Page #6: Examining a Hurricane's Structure
Category 5 hurricanes are not common occurrences (although the 2005 Atlantic season has proven to be an exception to this rule). As we now know, Katrina was a category 5 hurricane on the 28th of August in the Gulf of Mexico. Having such a well-developed storm available to study allows us to take a quick look at some classic elements of hurricanes.
Before we begin, let's be clear...the things we are about to discuss are not characteristics that are limited to category 5 hurricanes. They can (and sometimes do) occur in weaker hurricanes. However, they are very easy to see in the case of Hurricane Katrina.
Let's begin with the eye. Though we have talked quite a bit so far about Katrina's eye and eye wall--including eye wall replacement--we have not described how and why an eye forms in a tropical cyclone--and why it matters. Let's briefly do so.
The eye of a hurricane is the center of the storm and easily identifiable in well-developed cyclones. But why does it form? Image courtesy of University of Illinois.
First, let's remind ourselves what the eye looks like. Recall that Katrina's eye presented as a precipitation free warm spot on infrared imagery. An eye can be cloud free, but as we will see with Katrina, that is often not the case. The lack of cold, precipitating clouds in the eye of a hurricane suggests that the eye itself--unlike the eye wall--is a region of subsidence and descending air (keep in mind, the eye wall is a region of ascending air and upward motion). So, to understand why an eye forms, we have to understand why there is subsidence in the eye.
It turns out that there are several theories for why subsidence takes place and the eye forms--and they get pretty complex pretty quickly. Let's look at a couple of them in basic terms.
Let's begin with the simpler of the two. Recall from our discussion of concentric eye walls that the severe squalls that ring the eye of the hurricane are where the most convergence (coming together of air) takes place. As that air converges, it has nowhere to go but up. It ascends in the strong storms of the eye wall.
Once that air ascends, where does it go? There are two answers to the question... some air heads away from the center, beginning the process of establishing antiyclonic outflow (we'll discuss outflow in more detail in a moment). Other parcels sink on the inside of the eye wall, helping to create the eye.
This air warms as it sinks, lowering pressures in the storm, and adding to or maintaining the storm's intensity, and creating the "warm core"--temperatures at any given altitude inside the eye are warmer than those outside of the eye. This relationship can actually be used to estimate a tropical cyclone's intensity. And, of course, sinking air limits the ability of parcels to ascend in the eye, keeping the clouds in the eye relatively low, or even suppressing them completely.
There's a second mechanism that causes subsidence in the eye. Again, we start with air ascending in the eye wall. It turns out that in an effort to balance several of the forces at play in the eye wall (conservation of angular momentum, the pressure-gradient force, and the centrifugal force), an air parcel ascending in the eye wall will move outward--away from the center of the storm--as it ascends. This tends to happen relatively low in the eye wall because of the mechanisms involved. As it spirals outward, some air leaves the eye wall. Air from inside the eye will replace this air that leaves the eye wall.
So, there is a net transfer of air out of the eye at low levels (just above the boundary layer). Air from higher levels will sink from the top of the eye to replace that air that left...and sinking air is subsidence.
So there are two mechanisms for the subsidence in the eye--one that comes directly from the outflow in the eye wall convection, and another, more complex mechanism involving subsidence to replace air exiting the eye at lower levels.
It is pretty easy to see the eye on the images we've looked at for Hurricane Katrina. It is the precipitation-free area (clear area) at the center of the storm's circulation. (I'll explain the other annotations in a moment)
Multispectral image of Hurricane Katrina on August 28 showing impressive cirrus outflow. Image courtesy of CIMSS.
The other text on this image points to the cirrus clouds racing out from Katrina. Cirrus clouds "pointing" away from the center of the storm are sure indicators of outflow taking place in the upper reaches of the atmosphere:
As you may know, when air "spreads out" in the atmosphere, it's called divergence. A sure sign of divergence in tropical cyclones is anticyclonic (clockwise in the northern hemisphere) flow. This movie (again, courtesy of CIMSS), shows that. Watch the cirrus clouds (the thin wispy clouds) around the periphery of the storm. You'll notice that they turn clockwise--opposite the direction of clouds at the surface. We call the way that air exits a tropical cyclone--in an anticyclonic manner--"outflow."
