Jump to content
Sign in to follow this  
kold weather

Hurricane Tutorial

Recommended Posts

Right, I suppose since I have time I'll do a tutorial about hurricanes

Firstly lets start at what creates them.Generally what is needed is hot seas, 26-27C is generally the required temperature although if convection is good enough then even as low as 24-25C can sometimes do the trick,however this is rare and the only recent example is Lisa last year.In general all systems start of as a complex of T-storms over Africa that can fizzle out somewhat over on the ITCZ water,which means the intertropical Convergence Zone(the ins and outs of it I'm not 100% sure about),which generally exsits in the tropics between 15N-15S although this can shift way north or south of that and can give much needed rain to the desert regions of Africa if it shifts far enough north.

These T-storms then get blown along the trade winds (going from east to west in the tropics) into the atlantic where these T-storms may organise into a wave feature, generally in the early phases a open wave.This occurs when a surface low forms and there is some sort of circulation in the storms, however slight.Providing the atmopsheric condtions are good(I'll explain later)then some development can occur, although not always.

The next phase is when the circulation starts to become a seperate system and the low pressure center gains a closed circulation and pressure generally drops, it'll start to raise some intrest in the National Hurricane Center, or NHC and it'll more then likely gain the title of Invest. In the atlantic Invest systems are named from 90-99 and a L after the number, quite why they call it that I'm not to certain.Here are some more: (thanks to PK)

Here is the list of different invest letters:

A - Arabian Sea

B - Bay of Bengal

C - Central Pacific

E - East Pacific

F - Fiji (SWPAC)

L - North Atlantic

M - Mediterranean Sea

P - South Pacific Ocean

R - Meteo-France La Reunion (SWIO)

S - South Indian Ocean

T - South Atlantic

U - Australia

W - West Pacific

It is also possilbe they would use K for any more Black Sea systems.

Eventually when a certain amount of organisation is present and a closed circulation, with some convection present as well, preferably some deep, here's a image of a tropical depression, this is actually a pretty decent looking tropical depression, notice the deep convection:

Posted Image

Then you have the next phase up, the tropical storm. As this stage the storm is given a name, dpeneding on what tropical basin it'll be named with different names. IF any season has 22 named storms then there is a back-up list that is used,hich is the Greek Alphabet. This was used for the first time in the hyper-active 2005 season, which ended up having 26 named storms!

Anyway, this low will deepen more and it needs to keep its deep convection around its circulation. At the upper end of the scale there the large scale circulation becomes more easy to see with the eye, and sometimes the center of circulation is very evident and at the very top end around 60-70mph, it can start to develop a shallow eye system where the convection wraps around the ever deepening low, heres a image of a strong TS that had a obvious center:

Posted Image

Once the winds reach 74mph then the storm will be upgraded to a Hurricane. At the lower end of the scale there is category-1 and at the upper end the rare category-5, here's the ranking system, known as the Saffir-Simpson Hurricane Scale:

Category One Hurricane:

Winds 74-95 mph (64-82 kt or 119-153 km/hr). Storm surge generally 4-5 ft above normal. No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Also, some coastal road flooding and minor pier damage.

Category Two Hurricane:

Winds 96-110 mph (83-95 kt or 154-177 km/hr). Storm surge generally 6-8 feet above normal. Some roofing material, door, and window damage of buildings. Considerable damage to shrubbery and trees with some trees blown down. Considerable damage to mobile homes, poorly constructed signs, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages break moorings.

Category Three Hurricane:

Winds 111-130 mph (96-113 kt or 178-209 km/hr). Storm surge generally 9-12 ft above normal. Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures. Damage to shrubbery and trees with foliage blown off trees and large trees blown down. Mobile homes and poorly constructed signs are destroyed. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Flooding near the coast destroys smaller structures with larger structures damaged by battering from floating debris. Terrain continuously lower than 5 ft above mean sea level may be flooded inland 8 miles (13 km) or more. Evacuation of low-lying residences with several blocks of the shoreline may be required.

Category Four Hurricane:

Winds 131-155 mph (114-135 kt or 210-249 km/hr). Storm surge generally 13-18 ft above normal. More extensive curtainwall failures with some complete roof structure failures on small residences. Shrubs, trees, and all signs are blown down. Complete destruction of mobile homes. Extensive damage to doors and windows. Low-lying escape routes may be cut by rising water 3-5 hours before arrival of the center of the hurricane. Major damage to lower floors of structures near the shore. Terrain lower than 10 ft above sea level may be flooded requiring massive evacuation of residential areas as far inland as 6 miles (10 km).

