Questions (split from thread in Talkin' Tropics)

[Annie Oakley]

Ok-the stupidest questions of the day-but hey! I work in the area of disaster response and need to understand this-as do my co-workers....(thanks in advance!)
1) Why do all the 'waves' start in Africa?
2) What exactly is a 'wave'?
3) Is it a satellite that gives us knowledge of a 'wave'?
4) Or is it also mariners?
Please don't laugh at me lol!

[wxman57]

Ok-the stupidest questions of the day-but hey! I work in the area of disaster response and need to understand this-as do my co-workers....(thanks in advance!)
1) Why do all the 'waves' start in Africa?
2) What exactly is a 'wave'?
3) Is it a satellite that gives us knowledge of a 'wave'?
4) Or is it also mariners?
Please don't laugh at me lol!

Very good questions. I suggest moving this post over to the "Got a Question? I'm Listening," forum (in the Tropical Weather forums index). I don't have time to respond now (heading out on the bike with the wife) but could respond later.

[wjs3]

Annie:

Kudos to you for asking this. While you are waiting for Wxman57 to come back, I'd like to help. Like his, my time is limited, but there are couple of things I suggest you read to get you started on Tropical waves--and I apologize if you know fo these already:

Try this description of easterly waves from the NHC FAQ:

http://www.aoml.noaa.gov/hrd/tcfaq/A4.html

And a write up by my friend Phil as part of an assignment he did for a class we took:

http://www.personal.psu.edu/users/p/z/p ... 20WAVE.htm


Don't worry if these seem too technical right now. See if they help, though.

Wxman57, I don't want to hijack your thread, but am glad to help too if needed.

WJS3

[Annie Oakley]

Holy (Longhorn) Cow! Thanks so much for the information.......I have bookmarked it for further reading but at this point I do get the gist of it.-I thing 'easterly wave' is an operative word? lol....at any rate-I will continue to read and study....thanks a Texas million!

[wjs3]

No problem. Out of respect for Wxman57 (you couldn't be in better hands), I am going to let him take a crack at your specific questions. I'll be watching the thread, though, if I can help.

I urge you to pop over to this forum for any other questions...I think it's a much "quieter" place to get an answer to any of your questions than posting in talkin' tropics, FWIW.

Welcome to the boards!

WJS3

[pojo]

Ok-the stupidest questions of the day-but hey! I work in the area of disaster response and need to understand this-as do my co-workers....(thanks in advance!)
1) Why do all the 'waves' start in Africa?
2) What exactly is a 'wave'?
3) Is it a satellite that gives us knowledge of a 'wave'?
4) Or is it also mariners?
Please don't laugh at me lol!

I'm not laughing at you... You are still learning... we dont' stop learning. No questions are stupid.
Here is from a Hurricane Hunter standpoint....
1) The waves don't start in Africa.... actually these 'waves' are in accordance with a solid band of thunderstorms around the equator. This is called the InterTropical Convergence Zone (ITCZ). Lets put it this way. If you put your finger on a the equator of a globe and spin the globe.... your finger will go all the way around the world.... that's what the waves do also.
2) Air converges (rises) at the equator (technically the ITCZ) and with rising air, we get thunderstorms. Hence the reason for waves. With all the energy in the atmosphere, clusters and clusters of thunderstorms form, not just single thunderstorms. Some of these waves (thunderstorm clusters) have circuations associated with them... that's where the tropical activity comes in. The upper atmosphere has steering currents (ie. Jet Stream) that move the systems along the ITCZ... so it looks like the wave off the coast of Africa is moving on its own power... not true. It depends on the steering currents in the atmosphere to push the system to where its going.
3 & 4) The reason why I'm actually answering these questions together is that we not only rely on satellites, but we also rely on NOAA bouy reports besides mariners reports. In some instances, if the tropical disturbence approaches the US, aircraft reconnaissance (the NOAA and/or AFRES hurricane hunters) is dispatched to check out the storm and to gather valuable storm readings (ie. cloud temps, wind speeds, wind directions, pressure, etc) for the atmospheric models... when the models judge a storm, that's where you get the 'cone of uncertainty'. For example, Flossie has a 5 day cone of uncertainty. Our aircraft reports actually improve the forecast models by 30%. If we go investigate Flossie (we are scheduled to)... we may even bring the cone of uncertainty even tighter than it is currently.

I hope that helps.

Shannon

[Annie Oakley]

Thanks! All the information helps...Shannon you be careful up there!

[wjs3]

OK, Sorry to jump in, but I want to clarify some of what has been said. (All you PhDs out there, please don't cringe. I am going to water this down in a big way).

Waves are perturbations in the normal easterly flow in the tropics, and they DO originate over Africa. Sometimes they can make it a long way around the globe, but my undersatnding, from what I have read, is that they do not serve as seeding for tropical cyclonees far beyond the eastern pacific.

