PNNL

Abstract

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, i.e. 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, 18% higher droplet concentration (Nd), and 7% greater cloud top height compared to adjacent land clouds. Thus, clouds above the river “appear” polluted though river-breeze circulations caused by the thermal contrast between the river and the surrounding land influence these cloud property differences. 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 Nd is elevated above rivers and are also higher when carbon monoxide concentrations are elevated near Manaus. Lagrangian airmass tracking over Manaus shows that regional-scale river-breeze circulations impact Nd 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.