September 21, 2012

Agriculture: Tropical Cyclones are Welcome Visitors

When talk turns to tropical cyclones—hurricanes and their weaker siblings—the topic usually becomes winds, waves, and destruction. Or an exchange of war stories, like vacations cancelled, or harrowing tales from the beachfront lines. In some circles, anthropogenic climate change may enter the conversation.

But rarely does the discussion turn to the benefits of tropical cyclones.

For example, tropical cyclones often deliver widespread drought-busting rainfall during the crucial late-summer period when field crops are maturing. Across the Southeastern U.S., where most fields are non-irrigated, crops such as corn, soybeans, peanuts, tobacco, and hay benefit from tropical cyclone precipitation. From east Texas along the coast to Maryland, non-irrigated crops, like those above, combine to bring in a good ten billion dollars per year. While this economic activity is not all ascribable to tropical cyclone precipitation, neither is such precipitation inconsequential.

Evidence for this comes from a paper soon to appear in the Journal of Climate by Oliver Prat and Brian Nelson, two scientists from the U.S. National Climate Data Center in Asheville, NC.

Prat and Nelson analyzed satellite-observed precipitation associated with all tropical cyclones passing near or over the southeastern U.S. from 1998 through 2009 (there were 113 storms in total). Averaged over the entire hurricane season (June through November) and across their entire spatial domain (72W to 104W, 24N to 40N) tropical cyclone rainfall made up about 7% of the total precipitation. But this was not spread uniformly. Tropical cyclones contributed about 15-20% of the total seasonal precipitation within about 100 miles of the coast, 8-12% of the precipitation 100 to 200 miles from the coast, and less than 2% in areas further inland (Figure 1).


Figure 1. Percentage of total June through November precipitation contributed by tropical cyclones, 1998-2009 (from Prat and Nelson, 2012).

And of course this percentage varies from year to year (Figure 2), with a low of about 1.3% in 2009 and a high of nearly 14% in 2005. But as can be seen from Figure 2, almost every summer includes some tropical cyclone precipitation across most of the coastal states from Texas to Maryland. In other words, tropical cyclone precipitation is an integral part of the region’s climate and thus a vital part of its agricultural integrity.


Figure 2. Total precipitation contributed by tropical cyclones, 1998-2009 (from Prat and Nelson, 2012).

Prat and Nelson’s findings square with other recent investigations of the contributions of tropical cyclone precipitation across the eastern U.S., notably, Knight and Davis (2009), and Atallah et al., (2007) [disclosure: Knight, Davis, and Atallah are all former students and/or colleagues of ours so they must be right!]

So next time you are at a party and the conversation turns to hurricanes, after describing how you managed to survive some close encounters of third category, be sure to add that as the storms weakened and progressed inland, they went on to deliver very beneficial rainfall to the region’s agricultural interests and gave a boost the regional economy.

It’ll be a refreshing change from the normal talk of weather-related doom and gloom.

References:

Atallah, E., L. F. Bosart, and A. R. Aiyyer, 2007. Precipitation distribution associated
with landfalling tropical cyclones over the Eastern United States. Monthly Weather Review,
135, 2185-2206.

Knight, D. B., and R. E. Davis, 2009. Contribution of tropical cyclones to extreme
rainfall in the Southeastern United States. Journal of Geophysical Research, 114, D23102, doi:
10.1029/2009JD012511.

Prat, O., and B. Nelson, 2012. Precipitation contribution of tropical cyclones in the Southeastern United States from 1998 to 2009 using TRMM satellite data. Journal of Climate, doi:10.1175/JCLI-D-11-00736.1, in press.




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