Category Five Hurricane:

Winds greater than 155 mph (135 kt or 249 km/hr). Storm surge generally greater than 18 ft above normal. Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. All shrubs, trees, and signs blown down. Complete destruction of mobile homes. Severe and extensive window and door damage. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Major damage to lower floors of all structures located less than 15 ft above sea level and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5-10 miles (8-16 km) of the shoreline may be required.

It also has to be noted that the NHC (National Hurricane Centre) use 1 minute wind averages whereas all the other RSMC (Regional Specialised Meteorlogical Centres) use 10 minute wind averages. The difference between one minute and ten minute averages is 17%, therefore 100kts from the JMA (Japanese Meteorological Agency) would be 117kts from the NHC if they both saw the system the same way.(thanks to P.K for that)

All hurricanes tend to dump large amounts of rain, generally at the very least they will dump 2-3 inches at there worst if there is a fair amount of dry air present around the circulation, however they can give alot more, Hurricane Mitch ( a category-5 ) gave 35 inches when it slowed to just 4 knots close to Honduras and Nicaragua. The other and most dangerous is the storm surge with these storms which occurs on the right side of the storm where the winds come from directly the way the bay/beach is facing and the waves come crashing in, being blown in from the sea. It was this surge that made Katrina so deadly to New-Orleans. here's a image showing just why the storm surge is strongest on the right front quadrant of a systems, also below the surge height-category ratio:

Posted Image

Posted Image

In terms of hurricane structure, there is a large difference between category-1's and the rare category-5's. Here's a category-1 hurricane, Gaston last year which was upgraded at the end of the season due to winds reaching just over the needed speed:

Posted Image

It did have a shallow eye present that actually looked better over land thanks the friction of the land affecting the structure. There is a decent looking structure with a eyewall present, although it's by no means the best I've ever seen but there have certainly been some ugly category-1's, notice the feeder bands around the hurricane. However, although some category-1's are ugly, category-5's are always very magic to look at, here is hurricane Isabel from 2003,as a category-5:

Posted Image

A very noteable eye is present which is probably very clear with a very deep central pressure,impressive CDO(later!),impressive outflow,esp on the southern side and feeder bands which aren't amazingly noteable on this system but that is partly down to it probably being close to Annular (later as well!!!).

So while I'm talking about how a storm looks, lets have a quick look at the structure of hurricane Isabel:

Posted Image

A--This is the feeder band, this is where the strongest thunderstorms are often found and where the most tornadoes form from.The bands rotate around the center of the low pressure system,with the inner T-storms moving faster then the outer due to being closer to the center. These T-storms can themslevees be very strong, with very heavy rain, weak tornadoes and also sometimes some t-storm strength gusts.

B--This is the outflow, this has been pushed out from the eyewalls and generally consits of Cirrus clouds, the outflow can also give a clue as to wether shear is present on the system.

C--This is the central dense overcast, when in the tropical storm phase this is a good sign that a storm is getting stronger and is healthy and eventually the eye from underneath it. All this is the tops of the intense clouds that form in the eyewall and is generally the anvils of these storms, although there is rain, this section is generally not the strongest for rain nor wind but does give the hurricane part of its distinctive look.

D--This is the eyewall, where the generally most powerful cells are located and also the strongest winds. The reason for these storms is the cells are forced upwards rather then sideways due to the closeness of the eye, then eventually when they have reached the peak they can go then they fan out and form the CDO and eventually the outflow. In a hurricane generally the strongest of the 4 quadrants(NW/NE/SW/SE) is the NE. This is known generally as the RFQ,or right front quadrant which is also where the greatest surge is found as explained earlier. The reason for the name is its to the right of the eye where winds are from the south which also generally conicides in the general motion in US landfalling systems (this can alter depending on direction) here you have the tropical cyclone's foward motion (say 15kts) and its strength (115kts in this example) and you add the two and you get the actual windspeed found in the RFQ of 130kts,which makes it stronger then the rest of the system, tohugh worth remembering that the NHC will have factored this in, so if theysay a storm is 115kts, then that is actually how strong the storm is and you don't have to add the foward speed to that.

Intrestingly there is usually a disticnt lack of lgithning in hurricanes despite the strength of convection. There are however cases when systems are undergoing rapid development that lightning will be observed in the eyewall. One classic case of this was Hurricane Felix which rapidly developed into a category-5, recon observed lightning inside the eyewall, which seems to occur with rapidly developing systems.

