Keith234
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My question for today is why do hurricanes shed their eye and how can you forecast them? I can't figure it out. Thanks.
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LI Phil
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"Concentric eyewall cycles" (or "eyewall replacement cycle" ) naturally occur in intense tropical cyclones , i.e. major hurricanes (winds > 50 m/s, 100 kt, 115 mph) or Catories 3, 4, and 5 on the Saffir-Simpson scale. As tropical cyclones reach this threshold of intensity, they usually - but not always - have an eyewall and radius of maximum winds that contract to a very small size, around 10 to 25 km [5 to 15 mi]. At this point, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. During this phase, the tropical cyclone is weakening (i.e. the maximum winds die off a bit and the central pressure goes up). Eventually the outer eyewall replaces the inner one completely and the storm can be the same intensity as it was previously or, in some cases, even stronger. A concentric eyewall cycle occurred in Hurricane Andrew (1992) before landfall near Miami: a strong intensity was reached, an outer eyewall formed, this contracted in concert with a pronounced weakening of the storm, and as the outer eyewall completely replaced the original one the hurricane reintensified. Another example is Hurricane Allen (1980) which went through repeated eyewall replacement cycles -- going from Categrory 5 to Category 3 status several times. To learn more about concentric eyewall cycles, read Willoughby et al. (1982) and Willoughby (1990a).
It was the discovery of concentric eyewall cycles that was prtially responsible for the end of the U.S. Governements's hurricane modification experiment Project STORMFURY, since what the scientists had hoped to produce through seeding was happening frequently as a natural part of hurricane dynamics.
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Clark
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Excellent reply by Phil below, but I'd also like to add that concentric eyewall cycles are extremely hard to forecast until they are actually upon you. When one is there, you know that it is and approximately how long it might last, but they occur with no real rhyme or reason to them in these storms.
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Keith234
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I read a lot about project STORMFURY its very interesting. But I always have had this question, wouldn't making more condensation in a hurricane release more latent heat and therefore making it stronger?
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"I became insane with horrible periods of sanity"
Edgar Allan Poe
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scott
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It is my understanding that most concentric eyewall events occur in nearly all RI storms. Since they have made great strides in the prediction of potential RI storms it is therefore easier to forecast if a storm is prone to concentric eyewall regeneration cycles. But does anybody out there have any current analysis on exactly what intiates this cycle? Andrew was mentioned as an example of CERC. Case studies have shown that most RI events occur in darkness. Does anybody know why? Is the RI event actually the end of a CERC event? (i.e. Andrew occured at night. )Thanks
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Keith234
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Hi,
Let me anwser your question from what I have gathered over the years. I believe CERC's occur generally at night because that's where there is a vorticity max in the atmosphere and thus where the eye would start to generate from, and there is a process known as the nightime convection which leads to insolation ventilating back up into the upper atmosphere. This excessive energy cannot be placed in the center of the eye and is therefore placed somewhere else and a new eye forms and temporaily weakens the prexisting eye. Just my thoughts... I would let Clark take a stab at this one!
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"I became insane with horrible periods of sanity"
Edgar Allan Poe
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Clark
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I know it's well after the point where this is relevant, but I came across this when looking up the snow system post once again and now understand this topic a lot better, so I figured I'd add some more pertinent information.
Eyewall cycles are part of the natural evolution of a tropical cyclone. Many hurricane modeling studies have been performed using what is called the Sawyer-Eliassen non-linear balance model. In layman's terms, they took the typical atmospheric balances (in wind and mass) and developed a set of equations that captures the atmospheric motions and processes related to changes in the wind and mass (and heating) fields. Originally developed for mid-latitude storms, it has been successfully applied to hurricanes as well. This is relevant because the application of this model to hurricanes has shown that these cycles are a natural part of a hurricane's evolution and have several implications on the wind profile and future evolution of the storm.
These eyewall replacement cycles take place as a secondary wind maximum develops in the outer edges of the inner core of the hurricane. Enhanced convection is often found in association with these wind maxima; between the eyewall, or primary wind maximum, and the secondary maximum, you usually see a moat, or area of weakened rainfall rates. This results from the convergence at the surface needed to create and maintain the secondary (and primary) wind maxima. Rising motion that results near these maxima result in sinking motion outward -- both inside the storm and at larger radii -- thus resulting in weaker rainfall rates at ~50-200km radii in many instances (and perhaps the dry air surrounding the storm on water vapor imagery).
The natural tendency of this wind maximum is to contract with time. As it strengthens, however, the aforementioned sinking motion away from it begins to affect the eyewall itself. Eventually, the original eyewall begins to erode and a new eyewall is set up at a larger radius with the secondary wind maximum due to strong sinking motion. As a result, the storm has weakened (at least temporarily) and has grown in overall expanse (see from this year for a good case....a small storm to begin with becoming a monster). Another eyewall cycle may or may not take place from here on out.
