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Changes in simulated tropical rainfall due to increased resolution

 

Stephanie J. Johnson,  Richard Levine,  Andrew Turner,  Gill Martin, Steve Woolnough and the UPSCALE team (PI Pier Luigi Vidale)

NCAS-Climate, University of Reading Dept. of Meteorology and the Met Office

 

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Stephanie J. Johnson is a Research Scientist who works to improve forecasts of the Indian monsoon.

 

What are the new findings?

Simulations of the Earth’s atmosphere are used to predict weather and climate. We increased the simulation resolution by more than five times, to include more detailed features such as mountain ranges and small weather systems, and analysed changes in Indian monsoon rainfall and summer rainfall in the surrounding region. Compared to observations, rainfall over India increases by about 20%. Over Indonesia and islands between the Indian and Pacific oceans, it changes by 40% to over 100%.

Why are these findings important?

Tropical rainfall is difficult to simulate without large errors. Our results highlight locations where increasing resolution changes or improves tropical rainfall. Over India, the small increase in rainfall is an improvement, but is much smaller than the error. Over Indonesia, the larger increase in rainfall is an improvement in the error. North of Indonesia, the large decrease in rainfall worsens the error. Analysis of which newly resolved feature changes the rainfall suggests where high resolution is needed to accurately represent tropical rainfall, and this can also be used to improve lower resolution simulations.

How did we discover this?

This research used the UPSCALE dataset which contains eight simulations at three different resolutions. The simulations divide the atmosphere into a grid of columns from the Earth’s surface to the top of the atmosphere. The area of the column base, or the number of columns, determines the resolution. Using UPSCALE has enabled us to increase columns from 25,000 (≈200: km resolution) to 750,000 (≈40 km resolution).

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Above: The change in summer rainfall between the highest and lowest resolution simulations, shown as a fraction of the observed rainfall. Notice the large increase in rainfall over Indonesia, and decrease in rainfall north of the islands. Note also the smaller increase in rainfall over India.

 

Find out more:

  • see Stephanie Johnson's webpage
  • Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
  • Take a look at the Journal article

Johnson et. al 2015, Climate dynamics in press. DOI: 10.1007/s00382-015-2614-1

 

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This research was funded by the Joint Weather and Climate Research Programme, a collaboration between the Met Office, NCAS-Climate and the Natural Environment Research Council.