E--The most disticntive section of the storm, the eye. As most of you will know this is the area where there is calm due to a weak pressure gradiant. A good sign of the health of a eye is to check the temperature found when Recon goes and checks out the eye. The larger the difference the better the health of the eye, this difference can reach as large as 20C in very good eye formations.The reason for the difference between the eye and outside of the eye is due to the nature of the airflow in the eye.The eye sucks in air then it desends down the eye and stays cool down to lower levels, where outside of the eye air is being sucked inwards causing it to be warmer the inside of the eye. Lowest pressure in the atlantic outside of a tornado was found in a eye of a hurricane, which was hurricane Wilma in the 2005 season...

You can also get individual vorticies appearing within the eye such as here within the eye of Hurricane Isabelle:

Posted Image

Within these there can be pressure gradient of 8-15hPa over just 1km, leading to winds of 20-80mph above the background winds. (Quoted from: A Look at Mysterious Damage Streaks in Hurricane Andrew, Jeff Lindner, Texas A&M University, ATMO - 459 Tropical Cyclones, 2004)

The hardest thing to explain is when hurricanes become Annular. Very few hurricanes can reach this as Atmospheric condtions need to be nearly perfect, a good idea to see wether this is the case is wether the outflow is impressive right the way around the hurricane, which was infact the case in Isabel. Also the Annular storms tend to have very little in the way of rain bands, which as you can see from Isabel was also the case. Annular hurricanes are rare, however they tend to be very powerful in nature and the strongest hurricanes tend to be Annular in nature. Because of how hard it is to attain its very rare that a hurricane can become annular for more then 12hrs simply because the need for perfect enviormental condtions. The other intresting thing that needed to noticed with Annular storm is that they often come closer to the MPI then other storms. To understand what that means you need to know what MPI is, it means max potential intensity. This can never be achived simply due to the need for the atmopsheric condtions to be perfect with the SST's at a certain temp and atmospheric condtions perfect, most hurricanes max tends to be around the 150-250mph mark with pressure down to 880mbs, 100% of MPI can never be reached due to the lack of perfect condtions can never be achieved. In normal hurricanes they tend to have a much lower MPI then Annular hurricanes, usually in the region of 40-70% I believe. When the storms reach this sort of level they begin the EWRC, or eyewall replacement cycle.

I'll continue with annular hurricanes shortly, however right now it might be a good idea just to mention briefly about EWRC, as best I can anyway. When a hurricane reaches its max MPI it can in the environment its in, which generally is about 40-70% of its MPI, depending on shear and other factors, it will form a outer-eyewall as well as a inner-eyewal, this signals the start to the eyewall replacement cycle and explains why most major jurricanes under-go these cycles. So what occurs, well firstly two eyewalls formed as I stated earilier as well as a tight eye. This outer band starts to absorb more moisture then the inner eyewall and the inner eyewall starts to collaspe, with this more moisture gets into the eye and it clouds over and fills in and the at the same time the outer-eyewall gets stronger, where the convection becomes stronger the anvils push right across the eye, making the eye looking like it has gone. Generally about 6-10hrs later the outer eyewall will move towards the now ex-inner-eyewall and become the dominant inner eyewall and a new eye will form, which tends to larger then the last one. After this it'll generally under-go a period where its circulation will tighten back up and the eye will shrink again, as pressure decreases and winds go back up towards pre-EWRC. Sadly for western Florida Charley had just came out of a EWRC and bombed going from a moderate category-2 to a very strong category-4.

However there are times when hurricanes can reach continue higher then normal hurricanes and become Annular. This allows a storm to carry on getting stronger beyond the point where hurricanes would normally begin a EWRC event. All long lasting category-5's are likely to be Annular. Hurricane Isabel also became annular and managed to keep its apperance as one for nearly 24hrs. It lasted about 42hrs as a category-5 (the 3rd longest in record) before finally weakening as upper atmopsheric condtions weakened the system. Annular hurricanes are often called doughnut storms due to a lack of feeder bands and a large circle shape, also the main feature of these storms is the large outflow coming from all quadrants of the storms, showing that each quadrant was similar in terms of strength,another sure sign that Isabel was a Annular storm, here is the image of Isabel when she was Annular, notice those individual vorticies in there as well:

Posted Image

The key to storms becoming Annular is just how high of a percent of its MPI. A good example is Hurricane Epsilon. Despite having a max strength at 85mph, it still displayed strong annular features, though was probably just short of becoming such a system. Epsilon was under cool SST's which could support a min pressure of 970mbs. Epsilon got down to 979mbs. It got downto aobut 80% of its max and despite being only a 85mph hurricane, this meant it managed to probably gain Annular features, while it was obviously weaker compared to isabel, the annular features that were with Isabel are also present with Epsilon:

Posted Image

The other type of eye is dreaded by most forecasters is the pinhole eye. Much like normal hurricanes these storms do under-go eyewall replacement cycles, but for some unknown reason, when the eye re-emerges its a small and compact eye rather then the large eye thats normal. Quite why this is the case is unknown, though I do have a theory.

