes. Moreover, we have assessed when and where these shifts are the greatest and thus where perturbations to the aerosol environment and the land surface have the most significant impact for clouds, precipitation, and the broader Earth system.