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Modeling Extreme Precipitation in the Tropics

Rossow, W.B., Mekonnen, A., Pearl, C. and Goncalves, W. 2013. Tropical precipitation extremes. Journal of Climate 26: 1457-1466.
In an important study published in the Journal of Climate, Rossow et al. (2013) write that "some of the concern about possible negative impacts of a warming climate is focused on possible increases of precipitation extremes," and, therefore, they go on, as they describe it, to "exploit more than a decade of independent cloud and precipitation data products covering the whole tropics (15°S-15°N) to more clearly separate the contributions to average precipitation intensity and daily average accumulation rate made by the different types of deep convective systems," focusing on the period 1998-2008.

As a result of this effort, the four researchers determined that "the whole distribution of instantaneous precipitation intensity and daily average accumulation rate is composed of (at least) two separate distributions representing distinctly different types of deep convection associated with different meteorological conditions." In particular, they found that the extreme portion of the tropical precipitation intensity distribution "is produced by 40% of the larger, longer-lived mesoscale-organized type of convection with only about 10% of the ordinary convection occurrences producing such intensities." And when accumulation rates were considered, they found that "essentially all of the values above 2 mm/hour are produced by the mesoscale systems."

Unfortunately, Rossow et al. note that "today's atmospheric models do not represent mesoscale-organized deep convective systems that are generally larger than current-day circulation model grid cell sizes but smaller than the resolved dynamical scales," while further noting that these mesoscale convective systems "last longer than the typical physics time steps."

So what is the ultimate consequence of these model deficiencies? In the concluding sentence of their paper, the four researchers state that "until the full range of deep convective processes in the tropics is more realistically represented in climate models, they cannot be used to predict the changes of extreme precipitation events in a changing (warming) climate."

Archived 10 July 2013