It goes that once a hurricane starts to under-go EWRC it's environmental condtions/Heat content rapidly improve. This means that while the eye is forming it contracts like a normal eye would in such condtions. When it emerges its already smaller then a normal eye would be. Then depending on its gradiant it can continue to shrink its eye until its so tiny that the eyewall is forced to collaspe into the eye and it re-begins a EWRC.

A classic example of a pinhole eye was hurricane Wilma, which had one of the smallest ever. Due to the samll size of the eye it takes very little change in atmopsheric condtions for the system to deepen rapidly. Wilma was rather an anomaly and I consider it as a ultra-pinhole hurricane. As with normal pinhole eye hurricanes, it travelled over a region which had a greatly increased heat content compared to its origin. Yet the hurricane had yet to form a eye. So it started life already having a eye as small as most pinhole storms. It then did what most pinhole eyes do, tighten its gradiant. the fact that it already had such a small eye and tight gradiant was, I believe, the reason why it got stronger so fast and was the main facotr in allowing it to get so strong. Here's a image of pinhole eye Wilma at her ultimate, pressure in that 2 mile wide eye was probably below 880mbs:

Posted Image

There is also the intresting Sub-tropical depressions and storms. These storms tend to form from normal Barolinic low pressure systems.Generally what occurs is that these areas of low pressure develop convective systems and start to gain tropical properties,which includes a warm core. They generally are able to tighten there graidiant very fast, which causes the system to form very rapidly once the process starts to occur into a sub-tropical system. Quite why these sub-tropical lows form its uncertain and the exact processes that occur. It's also worth noting that sometimes sub-tropical lows can become completely tropical,like tropical storm Ana. There are also Hybird storms,where no-one is quite sure exactly what they are as they have both Barotropic and Barolinic properties,these sorts of systems can deepen rapidly due to the ablity to use both ways,one such system is the 'perfect storm' where a hurricane Grace mixed with a normal LP, causing a weird but powerful system to form and eventually give 70mph winds and massive seas. Anyyway to finish of the sub-tropical storm section,there has never been a sub-tropical hurricane, the strongest ever sub-tropical system had sustained winds of about 70mph, certainly a intresting but not well known part of the tropical knowleadge.Here is Sub-tropical storm Nicole late last year, notice just how far north it is:

Posted Image

There was also several systems in the 2005 season that formed in condtions that was previously suspected to be far to poor for tropical development. Hurricanes like Vince and Epsilon both shouldn't have been present under sea temps between 21-22c, yet they survived quite well under this as well as high shear. The models kept forecasting these sorts of systems to die off very quickly. However despite this they thrived with Epsilon getting close to possible Annular condtions. These systrems proved to forecasters just how little we do actually know. It turned out that two factors were in play. Being very northerly hurricanes (compared to normal anyway) this meant that cloud tops weren't as high as normal, which rendered the models simulations next to useless, because they based each storm to have tops of 200mbs, while both storms was somewhat more shallow then that at roughly 350mbs tops, with shallo convection evident on IR. Both times the models powered shear on-top of the hurricanes, yet both times because of the systems actual height it stayed safe from the powerful shear just above it. Secondly due to its northerly latitude and also time of year, the air was colder then normal above the hurricanes. This meant that despite colder sea temperatures, higher up was also colder which meant overall nearly as much instablity as in the tropics. There are also no doubt other factors that made Vince and Epsilon so hard to forecast even for the pros and just goes to show how much more there can be done to learn about these beasts.

Anyway now back to the basics again. The basic requirements a hurricane needs most is a area of low shear, warm waters preferably upwards of 26C, fairly moist air and a well stacked LP center. So when these things are present then you stand a good chance, providing a wave feature comes along, of getting a storm, however things can badly wrong when these things are forming.

