On the representation of warm-phase microphysical processes in numerical simulations of cumulus clouds

• Autor(a) principal: Lianet Hernández Pardo
• Data de Publicação: 2020
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações do INPE
• Texto Completo: http://urlib.net/sid.inpe.br/mtc-m21c/2020/10.27.17.28

This research analyzes the role of microphysical processes in the evolution of convective clouds, with emphasis on their representation in atmospheric models, taking advantage of in-situ measurements obtained as part of the ACRIDICON-CHUVA field campaign near the Amazon basin. Firstly, cloud sensitivities to changes in aerosol properties, such as total number concentration (Na), size distribution and composition, are explored. This analysis focuses on the droplet number concentration and effective diameter at the top of a shallow cumulus simulated with a bin-microphysics single-column model, for initial conditions typical of the Amazonian region. The impact of considering bin versus bulk aerosol size distributions is evaluated, in order to investigate the influence of entrainment and activation scavenging on the derived sensitivities. It is shown that the evolution of cloud-top droplet size distributions (DSDs) is generally very sensitive to changes in aerosol parameters, but the sign and absolute value of this sensitivity depends on the position within the multidimensional aerosol parameter space considered, as well as on the treatment of entrainment and aerosol scavenging. Following, idealized two- and three-dimensional Weather Research and Forecasting model simulations of an isolated warm cumulus cloud were performed to assess the role of DSD broadening mechanisms in bin microphysics parameterizations. As expected, collision-coalescence is a key process broadening the modeled DSDs. In-cloud droplet activation also contributes substantially to DSD broadening, whereas evaporation has only a minor effect and sedimentation has little effect. Cloud dilution (mixing of cloud-free and cloudy air) also broadens the DSDs considerably, whether or not it is accompanied by evaporation. Artificial broadening from vertical numerical diffusion appears to be relatively unimportant overall for these cumulus simulations. Finally, the spatial distribution of DSD relative dispersion () within observed and modeled convective clouds is analyzed, taking into account changes in Na. The spatial distribution of DSD shape in aircraft measurements of growing cumuli near the Amazon basin shows distinctive patterns depending on the aerosol loading. In cleaner clouds (Na < 900 cm&#8722;3), varies between 0.1 and 0.6, overall being inversely related to the ratio of the cloud water content (qc) and the adiabatic water content (qa). In polluted clouds (Na > 2000 cm&#8722;3), generally has values in the range 0.25 &#8722; 0.45, with no evident dependence on either height above cloud base or qc/qa. Bin-microphysics numerical simulations confirm that these contrasting behaviors are associated with the predominance of collision-coalescence in cleaner clouds, and in-cloud droplet activation in polluted ones. These findings can be useful for parameterizations of the shape parameter (&#956;) of droplet gamma size distributions in bulk-microphysics cloud-resolving models. It is shown that emulating the observed &#956;&#8722;qc/qa relationship improves the estimation of the collision-coalescence rate in bulk microphysics simulations.