Fog and Rain in the Amazon

Summary

The diurnal and seasonal water cycles in the Amazon remain poorly simulated in general circulation models. Simulations using existing models exhibit peak evapotranspiration during the wrong season and rain occurring too early in the day. A team of researchers supported by the Terrestrial Ecosystem Science and Atmospheric System Research programs and using data from the GOAmazon campaign show that those biases are not present in an approach opposite to that taken by general circulation models, in which they resolve convection and parameterize large-scale circulation as a function of the resolved convection.

The ability to simulate the seasonality of the hydrologic cycle in the Amazon using this approach is attributed to (1) the representation of the morning fog layer, and (2) more accurate characterization of convection and its coupling with large-scale circulation. The morning fog layer, present during the wet season, but absent in the dry season, dramatically increases cloud albedo, which reduces evapotranspiration through its modulation of the surface energy budget. These results highlight the importance of the coupling between the energy and hydrological cycles and the key role of cloud albedo feedback for climates over tropical continents. The study indicates understanding of tropical climates over land can be considerably advanced by using coupled land–atmosphere models with explicit convection and parameterized large-scale dynamics.

References

Anber, U., P. Gentine, S. Wang, and A. H. Sobel. 2015. “Fog and Rain in the Amazon,” Proceedings of the National Academy of Sciences (USA) 112(37), 11,473–477. DOI: 10.1073/pnas.1505077112.