PNNL

The Science                                

Tropical forests are important and sensitive ecosystems that rely on large amounts of rainfall. Increasing atmospheric carbon dioxide, or CO₂, changes the time of day when rain falls over tropical forests via two distinct physical mechanisms. Researchers illustrated these two pathways using the biogeochemically-enabled version of the Energy Exascale Earth System Model (E3SM). One such pathway involves the radiative response of the atmosphere to rising CO₂, while the other involves plant responses to increased CO₂ concentrations.

The Impact

Knowing how rainfall patterns in both space and time will respond to increasing atmospheric CO₂ is important for water management. This study sheds light on how the daytime cycle of rainfall over tropical forests responds to an increase in CO₂. The identified pathways reduce daytime rainfall relative to nighttime, making the distribution of rainfall more uniform across the span of a day.

Summary

Tropical rainforests receive more rainfall than almost any other land area on Earth. Understanding the sensitivity of rainfall in these regions to increasing atmospheric CO₂ concentrations is important for assessing the risks to the ecosystem and the people dependent on that rainfall. Currently, these areas have a strong day-night rainfall cycle. Using the biogeochemistry-enabled E3SM, scientists found that in response to rising CO₂ tropical forest rainfall becomes more uniform throughout the day. They identified two distinct physical pathways that govern this change in the rainfall timing. The first pathway starts with increased CO₂ leading to increased radiative heating. This results in a relative reduction in convective instability late in the day, producing less late-day rainfall (when climatological rainfall peaks). The second pathway starts with increased CO₂ leading to a reduction in evapotranspiration. This produces a warmer, drier boundary layer that leads to an increased lifting condensation level and a reduction in late-day rainfall. The physical realism of these pathways suggests these responses are likely not restricted to E3SM and could be active under higher CO₂ conditions.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, Ruby.Leung@pnnl.gov

Funding

This work was supported by the Department of Energy (DOE), Office of Science, Biological and Environmental Research program (BER), Earth System Model Development program area. Part of the work was performed under the auspices of DOE by Lawrence Livermore National Laboratory. Gabriel Kooperman acknowledges support from the BER Regional and Global Model Analysis program area. The data were produced using resources of the National Energy Research Scientific Computing Center, a user facility supported by the DOE Office of Science.