The first major thing that can happen although not often is when two storms are close, their outflow can choke each other and the pair can die due to the inflow not being enough to sustain thunderstorm development in the LP. Usually this situation is avoided by the cool Fujiwhara effect,which I'll try and explain later.

The next way is for the system to push into a area of shear. This shear is generally caused by a thing known as a ULL ,or upper level low although jet streams will also envoke massive amounts of shear onto systems, usually enough to kill it off. this therefore is the primary reasons why tropical cyclones rarely form in winter months in the deep tropics. The best thing for a storm is to have a ULH, or upper level high present over the top if it or close by. A good indicator of shear is to see wether the higher cirrus clouds are moving in a opposite direction to the tropical cyclone, if it is then shear is present.The stronger the circulation of the low pressure center the more it can cope with shear and very powerful systems can generally deal with shear quite well, however the smaller and less well defined systems can sturggle and it will limit any real growth of the system. Infact it can kill systems if shear is to strong and is responsible for some very quick weakening that some systems have undertaken.The shear can blow away the thunderstorms that form the cloud cover that cover the center and also de-tatch the LLC(low level circulation) from the MLC (Mid-level circulation) and leave it 'naked' without any convection.This generally means the death is near for systems although if it gets out the area of shear then it can re-organize again, although this generally takes a few days and is like starting from stratch. Lone LLC's can last a few days as they take a while to wind down, however it is certainly a dangerous thing for tropical cycnloes that shear!!!

It should be noted as well though that if the shear is placed where the outflow is present then in fact rather than weaken the system it could help enhance it by effectivly fanning the outflow.

Another one of the ways for hurricanes to die is simply for them to move into cooler waters. The hurricane will generally fade away slowly. Generally this option also ties in with it being quite far north in terms of latitude. The storm can merge with a larger low pressure system or front and travel along the jet stream towards Europe, this often causes the LP/frontal wave that absorbs the tropical cyclone to become stronger due to the latent heat and moisture that is present in those systems. The other thing that can happen is that it doesn't get absorbed and become extra-tropical,where it gains frontal systems and Barolinic properties (IE,normal Autumn Lp systems for the U.K) these systems can either go poof and not become anything other then a very shallow filling low pressure and die of completely or they can do quite well as a traditional low pressure system on their own.

The final way is for the hurricane to make landfall. In the atlantic basin the two most likely areas for a storm to landfall is the Caribbean Islands and of course the USA. The reason why land kills of tropical cyclones is down to its main source of energy, IE warmth from the Ocean is taken away from it and it cannot keep up its power. However nonetheless these systems can last a fair while even over land due to taking a fair while to wind down the circulation. This can take several days and in rare cases the remainant low pressure center may drift ut to sea and re-form, this is what happened to Ivan last year,here is the story of what occured that rare day:

Even as an extratropical low, the remnant circulation of Ivan was identifiable in both surface and upper-air data. Over the next 3 days, the low moved south and southwestward and eventually crossed the southern Florida peninsula from the Atlantic the morning of 21 September and emerged over the southeastern Gulf of Mexico later that afternoon. As Ivan moved westward across the warm water of the Gulf, the low began to re-acquire warm core, tropical characteristics as showers and thunderstorms started developing near the well-defined low-level circulation center. During the morning of 22 September, Ivan completed a large anticyclonic loop and by 1800 UTC reconnaissance aircraft reports indicated that it had become a tropical depression again over the central Gulf of Mexico. Ivan regained tropical strength 6 h later when it was located about 120 n mi south of the mouth of the Mississippi River. Tropical Storm Ivan turned northwestward and made landfall as a tropical depression in extreme southwestern Louisiana around 0200 UTC 24 September.

However as I said above, these systems are rare but when they occur they are quite amazing to behold. Sometimes the tropical cyclone also stalls over land, caused by a very slack flow in the lower-mid atmosphere, this in itslef can cause some large problems due to flooding, as these systems tend to be able to dump lots of rain. This occured with hurricane dennis in 2005, where it slowed to a halt and stayed there for a week before finally fizzling out completely.

From time to time hurricanes will stall in the ocean. What happens in this case is the sea temps will fall away, by as much as 1c. While doing so thing can become very complicated for a storm and it can appear as if its under-going EWRC after EWRC but never able to complete them due to ever decreasing temps and MPI. This causes a slow and steady weakening until it can get into warmer waters. This happened with Frances in 2004. It stalled just outside of the gulf stream and weakened slowly, trying in vain to under-go a EWRC. Yet eventually it got into the gulf stream. with a rise in sea temps again and just before landfall it was allowed to finish its EWRC and got slightly stronger and gained a 60 mile eye!