Their genesis is due to something called the Mid-Level African Easterly Jet. This is a band of strong easterlies that is at its most mature during the northern hemisphere summer. Without getting technical about it, it results becasue, well basically becasue of the sahara desert. The differential heating between the Sahara and regions to its south create a dynamic where by mid-level easterly winds are strong over a narrow area..the winds of the jet can drop off sharply as you head north or south from the core of the MLAEJ. Such a change in windspeed over a north south direction is horizontal wind shear. (We often talk about vertical wind shear when we talk about tropical cyclones--this is different).

The horizontal wind shear actually creates some "spin" in the air currents--this spin--or vorticity--is what gets a wave going.

So waves do originate form Africa.

Waves are a different animal from the ITCZ that Pojo mentions, though she's right--they often do enhance convection in the ITCZ or stay attached to the ITCZ, or even impact the ITCZ by drawing it north or south, but there are times and place where there are no waves and the ITCZ is still there (or waves separate from the ITCZ). For instance, during the southern hemisphere summer, the ITCZ is there (though south of where it is now)--but there are no tropical cyclones. One reason that some researchers suggest that this is the case is becasue in that season, there are no waves to provide the "spin" to get a tropical cyclone started. Anyway, my point is that waves are/can be separate entities from the ITCZ. The ITCZ always exists--and does not cause, nor is it caused by waves.

How do we identify waves? Yes, satellite imagery is one way. If you hang around the talking tropics forum, you will see a number of examples of folks discussing waves. Since the MLAEJ actually can get cranming over inland Africa, we can often see the convectrion/storms assocaited with them long before they come out over the Atlantic. Once they are over the water we can usually either see what's called an "inverted-V" signature in the low cloud field or, as Pojo points out, convection. Just to clarify, though, not every wave fires off substantial convection. Sometimes all we get is the inverted V.

A second way to detect waves is by surface and upper air observations. Meteorologists rely on the pattern in surface and upper winds in assessing where a wave is if all else fails. There are of course observations and soundings taken in western Africa, and the cape Verde Islands (http://www.nhc.noaa.gov/stations/08594.shtml is an example of the kind of analysis.), as well as buoy reports.

And, yes, there are of course mariner reports (ship reports) that can add to the information--with reports on pressure, wind direction, etc.

Hope this helps. I wish more people would ask questions like this. See how it gets us talking?

WJS3

[philnyc]

It is a great question, Annie. I see a lot's been said here, so I'm just throwing in the basics:

NOTE: For this discussion I'm only talking about the northern hemisphere. Reverse the winds for the southern hemisphere.

1. Pojo was right that waves form in the ITCZ all around the globe. But the ITCZ isn't solid (see image below), it has areas with sizeable clusters of thunderstorms and areas that are clear with no clouds or precip. The waves that form are areas of lower pressure, and they show up on a weather map as a wave shape - either an inverted V shape or a concave or convex line moving through the flow towards the west. These waves form due to convergence of high moisture-content air at the surface and complex interactions with the upper air. The waves are also shallow (go up to 18,000 feet or less) and so they are NOT steered by high level jets. They are steered by the low level easterly flow (westward moving) on the north side of the ITCZ.
2. Since most (60%) of the tropical storms and hurricanes in the N Atlantic form from easterly waves that originate over Africa, meterologists have come to call these open lows African Easterly waves, or easterly waves, or just waves. But no matter how they refer to them, they indeed specifically form over Africa due to the reasons wsj3 posted. Their dynamics are not easy to explain without some meteorological background. All you need to know is that these waves form on the north side of the ITCZ, usually over the East African highlands, and then move westward across the continent and come off the African coast from April to November. Winn gave some good links that explain why they sometimes form into hurricanes. Remember that the large majority of them NEVER do.
3. Satellites are used by far the most to detect them. However, they are pretty easily detected over land by weather observations, and over sea by ships or buoys, as the wind will shift from north or northeast to south or southeast as they pass.

Here is a satellite image from July 31st of this year that nicely illustrates a sequence of easterly waves moving westward across Africa and into the Atlantic, with one just coming off the African coast and another getting close to the Windward Islands. (None of these wound up developing into a depression, mostly because it was too early in the season)



Hope that helps.

[philnyc]

I just wanted to another link that should help:

http://www.aoml.noaa.gov/hrd/tcfaq/A4.html

Also, I hope you can just verify that what I said is OK, wxmann57 (being a pro hurricane forecaster) or correct me where I was wrong.

[wjs3]

Phil:

Thanks for clarifying the "tropical waves happen around the world" part of this. I honestly wasn't sure. I thought that by thetime you got to the westpac, most african waves were wiped out. I guess it makes sense that waves exist elsewhere...though I have to guess the the frequency of the African waves makes them uniquely intereresting in the tropics.

Winn

[wxman57]

Looks like y'all have it handled well without me. Got busy and forgot to come back to answer. Sorry.

[wjs3]

Glad to help. Thanks for suggesting this get split off. Wish it would happen more often.

Annie, anything else?