What is still unknown is the exact predictability of when one is going to occur as well as how fast they will occur and how far they will reach towards the center before resulting in the collapse of the original eyewall. Atmospheric conditions modulate whether or not another cycle will take place; if conditions become no longer favorable for TC development (e.g. shear, sea temperatures, etc.), another eyewall cycle may not take place and the storm may be left in a weakened state (or with a flat wind profile not entirely unlike an storm). This was likely the case with both Isabel in '03 and in '04 -- upwelling from Fabian (for Isabel) and itself (for ) resulted in unfavorable conditions for another eyewall cycle to take place, resulting in a weakened storm.
If we had full, real-time data about the evolution of the storm, we might be able to predict these secondary wind maxima as well as the evolution of the resulting concentric eyewall cycles (whether two or even three -- in rare instances -- eyewalls) to a better degree. However, due to their large sensitivity to the actual atmospheric conditions, we're likely still a ways off of actually being able to model them in a near-real-time basis. (Heck, maybe I'll be able to do it someday....it is sort of an extension of my current work on wind field expansion during transition.)
Hope this provides some insight, several months late...
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B.C.Francis
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I was just wondering, has it been a little quiet this year up in the central and eastern U.S. when it comes to spring time tornadic activity ? or am I jumping the gun. I know we had one touch down about a month back in the city of Palm Bay near Melbourne Fla. that did some signifacant damage to about 25 to 30 homes.Is there any changes in the Jet Stream or weather patterns on the mainland that would indicate a slow year for this kind of severe weather......Weatherchef
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Clark
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Severe weather season has been a bit subdued until recently, but across the nation, we were near-average for February and March and well-above average for January, this likely due to a lot of tornadoes in California. Over the past few weeks, we've seen activity increase over the south-central U.S., but the east coast itself isn't as favorable for tornado formation climatologically (nor has it been this year).
Simply put, tornado formation is favored in regions of strong vertical shear...especially in the lowest part of the atmosphere. This is created where you have winds at different directions between these levels of the atmosphere and enhanced by having slower winds nearer the surface and stronger winds further aloft. This pattern tends to happen when you have an upper-level low passing nearby but to the north and west, resulting in strong, nearly west winds aloft and southerly winds near the surface. Most of the storms this year -- and climatologically as well -- tend to move either along the coast, keeping the threat near to the coastline, or a good distance to the west of the Appalachians, keeping the threat further west. This is an oversimplification of the problem - you need favorable thermodynamical factors (such as heating) as well - but helps to highlight one of the main reasons for such a pattern.
Things will continue to become more favorable as we head further intro Spring, but moreso over the south central US.
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B.C.Francis
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Great information Clark. You are definately the man with the weather plan. In California whats going on there? I`ve read about those tornadoes popping up around that area. Over the years I very seldom heard of that kind of severe weather on the west coast. Were they short lived F-1`s? or were they more destructive. It also brings to my mind the rare twister that went through Salt Lake City back in August 1999. I`ve seen the film many times as it ripped through that citys down town section, awsome filming.....Weatherchef
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B.C.Francis
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I thought that this Web page might help answer any questions on how the numbers for landfalling cyclone probabilitys are calculated.....Weatherchef.......... web page
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B.C.Francis
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The web page address in my last post didn`t attach properly. Here`s the address....http://www.e-transit.org/hurricane/welcome.html.......Weatherchef
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Clark
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Sorry for the long-delayed response, but it's likely most of the tornadoes in California were on the weak end of things, based primarily off of sheer probability plus the lack of favorable large-scale conditions (instability, dynamics, and so on) to support the stronger tornadoes. The terrain can muck things up a little bit -- albeit the extent to which that can occur may be the subject of some debate -- but generally, there likely wasn't enough energy or vertical wind shear (e.g. wind changing speed and direction with height, generally from S at the surface to W or NW not far aloft & increasing in magnitude) to support any of the massive tornadoes.
It's pretty rare for regions outside of tornado alley (give or take some areas north and south) & into the Ohio River valley to see anything stronger than an F3. It happens, sure, but the majority of those events occur where tornado frequencies are the greatest to begin with. It's interesting to note too that the U.S. hasn't seen an "official" F5 since the May 3, 1999 tornado that passed just south of Oklahoma City.
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B.C.Francis
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Looks like we have a classic battle of the weather titans going on in the central and north central U.S.. Temperature differences of 45 degrees plus in some of those states with the collision of those two fronts. Super cells should be popping up just like yesterday with tornadoes likely wouldn`t you think? Plus isn`t it unusally late in the season for such a cold blast such as the one thats making it look more like late winter up there instead of spring? One day its 83 and the next day its 39. Makes me happy I live in Florida even though a tropical menace might be lurking off my coast later this summer....Weatherchef
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Clark
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Nah, it's not all that uncommon this time of year -- maybe more likely in mid-late April than early-mid May -- but this is prime time for tornadoes across the midwest. It's these seasons that make me glad to live in Florida as well: relatively nice weather without the threat of tornadoes, plus hurricane season on its way. I'd like to chase tornadoes at some point, but hurricanes are my life. Of course...it'd be nice if both stayed away from populated areas, but unfortunately that doesn't always happen.
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