The relative and joint effect of rivers and urban area on a squall line in the Central Amazonia

In the Amazon, large cities are typically bordered by rivers and dense forest areas, which influence and are influenced by mesoscale convective systems. In the Amazon, the growth and establishment of cities is strongly dependent on the presence of the rivers; thus, understanding the influence of the river-city system on mesoscale systems is relevant. In this work, we used the WRF model to assess the impact of the presence of both the river and the urban area on the development of a Squall Line (SL) that reached the city of Manaus (Brazil). Seven microphysical schemes were evaluated, aiming to verify the performance of the model to represent general characteristics of the SL. After evaluating WRF skill, sensitivity experiments, using four ensembles perturbing the initial conditions, about the effect of changes in the surface were conducted. Results showed the regional effect of the river influences the arrival of the SL at Manaus. The urban presence also favored the intensification of SL near Manaus. Furthermore, the absence of the river produced a weaker SL, which arrived earlier with maximum intensity after passing through Manaus. In turn, the river influences the accumulated rainfall over the urban area. Without the city, the accumulated rainfall reduced by 8.85%; without the river, the reduction reached 49%; and without both, a decrease of about 59.5% was observed. The effect of the river on the accumulated rainfall over the city depends on the urban structure itself. Therefore, this study reveals that the presence of the river intensifies the SL over the city, but the urban structure dictates the intensity of this effect.