WJS3

[philnyc]

Phil:

Thanks for clarifying the "tropical waves happen around the world" part of this. I honestly wasn't sure. I thought that by thetime you got to the westpac, most african waves were wiped out. I guess it makes sense that waves exist elsewhere...though I have to guess the the frequency of the African waves makes them uniquely intereresting in the tropics.

Winn

Winn, the African easterly waves rarely ever make it to the westpac, although a few do. I'm trying to say that tropical waves form in other places in the world under different circumstances. It's actually the more concentrated study of tropical waves in the Atlantic by the North American and European countries that got the word "tropical wave" to become synonymous with a N Atlantic African wave. Tropical waves that form elsewhere in the world are still tropical waves, they're just not African tropical easterly waves. No where else in the world produces so many hurricane-producing waves that originate over land.

[wjs3]

Thanks, Phil. Do you have any reading on processes that form waves in other part of the world?

Winn

[Annie Oakley]

Thanks to all of you for the information...I am bookmarking links etc. I do have a better understanding of the African easterly wave concept and now when I read the forums it is making better sense to me.

Now, for another question regarding 'models'......I notice there are several sources for models. My questions at this point are:

1) When did 'official' models come into being-was it the NHC that developed them originally or NOAA? Or someone else?
2) Are their some sort of 'official' model sources throughout the scientific community-including Universities etc? In other words, not just anyone can create a model, right?
3) How are these models developed? The creators/scientists must have sophisticated instruments, equipment, inside knowledge, or resources is what I am thinking.

I probably asked too much this time! But I will be returning to this thread in between forum readings to learn more. I absolutely love this site. I can't step away from the computer ha ha. Again, thanksin advance.

[philnyc]

Thanks, Phil. Do you have any reading on processes that form waves in other part of the world?

Winn

Winn,
Here's a very good paper on tropical cyclogenesis in the westpac:
http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0469(2002)059%3C2183%3AMRGWAW%3E2.0.CO%3B2

The opening quote is noteworthy: "off-equatorial easterly waves, now often called tropical depression (TD)-type disturbances, with predominant periods of 3–6 days; mixed Rossby–gravity (MRG) waves, also most common in the 3–6-day period; and a third type of disturbance with a 6–10-day period that has not been identified with a specific wave type."


You can see how they refer to them as waves, and sometimes even easterly waves, because that is what they are -waves moving in an easterly flow. The western Pacific waves form usually due to the monsoon trough interaction with the strong Subtropical Ridge, and this interaction exists during the warm season all the way from the westpac to India and the Arabian Sea. The monsoon trough is another subject that needs a lot of explanation which I won't do here, but it's good to remembe that a number of westpac typhoons and many of the monsoon depressions that form over India are the result of this interaction. Here's a paper I did last year on the 2006 Indian monsoon that has some discussion of the the monsoon trough - scroll down to monsoon trough and monsoon depressions:

http://www.philip-lutzak.com/PENN%20STATE%20WEB%20PAGE/Meteo%20241/PROJECT_2%20THE%202006%20INDIAN%20MONSOON%20AT%20ORISSA.htm


The bottom line is that these waves are not limited to the Atlantic, but occur all over the world. African easterly waves are just a special case.
I haven't studied other areas of the world yet, but it's on the list. One can envision that the southern hemisphere also has waves from which tropical cyclones may form. As I said, I haven't researched that yet.

[wjs3]

Phil...

Thanks, I'll review and have written a knowledgeable "friend" about the issue of seeding disturbances in ohter basins and the presence of Rossby waves in the tropics (whch you're right, must exist). Aren't those easterlies in the Southern Hemisphere too?

http://www.nhc.noaa.gov/tafb/QUNA00.jpg


WJS3

[wjs3]

Thanks to all of you for the information...I am bookmarking links etc. I do have a better understanding of the African easterly wave concept and now when I read the forums it is making better sense to me.

Now, for another question regarding 'models'......I notice there are several sources for models. My questions at this point are:

1) When did 'official' models come into being-was it the NHC that developed them originally or NOAA? Or someone else?
2) Are their some sort of 'official' model sources throughout the scientific community-including Universities etc? In other words, not just anyone can create a model, right?
3) How are these models developed? The creators/scientists must have sophisticated instruments, equipment, inside knowledge, or resources is what I am thinking.

I probably asked too much this time! But I will be returning to this thread in between forum readings to learn more. I absolutely love this site. I can't step away from the computer ha ha. Again, thanksin advance.

Annie:

I suggest you start a new thread for this (I don;t know...can it be split?)

Nevertheless, try these couple of links to get you started on models:


viewtopic.php?f=61&t=9467

And


http://www.aoml.noaa.gov/hrd/tcfaq/F2.html


Start with those basics.

WJS3

[Annie Oakley]

WJS3, you can bet your boots I will check out those links! Thanks. My learning curve is rising rapidly on this website. Keep up the good work! Adios...........for now lol.