Analyzing the effects of aerosols and moisture on the organization of shallow cloud fields in the Amazon

Abstract

Our main hypothesis deals with interaction between thermodynamic and microphysical processes and how they help organize locally generated convective cloud fields over the Amazon. The organization of cloud field here is viewed as the physical distribution of the clouds over a certain domain, as well as the size distribution and the distance between the clouds themselves. Several small clouds spread out over a domain indicates an unorganized cloud field, while fewer and larger clouds in close proximity indicates the opposite. It follows that a highly organized cloud field is prone to generating deep convection, therefore favoring the so-called shallow-to-deep transition. This transition has been historically misrepresented in climate models where deep convection is triggered too quickly compared to observations. This leads to errors in climate projections because of the consequent errors in the hydrological and energetic cycles. In this study, we aim to shorten this gap by way of increasing our understanding of how clouds organize themselves in domains with no large-scale influence. In other words, what are the thermodynamic and microphysical factors that determine whether the cloud field will be organized or unorganized. Here we will study two factors that most likely play important roles based on the literature. The first and primary one is the moisture availability, which, given a fixed temperature profile, determines the atmospheric static instability. Secondly, there is evidence showing that aerosol pollution may hamper or favor cloud organization depending on the level of pollution and meteorological context. We will study the interplay between those two factors to quantify their individual and combined effects on cloud organization. This will be achieved by a series of idealized simulations with the RAMS (Regional Atmospheric Modelling System) model. The reference simulation will be achieved by providing typical thermodynamic profiles of shallow-cumulus cloud field days to the model. From there, we will vary the boundary layer moisture amount and aerosol pollution levels both individually as well as combined to analyze the effects on the cloud field organization. This organization will be quantified using indexes available on the literature, therefore allowing direct correlations with the pre-convective environment proprieties. The overall goal is to improve our knowledge on how the thermodynamic and microphysical conditions can induce cloud field organization and deep convection initiation. This will lay the groundwork for future improvements in climate model parameterizations.