Enhanced Deep Convection and Lightning Activity along the Amazon River tributaries

The Amazon basin exceptional land cover drives the convection in the worlds largest rainforest. Its enormous tropical forest is cut from the Andes to the Atlantic Ocean by the Amazon River, the second longest river in the world with its width ranging from 9 to 30 km. The basin is also cross cut by its approximately 1,100 tributaries. Several of these tributaries are also among the longest rivers with widths from several hundreds to a few kilometers. Rivers with these extents are known to produce breeze circulations that play major roles on the diurnal cycle and distribution of precipitation, but little is known on their impact on thunderstorm and lightning activity in the basin. In this study, we use TRMM and GOES-16 satellite data to show that specific rivers have an important and determinant role in the development of deep convection and very high flash rate density along the Amazon basin. Particularly, rivers with large widths that run nearly parallel to the main large-scale easterly windflow, during the daytime (i.e., breeze from the river towards the land) work as natural barriers for the easterlies, promoting uplift and enhancing afternoon convection inland at the eastern side of the river. These rivers are among the main Amazon River tributaries: Rio Negro, Solimoes, Japura, Purus, Madeira and Trombetas. The river breeze role is seasonal dependent of the land surface temperature contrast and the moistening of the planetary boundary layer. During the transition between the dry-to-wet seasons, river breeze contributes to deeper convection, showing higher cloud tops and stronger reflectivity profiles. More than 70% of afternoon lightning production along the Rio Negro river, including the location of Brazils top lightning hotspot (~120 km northwest of Manaus). Less wider rivers, such as Uatuma River nearby the Amazon Tall Tower Observatory (ATTO), also show enhanced lightning activity on its eastern side.