Aerosol and thermodynamic effects on the formation and evolution of Amazonian clouds as observed from aircraft measurements

• Ano de defesa: 2017
• Autor(a) principal: Micael Amore Cecchini
• Orientador(a): Luiz Augusto Toledo Machado
• Banca de defesa: Daniel Alejandro Vila, Maria Assunção Faus da Silva Dias, Manfred Wendish
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Instituto Nacional de Pesquisas Espaciais (INPE)
• Programa de Pós-Graduação: Programa de Pós-Graduação do INPE em Meteorologia
• País: BR
• Link de acesso: http://urlib.net/sid.inpe.br/mtc-m21b/2017/03.30.17.37

Convective clouds over the Amazon are a key component of the South America and global climate systems. Nonetheless, they are poorly understood and models struggle to represent them appropriately. One of the primary reasons for that is the lack of data available for this type of cloud. Satellite retrievals have high underlying uncertainties for continental regions, especially considering the variability of surface reflectance between forested and deforested regions. The inherent lack of infrastructure also hampers the deployment of continuous field experiments. Additionally, the underlying physical processes in tropical clouds and their relation to aerosols is an open scientific question. This study aims at contributing to filling this gap by reporting on recent aircraft measurements over the Amazon, encompassing forested, deforested, urbanized, and maritime regions. One of the focus was on studying the interactions between the pollution plume generated by Manaus, a 2-million-inhabitant city surrounded by hundreds of km of rainforest, and the surrounding clouds. This was achieved by a low-altitude research aircraft that continuously penetrated the plume region and its surrounding cleaner air downwind from Manaus. Another research aircraft performed long range flights from remote regions on northern and northwestern Amazon, to the biomass-burning-polluted Arc of Deforestation in the south, and off the coast of Amapá State for the maritime reference. Both small and large scale approaches showed a primary role of aerosols on the warm-phase microphysical characteristics of Amazonian clouds regardless of thermodynamic conditions. Sensitivity calculations demonstrated that when aerosol number concentrations increase by 100%, there is an +84\% and -25\% response on cloud droplet number concentration and effective diameter, respectively. On the other hand, when updraft speeds strengthen by the same amount, droplet number concentrations increase by 43\% and almost no effect is seen on the effective diameter. It shows that aerosols have significant impacts on the clouds microphysical structure, while updrafts modulate the amount of condensed water. In this study, it is proposed that the aerosol-cloud interactions can be studied by using the Gamma droplet size distribution (DSD) parameterization and its phase state. The phase state is defined by the three parameters that define the Gamma curve and can be visualized in an 3D plot. Polluted and clean clouds were found to populate different regions in this space, where each point represents one DSD measurement. By sequentially connecting points associated to increasing altitudes, it was possible to infer the DSD evolution during the clouds development and define trajectories in the phase space. The trajectories of polluted and clean clouds where substantially different given the different balance between condensational and collection growth mechanisms. It is suggested that those growth processes can be represented by pseudo-forces in the 3D phase space because they are able to generate displacements. The pseudo-force balance in clean clouds were found to favor their development into the phase space region favorable for fast glaciation, while polluted clouds remained outside of it. As a consequence, clean clouds readily glaciate above the 0 $^{o}C$ isotherm while supercooled droplets persist in polluted system. This was confirmed by hydrometeor sphericity measurements. Overall, this work contributes to the understanding of Amazonian clouds by both providing statistics of their microphysical properties and by suggesting a new way to study the underlying physical processes in the Gamma phase space. The two approaches can be useful to infer specific modeling weaknesses regarding tropical clouds as well as steer the development of new parameterizations.