Abstract. The remote atmosphere over the Amazon can be similar to oceanic regions in terms aerosol conditions and cloud type formations. This is especially true during the wet season. The main aerosol-related disturbances over the Amazon have both natural sources, such as dust transport from Africa, and anthropogenic sources, such as biomass burning or urban pollution. The present work considers the impacts of the latter on the microphysical properties of warm-phase clouds by analyzing observations of the interactions between Manaus city and its surroundings, as part of the GoAmazon2014/5 Experiment. The analyzed period corresponds to the wet season over a tropical rain forest (i.e., Feb to Mar 2014 and corresponding to the first Intensive Operating Period (IOP1) of GoAmazon2014/5), and the droplets observed are in the range 1 μm ≤ D ≤ 50 μm. The wet season largely presents a clean background atmosphere characterized by frequent rain showers. As such, the contrast between background clouds compared to those affected by the Manaus pollution can be observed and detailed. The focus is on the characteristics of the initial microphysical properties in cumulus clouds predominantly at their early stages. The pollution-affected clouds are found to have lower effective diameters and higher droplet number concentrations. The average differences range from 10% to 40% for the effective diameter and are as high as 1000% for droplet concentration across different vertical levels (0 to 3200 m). The growth rates of droplets with altitude are slower for pollution-affected clouds (2.90 compared to 5.59 μm km-1), as explained by the absence of bigger droplets at the onset of cloud development. Clouds under background conditions have higher concentrations of larger d roplets (e.g., > 20 μm) close to cloud base, which would contribute significantly to the growth rates through the collision-coalescence process. The droplet size distribution (DSD) overall shape do not appear to be predominantly determined by updraft strength, especially beyond the 20 μm range. The aerosol conditions play a major role in that case. However, the updrafts modulate the DSD concentrations and are responsible for the vertical transport of water in the cloud. The larger droplets found in background clouds are associated with weak water vapour competition and a bimodal distribution of droplets in the lower levels of the cloud, that enables an earlier initiation of collision-coalescence process. This study shows that the pollution produced by Manaus affects significantly warm-phase microphysical properties of the surrounding clouds by changing the initial DSD formation. The corresponding effects on ice-phase processes and precipitation formation should be the focus of future endeavors.
A stochastic min-driven coalescence process and its hydrodynamical limit
