A cool surface of the Amazon River alters daytime precipitation and moisture convergence

 

Submitter

Wang, Die — Brookhaven National Laboratory

Area of research

Cloud Processes

Journal Reference

Wu M, J Lee, D Wang, and M Salameh. 2021. "Suppressed Daytime Convection over the Amazon River." Journal of Geophysical Research: Atmospheres, 126(13), e2020JD033627, 10.1029/2020JD033627.

Science

We provide a comprehensive picture of the precipitation characteristics modulated by the cool Amazon River surface using both high-resolution satellite products and regional climate modeling.

Impact

The results will have implications on how land cover change may influence the hydroclimate in moist tropical regions. In addition, these results can also help interpret the ground-based ARM meteorological observations during the GoAmazon2014/15 field campaign.

Summary

We investigated the interaction between surface conditions and precipitating convection by comparing the Amazon River against the surrounding forest. The river surface is substantially cooler than the surrounding forest during the day and warmer at night. We analyzed 20 years of high-resolution satellite precipitation data and confirmed daytime rainfall reduction over the river for the whole Amazon Basin. The percentage reduction is strongest during the dry-to-wet transition season. In addition, the percentage reduction of individual tributary is significantly correlated with the Laplacian of surface temperature, which causes thermally driven surface divergence and suppresses local convection. Additionally, nighttime rainfall is enhanced over tributaries near the Atlantic coast during the wet season. A regional climate model then simulates the local rainfall anomalies associated with the river. Above the river, moisture diverges near the surface and converges above the surface before the daytime rainfall, partially driven by the horizontal gradient of humidity. Unlike the river, moisture convergence within the boundary layer is more critical for the rainfall above the forest region. Our studies suggest that strong thermal contrast can be important in deriving heterogeneous convection in moist tropical regions.