My final (at last some might think!!) thing I'd like to talk about is the track of these systems and why they track like they do.

Firstly in this case there are four main players, ULL's, fronts and troughs, sub-tropical ridges and finally the trade winds. The first thing that may happen is that another tropical cyclone may cause a weakness to form and when that occurs the other tropical cyclone tends to head into the weakness. The reason for this is tropical cyclones when they are out of the main steering flow head towards where there is the lowest pressure, which in this case is the weakness, IF the storm is close enough of course!

If all things are balanced then another thing turns up, with in general the storm slowly turning poleward and westwards in a effect called the Beta effect, its difficult to explain this as it requies some previous knowleadge and I've not stuided in that exact area of weather, however I believe its down to the rotation of the earth on it's axis, here's a quote from the web about it and explains what occurs:

The Beta effect is cause by differential planetary vorticity advection and it causes hurricanes to move westward and northward even if the hurricane is imbedded in calm winds. The stronger the storm the stronger the effect.For westward moving storms this causes them to move faster and to the left of the winds, in eastward moving storms this effect induces movement to the right and slower than the winds.

In other words, the stronger the storm the more Beta effect is produced and is often why small storms tend to head westwards in blanced situations, AKA systems from Africa, where larger storms tend to head more north-westwards/northwards.

Then you get troughs. Now they tend to pick up storms as the general flow that the storm is in turns more to the north/north-east on the eastern side of the trough. Storms that are close enough to the trough will tend to swing northwards pretty fast, or IF a strong trough comes through its axis may take the system of to the north-east, which was what occured to hurricane Charley last year. Notice the sharp turn to the north-east once out of Cuba caused by the trough digging down southwards, this was a unusually strong trough for August, something much more common in Autumn systems, here's Charley's track:

Posted Image

Then you have the possiblity that the ridge to the north of the storm is strong, in which case the storm will generally head westwards anyway. In general if the flow is weak then a strong storm is more likely to continue the way its going simply due to in creating its own environment, where as small storms need to go along with the flow!!!!

Once a storm generally reaches higher latitudes then the storm will pick up the westerlies rather then the easterlies that it came along the atlantic with and so thats when a storm will generally have to re-curve, I call that the re-curve point but it doesn't have any real name!!!

Then there is the complex Fujiwhara effect (or Fugiwhara,depends on what way you say it!!) where if two close tropical cyclones come close enough then they will start to circle one another as the western system heads southwards and the eastern northwards and so it continues that way until one storm gives under the shear that is created by the other storm on the other side. This feature is what stops tropical systems from coliding, because if they did they would probably wipe each other out. This set-up is fairly rare but there are times when double systems that are similar will under-go Fujiwhara and it must be remembered, this set-up can be applied to Barolinic storms as well. Here's an example of this, Hurricane Ione and Kristen circulated around each other. Due to Ione being the stronger the hurricanes in the most part it was Kristen who circulated around Ione, while Ione only slightly strayed from its path thanks to the tug of Kristen.

Posted Image

Well there you have it, that'll do me for now and its enough of my nattering,sorry about the extreme length of this post,but I hope it covers many of the major things about hurricanes, and most of my knowleadge of these things have gone into this so hope its a good read.

(EDIT 12/04/06- Have changed a few things, added in other stuff and general corrections.)

(EDIT 21/02/08- Changed a few parts going to add a few more bits soon.

Edited by kold weather

Share this post


Link to post
Share on other sites

I'm in a picky mood this evening!

Sea temps in excess of 26C are required.

regards

John

Share this post


Link to post
Share on other sites

Hi john,not quite right actually,Lisa became a hurricane under sea temps of just 24C(i believe it was the furthest north hurricane ever to form I believe)(granted it had help from a convective burst),however to get the process underway,yeah generally 26C is the benchmark to look for.

ps,but don't worry,I don't blame you for being in a picky mood!

Edited by kold weather

Share this post


Link to post
Share on other sites

A couple of points to add:

Here is the list of different invest letters:

A - Arabian Sea

B - Bay of Bengal

C - Central Pacific

E - East Pacific

F - Fiji (SWPAC)

L - North Atlantic

M - Mediterranean Sea

P - South Pacific Ocean

R - Meteo-France La Reunion (SWIO)

S - South Indian Ocean

T - South Atlantic

U - Australia

W - West Pacific

It is also possilbe they would use K for any more Black Sea systems

It also has to be noted that the NHC (National Hurricane Centre) use 1 minute wind averages whereas all the other RSMC (Regional Specialised Meteorlogical Centres) use 10 minute wind averages. The difference between one minute and ten minute averages is 17%, therefore 100kts from the JMA (Japanese Meteorological Agency) would be 117kts from the NHC if they both saw the system the same way.

