Gust or Bust: Blustery Winds Important for Modeling Tropical Rainfall

February 25, 2019

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In areas with weak surface winds, additional evaporation from the ocean’s surface is a major energy source for driving tropical patterns that create rainfall.
In areas with weak surface winds, additional evaporation from the ocean’s surface is a major energy source for driving tropical patterns that create rainfall.
Image courtesy of Pacific Northwest National Laboratory

The Science

Wind gusts beneath rainstorms over the ocean cause more water to evaporate from the ocean's surface. How does this increased evaporation affect the climate? How does it affect precipitation levels? A research team found that the additional surface evaporation is an important energy source for tropical circulations. These circulations create rainfall in areas with weak surface winds, such as the Tropical West Pacific, which in turn affects large-scale atmospheric motions influencing precipitation and temperatures over the United States and other distant regions.

The Impact

Accurate models of rainstorms are vital to understand how rain patterns affecting crops, floods, and droughts will change in a carbon-dioxide-warmed world. In the northern Tropical West Pacific, a region with very strong rainfall and weak surface winds, summertime precipitation is often underestimated in Earth system models. The models show less rain than is seen in the field. This work shows how to get simulations that are more accurate: add in surface wind gusts under rainstorms over the ocean.

Summary

Earth system models struggle to represent some aspects of tropical circulations, leading to offsets from observations in tropical rainfall. In particular, models often underestimate summer rainfall in the northern Tropical West Pacific. The simulated rain shortage is related to the low amount of evaporation arising from light surface winds in the region. General circulation models typically neglect wind gusts generated below evaporating rain, but these gusts create circulations that increase surface evaporation. Using the Energy Exascale Earth System Model, researchers determined that the increased surface evaporation from these "gusty winds" acts as a local source of moist energy to the atmosphere. This energy intensifies the circulation and total surface rainfall amount. Clarifying the role that surface evaporation plays in increasing rainfall has improved our understanding of tropical circulations.

Contact

Biological and Environmental Research Program Manager
Dorothy Koch
Department of Energy, Office of Science, Earth System Modeling
Dorothy.Koch@science.doe.gov

Principal Investigator
Philip Rasch
Pacific Northwest National Laboratory
Philip.Rasch@pnnl.gov

Funding

This research was supported as part of the Energy Exascale Earth System Model project, funded by the U.S. Department of Energy (DOE) Office of Science, Biological and Environmental Research. The research used computational resources of the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. The global ocean heat flux and evaporation products were provided by the Woods Hole Oceanic Institute Objectively Analyzed Air-Stream Fluxes for the Global Oceans project funded by the National Oceanic and Atmospheric Administration Climate Observations and Monitoring program. The Global Precipitation Climatology Project satellite gauge combined precipitation data were developed and computed at the National Aeronautics and Space Administration Goddard Space Flight Center's Mesoscale Atmospheric Processes Laboratory-Atmospheres as a contribution to the Global Energy and Water Exchanges Global Precipitation Climatology Project.

Publications

B.E. Harrop, P.L. Ma, P.J. Rasch, R.B. Neale, and C. Hannay, "The role of convective gustiness in reducing seasonal precipitation biases in the Tropical West Pacific." Journal of Advances in Modeling Earth Systems 10, 961 (2018). [DOI: 10.1002/2017MS001157]

Related Links

Pacific Northwest National Laboratory science highlight: Gust or Bust: Blustery Winds Important for Modeling Tropical Rainfall