Journal Article

The Amazon River‐Breeze Circulation Limits Detection of Aerosol‐Cloud Interactions in Warm Clouds

Holz, Robert, Platnick, Steven, Fast, Jerome, Christensen, Matthew W., Tai, Sheng‐Lun, Wind, Gala, Varble, Adam C., Meyer, Kerry

2026

Overview

Increased aerosol concentrations can brighten low‐level clouds and extend their lifetimes, but aerosol–cloud interactions (ACI) remain highly uncertain and difficult to quantify. We show that part of this uncertainty is caused by topographical influences on clouds, that is, those arising from land–water contrasts. This is demonstrated using satellite retrievals in regions with extensive river networks, such as the Amazon Basin. 15 years of MODerate resolution Imaging Spectroradiometer (MODIS) satellite data show cloud formation over the Amazon River basin is suppressed by 26% with warm low clouds above the river exhibiting a 22% smaller droplet effective radius and 18% higher droplet concentration (NdN_(d) ) compared to adjacent land clouds. Thus, clouds above the river may appear polluted but are actually influenced by river‐breeze circulations driven by the thermal contrast between the river and the surrounding land. These responses are robust in both wet and dry seasons, and tests using an improved MODIS retrieval product show cloud differences are unlikely due to retrieval artifacts. In situ measurements from the Green Ocean Amazon Experiment (GoAmazon) confirm that NdN_(d)is elevated above rivers and are also higher when carbon monoxide concentrations are elevated near the large city of Manaus. Lagrangian airmass tracking over Manaus shows that regional‐scale river‐breeze circulations impact NdN_(d)as much as the urban aerosol plume, complicating ACI attribution and highlighting the need to isolate land‐surface effects to assess ACI in continental regions. Plain Language Summary Aerosols, tiny particles suspended in the air, play an important role in cloud formation by acting as seeds for cloud droplets to grow, thus affecting cloud brightness. Cloud brightness and size regulate how clouds cool the atmosphere by reflecting sunlight, but isolating how aerosols influence clouds over land is challenging because surface properties also affect cloud properties. Using 15 years of satellite data from the Amazon river basin, it was found that river‐breeze circulations, caused by temperature differences between cooler river surfaces and warmer surrounding land, significantly affect cloud properties. Clouds over rivers have smaller droplet sizes and more numerous droplets compared to clouds over nearby land. These clouds can look “polluted” by high aerosol concentrations even when their characteristics are shaped by natural processes rather than human activity. Data from the GoAmazon experiment supports these findings, and tests with improved satellite retrieval products confirm that these cloud differences are not due to satellite data errors. This research emphasizes the importance of separating the natural effects of rivers on cloud properties from aerosol impacts to accurately attribute human‐related influences, such as those from cities and fires, and improve our understanding of how aerosols affect clouds and the energy budget. Key Points Daytime low‐cloud fraction is strongly suppressed over Amazon and other major rivers compared to adjacent land Urban aerosols from Manaus significantly increase cloud droplet concentrations, locally, and up to 200 km from the city River‐breeze circulations influence low‐cloud properties as much as the aerosol sources, supporting greater drop concentrations over rivers

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Aerosol-Cloud Interactions / Aerosols / Air circulation / Aircraft / Algorithms / Carbon monoxide / cloud physics / Clouds / Datasets / Dry season / GoAmazon / In situ measurement / Land area / MODIS / Precipitation / Rainy season / River basins / River networks / Rivers / river‐breeze circulation / satellite remote sensing / Satellites / Unmanned aerial vehicles / warm clouds / Water pollution