|
|
Joined: Sep 2001
Posts: 6,296
Carpal Tunnel
|
OP
Carpal Tunnel
Joined: Sep 2001
Posts: 6,296 |
Dear Bean,
Thanks for those definitions--clearly written and easy to understand!
You wrote:
Advect - (verb) to be "dragged along" with the current. Little oceanic suspended things, like plankton, are advected - they move where the current goes - they can't swim of their own accord. Velocity can also be advected. (Now try and wrap your minds around THAT - I'm still not pleased with it myself!) The noun is "advection".
...When velocity is advected, is it anything like the draft effect in racing? Seriously. Is it akin to two forces enhancing each other's force once they work in tendem?
Curious,
WW
|
|
|
|
|
Joined: Oct 2000
Posts: 5,400
Carpal Tunnel
|
|
Carpal Tunnel
Joined: Oct 2000
Posts: 5,400 |
Re:Fjord - I'm sure most of us know this one but the word has a great ring to it - those steep ocean inlets like in Norway - and Newfoundland, and Labrador!
Ahem, cough, cough, NY's Hudson river is the souther most fjord in north america. the harbor is silting up rapidly, and turn the harbor into an estuary-- but it is still technocally a fjord.
a fjord is a sunken river, into which the sea has pushed..
in NY, salt water can be detected all the way up to poughkeepsie, 60 some odd miles up river..
the river is (stacked?) the fresh water floats on top of the denser salt water-- something like the thermocline--only its a salt/fresh water change.. (with brackish water too)
even a casual visiter can experience the same effect we all associate with fjords.. steep cliff, narrow or almost no river bank, in Manhattan.. the Metpolitan museums Cloisters in upper manhattan are a prime place to see the steep cliffs that bank the hudson. 20 or 30 miles upriver, at bear mountain,Hi Rubrick!, the river is narrow, runs fast and is over 265 feet deep! that is one sunken river!
It no wonder hudson thought he had found the great north west passage! I'll look for some photos of hudson from the cloisters.. but if you ever say the movie, "looking for Richard"-- you have seen them. most of the locations shots where in NYC, and several were in the cloisters.
|
|
|
|
|
Joined: Sep 2001
Posts: 872
old hand
|
|
old hand
Joined: Sep 2001
Posts: 872 |
The whole thing lasted only 15 to 20 minutes, but when it was over, several boaters had lost their lives.-consuelo
In 1978 I was in a microburst. We were changing clothes to visit Beech Spring Cave near Lake Guntersville on the Tennessee River when -in a sudden- violent winds and torrential rains hit. The six of us sought refuge in a big Dodge van that rocked with alarming portent of overturning.
The deluge passed in about 15 minutes. But strong gusts of winds keep us imprisoned for 15 more.
To lighten the atmosphere as we waited out the tail of the storm, I offered Caver Tom ten dollars to eat the one inch long green horse-fly buzzing in a bottle who had been deftly caught by my son Danny as we waited. Tom is the quintessenial retro-woodsman. He walks through the woodlands as others would vist the salad bar at Denny's Restaurant. But the horse-fly was exceptionally ugly and Tom hesitated. Soon, though, the ante was raised to 40 Dollars. Our guests from out-of-town had never seen anybody eat a horse-fly before.
Tom ate the horse-fly, which became the hit amusement of the trip. The wind stopped and we had a pleasant visit to the cave, then we returned to Birmingham.
The next morning the headlines of the Birmingham News reported that 21 people had drowned when a freak microburst had overturned an excursion houseboat out of Huntsville. This was the worst maritime disaster in Alabama since the days of the Steamboats. The ill-fated sight-seeing boat overturned about the same time and less than a quarter of a mile from our van where Tom ate the horse-fly.
|
|
|
|
|
Joined: Sep 2000
Posts: 4,757
Carpal Tunnel
|
|
Carpal Tunnel
Joined: Sep 2000
Posts: 4,757 |
a fjord is a sunken river, into which the sea has pushed
Guided only by hazy memories of old geography lessons, isn't it typically a coastal feature created by the flooding of an ice-formed 'river' valley?
|
|
|
|
|
Joined: Sep 2001
Posts: 6,296
Carpal Tunnel
|
|
OP
Carpal Tunnel
Joined: Sep 2001
Posts: 6,296 |
Heh, Mav! Just a heads-up ('cause I done gone and done the same durn thing too many time):
You jes' responded to the fjord stuff on the white squalls tuft of the thread. Me thinks that's what all the folks be talkin' about when they say we screw up the threads for those folks what's read 'em different than us! (Me, it done take me six months to finally unnerstan' what all this fussin' done been about!)
And to milum:
Thanks to you fer one heckuva story--that poor leetle horsefly must've felt he been caught in the jaws of a macrospout, huh? What yer Danny think of his specimen bein' et?
Best regards,
DubDub
|
|
|
|
|
Joined: Jan 2001
Posts: 1,156
old hand
|
|
old hand
Joined: Jan 2001
Posts: 1,156 |
the river is (stacked?) the fresh water floats on top of the denser salt wate
Another great oceanography word - I'm getting the feeling you're thinking of stratified. (My brain is still fuzzy from the cold meds and I had to think for a while on that one!)