You can also get individual vorticies appearing within the eye such as here within the eye of Hurricane Isabelle: http://www1.ncdc.noaa.gov/pub/data/images/isabelloop1sm.gif. Within these there can be pressure gradient of 8-15hPa over just 1km, leading to winds of 20-80mph above the background winds. (A Look at Mysterious Damage Streaks in Hurricane Andrew, Jeff Lindner, Texas A&M University, ATMO - 459 Tropical Cyclones, 2004)

I also don't think this was mentioned, but the stronger the system the deeper the system and so the higher level winds that help to steer the system. It does have to be noted however that storms can be very small, such as Cyclone Tracy which has been compared to a very large tornado, and so don't need such a low central pressure for winds of the same strength of say Typhoon Tip. (Size of these systems can be compared at http://www.srh.noaa.gov/srh/jetstream/trop...yphoonsizes.jpg

I'm sure I'll think of more a bit later.

Share this post


Link to post
Share on other sites

Thanks P.K,intresting to note they have M for the med,of course there are some systems that can form there,although they are rare.I'll be sure to add those points,thanks,as i said there is plenty I've left out!!!

(Time to edit information in!!)

(ps,thanks for the Isabel image,fits well with my study on the structure of the hurricane.)

Edited by kold weather

Share this post


Link to post
Share on other sites

The M and T sufficixes were first used by the Met Office (You say they but that is a list I made the other day of all the ones I've seen :rolleyes: )

Few other things you could add: Annular hurricanes, Dvorak technique. :)

Share this post


Link to post
Share on other sites

Has any Tropical System done what Ivan did, but become a Hurricane again instead of a Tropical Storm.

Share this post


Link to post
Share on other sites

Hi P.K,was actually going to cover Annular hurricanes with Isabel again,I'll get to it sometime over the next 1-2,the Dvorak technique is also very much a good idea to cover,Ah!!!

so many things that need to be yet done,already this has to be the longest post I've ever done!!!

Share this post


Link to post
Share on other sites

Has any Tropical System done what Ivan did, but become a Hurricane again instead of a Tropical Storm.

<{POST_SNAPBACK}>

Not that I'm aware of. I thought Ivan was a first.

Hi P.K,was actually going to cover Annular hurricanes with Isabel again,I'll get to it sometime over the next 1-2,the Dvorak technique is also very much a good idea to cover,Ah!!!

I'm sure I can think of some other things as well. :rolleyes:

Share this post


Link to post
Share on other sites

I get the feeling KW is being made a fool off??

The last few posts he's put up certainly helped me anyway. I guess the hurricane tutorial is for novices/not-so-clued up/newbies wanting to learn more; not nessarily read about overlooked facts.

Keep up the good work kold. :rolleyes:

Share this post


Link to post
Share on other sites

Thanks Mondy,indeed it was aimed at people with little knowleadge of the tropics,I don't mind people suggesting things as thats how things get better,certainly gotta thank P.K for helping me add some things I've forgotten!!!

Share this post


Link to post
Share on other sites

Thanks, KW - very interesting, but can anyone explain in more quantitative terms what it is that's so magical about a water temperature of 26/27 C. I mean, why can't you have a smaller system forming over water at, say, 23C?

I know it's all to do with energy but I'd like to see some more numbers!

Share this post


Link to post
Share on other sites

From the NHC:

Subject: A16) Why do tropical cyclones require 80°F (26.5°C) ocean temperatures to form ?

Tropical cyclones can be thought of as engines that require warm, moist air as fuel (Emanuel 1987). This warm, moist air cools as it rises in convective clouds (thunderstorms) in the rainbands and eyewall of the hurricane The water vapor in the cloud condenses into water droplets releasing the latent heat which originally evaporated the water. This latent heat provides the energy to drive the tropical cyclone circulation, though actually very little of the heat released is utilized by the storm to lower its surface pressure and increase the wind speeds.

In 1948 Erik Palmen observed that tropical cyclones required ocean temperatures of at least 80°F (26.5°C) for their formation and growth. Later work (e.g., Gray 1979) also pointed out the need for this warm water to be present through a relatively deep layer (~150 ft, 50 m) of the ocean. This 80°F value is tied to the instability of the atmosphere in the tropical and subtropical latitutes. Above this temperature deep convection can occur, but below this value the atmosphere is too stable and little to no thunderstorm activity can be found ( Graham and Barnett 1987).