Outflow is very, very important in a tropical cyclone. It removes all the air that converges in the low pressure center of a tropical cyclone. We've talked about all the convergence that takes place in the eye wall of a hurricane. If all of that air didn't go up and eventually away from the storm (diverge), the low pressure would "fill" and the storm would weaken. Observing outflow is an important way to assess the health of a tropical cyclone.
The detailed mechanics of the creation of outflow are beyond the scope of this discussion. Some of them are simple--for example, recall the air converges in the eye wall and ascends. It must diverge from there (aloft), and such divergence is outflow. But there are much more complex mechanics at work (again, involving the conservation of angular momentum and balance of forces in the eye wall that force rising air parcels to increase the radius of their circulation as they ascend--this begins a complex process that leads to anticyclonic outward motion once the parcels ascend).
Let's go back into the eye to take a look at another couple of pretty neat features we can clearly see in Katrina.
Recall that while some ascending air exits a tropical cyclone, some also sinks and helps to create the precipitation free (and sometimes cloud free) eye. (Other mechanisms that include the conservation of angular momentum by some air parcels are at play too). Some might have the impression that the subsiding air results in an eye that is a quiet, calm area in the storm. However, calm is perhaps too strong of a word.
First, imagine for a moment the eye and eye wall in Katrina at the time of the image above. In the eye wall, air is converging and ascending (spiraling outward slightly as it ascends), creating and maintaining convection. Surface winds are, of course, blowing in toward the center. The hurricane's maximum winds are almost always in the eye wall convection.
Now, imagine just inside the eye wall--in the eye of the hurricane itself. Subsidence, as we described, suppresses convection and ascent, so winds are not as strong in the eye. The area in the eye, in contrast to the eye wall, is relatively calm. Of course, there is no real "wall" separating the relative calm of they eye and the strong winds outside of it. As such, there can be a mixing of vorticity (air parcels with a lot of spin) from the very turbulent eye wall into the relative calm of the eye.
Though the explanation is much more complex than I have just laid out, the end result is that areas of vorticity, or "spin" can pop up in the eye as momentum is carried form the eye wall into the eye itself. These are called mesovortices, and they make for some amazing pictures of hurricanes.
Here is a high resolution satellite image of the eye of Hurricane Katrina on the 28th of August. I've cropped it significantly to only look at the eye and eye wall, with the mesovortex located inside it:
Small close up of Hurricane Katrina's eye with a mesovortex in it on August 28, 2005. Image courtesy of CIMSS.
Here's another image from more directly overhead that shows the mesovortices in Katrina's eye.
Multispectral image of Hurricane Katrina's eye with a mesovortices in it on August 28, 2005. Image courtesy of CIMSS.
Look back at the two images above and focus on the shape of the eye from top to bottom. You'll notice (especially in the top image) that the diameter of the eye is larger at the top than at the bottom--the eye wall slopes outward as altitude increases. This is called a "stadium effect" for its resemblance to a giant sports arena--smaller at the bottom, wider at the top.
The description has to do with conservation of angular moment, supergradient winds, and centrifugal force (whew!)--the same things that are described above that help establish outflow. The attempt to balance these forces ends up, as mentioned above, in an increase in the radius that a parcel follows as it ascends. Hence, the eye wall slopes outward as radius increases with altitude. Regardless, the stadium effect is a signature of a strong tropical cyclone.
Here's a photo taken by aircraft reconnaissance of Hurricane Katrina (Courtesy of AOML). From it you should get a sense for the stadium effect. The plane is in the eye, and the vertical clouds are the eye wall. Note how the eye wall angles away form the plane with altitude.
So the eye of a hurricane isn't as calm as it might seem.
And, while we've paused for a moment to kind of admire some of the awesome features of Hurricane Katrina, let's not lose sight of the fact that a category 5 hurricane, like Katrina, while beautiful in some ways, is a potentially devastating force of nature. And Hurricane Katrina, at this time in the storm's life was impressive, but also horrifying, especially for those in Louisiana and Mississippi who had become the focus of track forecasts issued by the National Hurricane Center.
Let's next examine what forces brought Hurricane Katrina to landfall on the Gulf of Mexico coast. What are the forces that guide tropical cyclone tracks?
PAGE 6: Examining a hurricane's structure