Both temperature and salinity (the salt content of the water) affect the density of the water - and that affects which water mass will be on top. Estuarine circulation describes what happens when a river empties fresh water into a salt water estuary. Basically what happens is that the fresh water "waters down" the salt water near the surface (this watering down is called entrainment, another great word). Then that whole, larger mass of lighter, less dense "fresher" water will leave the estuary on the surface. But because of conservation of mass, there will have to be an inflow near the bottom of the estuary - otherwise the estuary would drain! So you actually have an outflow of less dense water, and to balance that, an inflow of denser water. The key thing is that the volume of the outflow from the estuary is much greater than the volume of water that the river puts in, because of the entrainment.
There is also (surprise, surprise) inverse estuarine circulation, which happens when the river outflow is more dense than the water already in the estuary (this can happen with certain combinations of temperature and salinity of both waters - the most common example is the Mediterranean water entering the Atlantic Ocean). Then the river water sinks and flows along the bottom, and the inflow is on the top.
The characteristics of the Mediterranean outflow were useful during WW II. Apparently the submarine captains had to be aware of the flow in and out of the Mediterranean and they'd go to the depth where the water was going in the direction they planned to go, then cut the engines and let the current take them (so as not to be detected). Or something like that. (Anyone out there care to confirm or deny this one?)
|
|
|
|
|
Joined: Jan 2001
Posts: 1,156
old hand
|
|
old hand
Joined: Jan 2001
Posts: 1,156 |
The problem with tides is that they are greatly affected by local bathymetry and are not usually perfecty sinusoidal. More great words: most regions have either diurnal (daily) or semi-diurnal (twice-daily) tides. Oh, it gets even worse! Usually, if you are in a place with semi-diurnal tides, one high tide is higher than the other, and one low tide is lower than the other! So you get: high high water, low high water, high low water, and low low water. ARGH!!!!!!
When we were on the Fraser River in BC last summer for a field trip, we were a good 100 km inland on the river, but the river is so huge that we still observe tides there. They were NOT sinusoidal. (Of course, thanks to Fourier, we know we can always represent them as sums of sinusoidal components - but that's another story!) They consisted of a sharp rise to high water and a slow drop to low water. The river nearly ground to a halt just before high water - compared with a speed of about 1m/s when the tide was going out. It was quite something to observe!
|
|
|
|
|
Joined: Jan 2001
Posts: 1,156
old hand
|
|
old hand
Joined: Jan 2001
Posts: 1,156 |
When velocity is advected, is it anything like the draft effect in racing
Like I said, I'm not too fond of this idea myself. Imagine yourself observing a particular point in the ocean. You're trying to describe mathematically the changes in water velocity around you. A number of forces are at work. There's the pressure force, shear and stress forces, good ol' Coriolis...all those forces cause the velocity of the fluid around you to change. Now remember, for example, the eddies I mentioned above. They're localized "swirly bits" of ocean, and they can move around (kind of like a tornado but much less dramatic). If one moves toward you, it will bring along with it the velocity of the fluid in it. So the water where you're observing will suffer a change in velocity that's not due to any of the other forces I just mentioned, but just velocity that was "dragged in" from somewhere else. And a change in velocity is called acceleration. (Furthermore, wherever the eddy came from will have had velocity advected away from it.) So that's what I take to mean "advection of velocity". In our fluid equation there are, of course, mathematical terms to describe it!
|
|
|
|
|
Joined: Oct 2000
Posts: 5,400
Carpal Tunnel
|
|
Carpal Tunnel
Joined: Oct 2000
Posts: 5,400 |
>>isn't it typically a coastal feature created by the flooding of an ice-formed 'river' valley?
Yeah, but crossing thread, i did mention NY is at the southern boundry for the last Glacier-- (glacier=Ice)--(the bit about the terminal moraine..) an yes, it was the last glacier, in the last ice age that scoured out the river bed..
the glaciers went further south inland.. but here on the east coast, NY markes the southermost extention of the ice covering, and as result, is home to the southern most fjord in North America. (i think there are even more southerly ones on the east coast of asia, either on island or mainland, but hey, that's not were i hang my hat, so i don't know the details!)
|
|
|
|
|
Joined: Dec 2000
Posts: 13,803
Carpal Tunnel
|
|
Carpal Tunnel
Joined: Dec 2000
Posts: 13,803 |
Yeahbutİ they're close enough that sinusoidal is a half decent first order approximation and it explains the phenomenon that rkay asked about.
If you want to get complex about it, the driving forces are two sine wave shaped forces of slightly different frequency and vastly different amplitude. Resistance and impedance are local variables but the whole thang could be modeled quite easily on an analog computer, at least to a second or third order approximation.
|
|
|
|
Forums16
Topics13,913
Posts229,344
Members9,182
| |
Most Online3,341
Dec 9th, 2011
|
|
|
0 members (),
782
guests, and
2
robots. |
|
Key:
Admin,
Global Mod,
Mod
|
|
|
|
