Lightning and the Role of River Breezes in the Amazon

Lightning and rainfall do not have a linear relationship. Rainfall can be produced by modest lifting and moderate updrafts over large areas, but lightning production is a response from deeper lifting and strong updrafts. The Amazon basin itself is an example of this “mismatch” on rainfall and lightning along the year: there is a very well defined wet (November to May) and dry (June to October) seasons and also a very well defined lightning season that occurs during the transition between the dry an prior the onset of the wet season (September to November). Besides the onset of the wet season, other features can lead to deep convection and lightning in the Amazon. One is the deforestation, which deepens planetary boundary layer, increasing cloud base heights and leading to shallower warm cloud depth and anomalous higher conditionally thermodynamical instability, and another is the indirect effect of pollution from biomass burning and urban areas in clouds, which can suppress warm rain and rainout increasing its chance to grow as deep convection. In addition, the presence of large bodies of waters such as rivers and lakes can alter local circulation.
In this study, we show that some specific rivers have an important and determinant role in the development of deep convection and very high flash rate density along the Amazon basin. Rivers that have a large width and are nearly parallel to the main easterly windflow are able to produce a strong river-land breeze circulation that works as natural barriers for the easterlies, promoting uplift and enhancing afternoon convection. This feature is seasonal dependent and contributes to more than 70% of afternoon lightning production during the transition between the dry-to-wet seasons. In fact, the enhanced deep convection by river breeze is the main reason for thunderstorm development at the location of Brazil’s top lightning hotspot located approximately 120 km northwest of Manaus.