Answers: Hurricane rainbands
Q: Cross-cut figures of hurricanes show a structure like a jelly roll turned on end and sliced through the middle. Are these rings all connected in a continuous spiral or are they multiple rings? (I suspect they are continuous.) Also, how do they develop in the evolution of a hurricane? That is, why isn’t the hurricane just a big homogeneous storm? And, at what stage do they appear? At the onset, or somewhere along the way from a disturbance to the full blown hurricane?
Paul, Bethesda, Md.
A: As with almost any question involving the weather, the answer to your questions are complicated.
To begin, meteorologists call the “rings” you mention “rainbands” or “spiral bands” because they consist of the thunderstorms (with heavy rain) that make up a hurricane and they spiral in toward the eye along the path of wind that is blowing toward the storm’s center.
They begin as “some disorganized spiral rainbands (that) surround the center of a weak low pressure system. Later … some of the rainbands have amalgamated into an eyewall, and a large rainband spirals into and connects with the eyewall.”
The quotation above is from an extensive report on the big 2005 “Hurricane Rainband and Intensity Change Experiment” (RAINEX) published in the November 2006 issue of the Bulletin of the American Meteorological Society (BAMS). (The full report is available as a pdf file.) While it’s technical anyone with a basic understanding of meteorology will understand much of it. It’s also a good example of the kind of research conducted by scientists who fly into hurricanes.
The eyewall is the ring of thunderstorms around the nearly calm center of a hurricane and is the location of the storm’s fastest winds. A good share of the air that flows into a hurricane rises in the eyewall with most of it flowing away out of the top of the storm. Some of this air sinks into the eye, evaporating clouds in the eye and keeping the eye warmer than the surrounding air.
An important reason for scientific interest in rainbands is, as the report says: “In some intense storms, principal and/or secondary rainbands may evolve into a secondary eyewall … which becomes symmetric with an older inner eyewall. The secondary eyewall then becomes the primary recipient of radially inflowing low-level warm, moist air. The inner eyewall collapses as it is cut off from the main low-level inflow.”
In other words, learning more about hurricane rainbands is an important part of learning enough about hurricanes to enable forecasters to make better predictions of when a hurricane’s wind speeds are gong to rapidly increase.
I sent your question to Jeffrey Halverson, a professor and hurricane researcher at the University of Maryland Baltimore Campus, and noted that I had been able to find little in the professional literature–and nothing in the popular literature about hurricane rainbands (including in my own books). He answered:
“The metaphysical question as to why hurricanes form rainbands in the first place, is still somewhat of an enigma. It’s OK for hurricanes to still harbor secrets!
“Aside from the fact that they are not necessarily continuous, and the hurricane is organized around both rainbands and concentric circles…I usually tell students that there are plenty of other analogs in nature, and that the logarithmic spiral seems to be a ubiquitous form of self-organization, ranging from spiral galaxies all the way down to the humble nautilus shell.
“Like you, I appreciate that few papers in the literature address this problem, and the few that I have seen describe them as gravity waves. But it’s more intriguing that nature prefers to organize many of its sub-systems around the mathematical constant phi (the Golden Ratio), which is the basis of the Fibonacci sequence, and logarithmic spirals in general.”
Halverson is on the science team for the Hurricane and Severe Storms Sentinel, a NASA multi-year mission that will use two Global Hawks (unmanned airplanes) to study hurricane genesis and intensity change over the Atlantic.
Chapter 10 of The AMS Weather Book: The Ultimate Guide to America’s Weather is a good introduction to hurricane science.