What is it about?
Tropical cyclones (a.k.a. hurricanes, typhoons, or cyclones, depending on the ocean basin in question) are observed to span a very large range of sizes. Some are quite small, such as Cyclone Tracy, which inflicted severe but localized damage on Darwin, Australia in late December 1974. Others are massive, such as Supertyphoon Tip in 1979. What factors control the highly variable size of these dangerous storms? When conducting numerical model experiments, we noticed that the size of tropical cyclones that developed in our idealized simulations varied significantly depending on the relative humidity (how close the air was to saturation) in the environment surrounding the storm. More humid environments yielded larger storms, in terms of the wind, cloud, and precipitation field. Storms in drier environments could still be very strong, but tended to have much less precipitation in the spiral bands surrounding the inner core of the storm. This, in turn, is related to the size of the wind field: We showed that precipitation in the outer bands resulted in lowering of the pressure there, and expansion of the storm wind field. Furthermore, we found that in more humid environments, the tropical cyclone formed additional "eye walls", and that these underwent "replacement cycles" that caused fluctuations in storm intensity.
Featured Image
Why is it important?
Many tropical cyclone impacts are related to their size. Previous studies have shown that the strength of the "storm surge", which is the onshore push of ocean water that accompanies many landfalling tropical cyclones, is related to storm size. Think about trying to slosh water against the side of a bath tub with just one finger (a small storm). Then, think about doing the same with your whole arm (a large storm). Also, larger storms affect larger areas, and for longer duration, which can mean greater accumulated rainfall and damage from prolonged strong winds. The eyewall replacement cycles we found only took place in the most humid simulations. This could have important implications for the prediction of tropical cyclone intensity, which is a very difficult challenge. Previous studies had shown that Atlantic tropical cyclones tended to be smaller than those in the western North Pacific basin. Dry air originating over the Sahara Desert often moves westward over the North Atlantic (the so-called "Saharan Air Layer"), creating drier conditions there. There is no counterpart to this in the western North Pacific. Our findings were thus consistent with the earlier observations of size differences between Atlantic and Pacific tropical cyclones, and offer a useful explanation of this difference. For these and other reasons, it is important to know the size of tropical cyclones, and to understand what controls this aspect.
Perspectives
When my family and I first moved to North Carolina in 1999, we arrived in time to experience the large and destructive Hurricane Floyd. A few years later, I was impressed with the powerful but diminutive Hurricane Charley (2004). Contrasting these storms made me wonder what controlled their size, and I found few published studies on this topic. A few years later, first author Kevin Hill was a PhD student in my group, and we were studying hurricanes and climate change. In this work, we were interested in how these storms responded to changes in the larger-scale environment in which they were embedded. In conducting idealized storm simulations, we noticed the relation between storm size and the relative humidity in the environment. Although it was not part of the original research plan, we recognized that this was an interesting and important result, and conducted additional experiments to clarify this dependence. Sometimes, it is important to allow sufficient flexibility to pursue interesting findings, even if they are not part of the original research plan.
Gary M Lackmann
North Carolina State University
Read the Original
This page is a summary of: Influence of Environmental Humidity on Tropical Cyclone Size, Monthly Weather Review, October 2009, American Meteorological Society,
DOI: 10.1175/2009mwr2679.1.
You can read the full text:
Resources
Contributors
The following have contributed to this page