See Question G3 for how this value might change if a significant global warming occurs.

http://www.aoml.noaa.gov/hrd/tcfaq/A16.html

Share this post


Link to post
Share on other sites

thanks for that P.K,you've saved me abit of work there and digging for that sort of extract,currently trying to explain Annular systems and EWRC and how its all linked in,very hard to explain!!!!

Share this post


Link to post
Share on other sites

Thanks KW. Coincidentally I found this news item published today.

2005 set to be historic year for hurricanes

PARIS, (AFP) - 2005 is set to be one of the worst years on record for hurricanes, scientists say, amid spectacular new evidence about the power of these storms and fears that global warming is intensifying them.

Less than halfway through the six-month tropical storm season, experts are already warning that the brooding western Atlantic may serve up as many as 21 severe storms and hurricanes this year.

If so, that would be more than twice the average annual tally since records began in 1851.

"The 2005 hurricane season could rival historically significant years such as 1887, which had 19 named storms; 1933, which had 21 named storms; and 1995, which had 19 named storms," says Barry Keim, assistant professor of geography and anthropology at Louisiana State University, and a climatologist for the state.

Full Article : http://uk.news.yahoo.com/07082005/323/2005...hurricanes.html

Share this post


Link to post
Share on other sites
Guest Mrs murphymoo

Just to confirm, KW has updated the original post with the points raised, and I have edited the post for him to reflect the changes.

Thanks for doing this Kold, much appreciated and a very interesting read.

Share this post


Link to post
Share on other sites

I would of liked to see a front chart of Sub-Tropical Storm Nicole.

Share this post


Link to post
Share on other sites

Kold you have surpassed yourself with the above, well done.

Share this post


Link to post
Share on other sites

Just seen the last 2-3 posts,thanks Adi and Mrs.Murphymoo,it took me a entire eveing to write and a good 2 hours of the next day to do the rest.(there was me thinking it was gonig to be a 30-40 minute job)

Some thanks from me to P.K as well for reminding me off things to do,as it happens it al lfitted in quite nicely...and gave me another hour or so of typing!!! :D

as for the chart SB of Sub-tropical storm nicole,i'll do abit of digging on google and see if there are any charts that show this,but I don't think there wil lbe but you never know.

Share this post


Link to post
Share on other sites

thanks I luv snow,turns out there is 4000 words in that post,by far my longest post on this site.

Share this post


Link to post
Share on other sites

Thanks for the lesson Kold Weather.

There was a lot of useful information that I had not a clue about before the read.

Well done.

John

Share this post


Link to post
Share on other sites

Hi, just a quick post about SST's and what is required. I think it all depends on how the storm started. If it started in the classic T-storm way and developed into a pure tropical system from a wave then I believe they do need sea temps of about 25-26C.

However systems that start their lives as extra-tropical systems, IE non-tropical tend to be able to handle cooler condtions much better, I suspect thats because in the main they never go completely tropical but becaome more of a hybird, not sub-tropical storms but something between the two, its certainly very complicated and where do you draw the ling between tropical, subtropical and extra-tropical is certainly a very intresting question.

Its just a point that I've been thinking about since Epsilon and this statement:

EPSILON HAS CONTINUED TO STRENGTHEN AGAINST ALL ODDS. THE CLOUD PATTERN IS REMARKABLY WELL-ORGANIZED FOR A HURRICANE AT HIGH LATITUDE IN DECEMBER...EMBEDDED IN A STRONG UPPER-LEVEL WESTERLY WIND ENVIRONMENT AND MOVING OVER 21-22 DEGREE CELSIUS WATER.

However I also think that Epsilon may well have been Annular despite apperently bad condtions, it certainly handled those condtions like a Annular would and it did probably come very close at maxing out its MPI, also this is what the NHC had to say about it:

EPSILON RESEMBLES ONE OF THESE

ANNULAR HURRICANES OR "TRUCK TIRE PATTERN" DESCRIBED IN THE

SCIENTIFIC LITERATURE. ALTHOUGH THIS CLASSIFICATION IS FOR

HURRICANES IN THE DEEP TROPICS...EPSILON HAS INDEED MAINTAINED ITS INTENSITY...MORE THAN ANTICIPATED...LIKE THESE TYPE OF ANNULAR HURRICANES.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

×