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Tropical Precipitation Extremes: How Well Are They Modeled?
Rossow, W.B., Mekonnen, A., Pearl, C. and Goncalves, W. 2013. Tropical precipitation extremes. Journal of Climate 26: 1457-1466.

The authors write that "some of the concern about possible negative impacts of a warming climate is focused on possible increases of precipitation extremes." And within this context, they indicate that "for the case of large precipitation amounts that cause flood damage, larger total accumulation can be produced by [1] events with larger average precipitation intensities, [2] events with more moderate average intensities but longer durations (and slower motions), or [3] even more frequent events of moderate average intensity."

What was done
Rossow et al. demonstrate that "the distribution of precipitation intensity is not determined by precipitation as a single type of phenomenon with a range of values," but that it "comprises the separate distributions of at least two distinct types of deep convection (with the collection of 'non-deep-convective' or shallow convection types contributing most to the smaller intensities)." But contrary to what reason would suggest if this be true, they say that "all of the climate GCMs currently parameterize tropical deep convection as a single process, localized to individual grid cells (on the order of 25-200 km in size) with short lifetimes (on the order of minutes to a few hours) that most resembles ordinary cumulonimbus [italics added]."

What was learned
On the basis of these observations, the four researchers contend that "the observed distinctive behavior of the different deep convective storm types undercuts the simple projection of changes of extremes based on the large-scale balances or by a simple scaling [italics added]." And they say that "these results draw attention to the need to understand why different deep convective storm types exist, how they interact with each other and with the larger-scale circulation, and what role they each play in the atmospheric general circulation."

What it means
Cutting to the chase, Rossow et al. conclude 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."

Reviewed 17 July 2013