Fish-bone convergence lines

[NZ Thunderstorm Soc]

I might be dumb and fully understand weather charts but what does the 'fish-bone' symbol mean in the Tasman sea between the high over NSW and the high located to the west of the North Island as indicated on today's MetService's noon chart?

JohnGaul
NZThS

[Lacertae]

Fishing nets may be ?

[David]

http://www.metservice.com/public/maps/h ... l-map.html

Scroll down to near the bottom of the page, you'll see this means a convergence line.

[Willoughby]

http://www.metservice.com/public/maps/h ... l-map.html

Scroll down to near the bottom of the page, you'll see this means a convergence line.

Interesting! Never seen that before on a weather chart. Over the tropics they're quite common but RSMC's just plot the trough marker (dashed).

[NZ Thunderstorm Soc]

Ah huh!
Now I know
A convergence line
Thanks.
I have seen this line in charts for tropical regions but never in other regions.

JohnGaul
NZThS

[jamie]

Often see it on the metservice charts right at the top during the summer. First time i have ever seen one a) in the tasman and b) that far south.

[NZ Thunderstorm Soc]

Interesting as it extends south from a trough line

JohnGaul
NZThS

[Michael]

Most years theres something major to break up the pattern about mid April in dry years,it happened last year and about the same time in dry '78.[quote=]looking ahead,looks like a big north tasman low to move over the NI 10 days from now.....will be interesting![/quote]

but anticyclones have been at stake.

JohnGaul
NZThS

[NZ Thunderstorm Soc]

I see on the 6pm chart, the so called line is now a trough line. Maybe a computer glitch?

JohnGaul
NZThS

[Lacertae]

Why Michael is signing his message "JohnGaul NZThS" ?!?
Bring an exorcist, there's an emergency here !!!

[NZ Thunderstorm Soc]

Don't know what happened there, Ben
...just simply replied to his message

JohnGaul
NZThS

[NZ Thunderstorm Soc]

Don't know what happened there, Ben
...just simply replied to his message

JohnGaul
NZThS

Being dumb, I think I pushed thee wrong button

JohnGaul
NZThS

[Myself]

Regarding the convergence line:


Broadly speaking,

1) All troughs are convergence lines
2) All fronts are troughs

The fact that a convergence line has been drawn shows that neither a trough nor a front could be drawn there. There appears to be very limited pressure gradient there so it's probably difficult to see any cyclonic curvature (the hallmark of a trough) of the wind field. Given the location, it's likely also that there is no baroclinicity, hence a front cannot be drawn.

That CL is between northwesterlies strengthening ahead of the cold front and northeasterlies around the periphery of the high. Despite the weak pressure field the winds might actually be fairly strong due to the supergeostrophy on the outskirts of the high and the increase in wind ahead of the front.
So it seems a reasonable feature to draw.

As for never seeing them on NZ charts, well as I have shown they are there all the time that you see a trough or front, it's just that if there is cyclonic curvature of the wind field/pressure field or thermal advection then they will draw a front or trough and not a CL.
But they also exist in their truest form as meso scale features anyway. If you drew a close up NZ chart, you would see them all the time, for example in Canterbury along the axis of convergence between, say, southwesterlies and northeasterlies.

They are more common in the tropics, true. But, the SPCZ is always drawn as a stationary front, and the ITCZ is usually drawn as a trough.

There are also other features that can be drawn on charts that the public will never see. eg upper troughs (drawn as parallel trough lines with the height written on them), upper fronts (look just like surface fronts only they are not coloured in), lines of convection, bands of CBs....

Usually there is little point drawing upper fronts because if you know the theory you know where the upper front will be in comparison to the surface front. However it can be useful to mark out the situation, for example to distinguish between ana and kata cold fronts. But again, we as members of the public will never see these on MS charts.

If you get two people to draw an analysis or prognosis chart then you will get two different charts. A lot of it is subjective. It's an artform and also a high form of meteorological science. There is a lot going on there and a lot of scientific thought has been invested in the charts.


The CL has disappeared on the latest chart, not because of a "computer glitch" but because it's a new chart. For whatever reason, the analyst has decided to draw a trough now instead of a convergence line.

[NZ Thunderstorm Soc]

Thankyou for being yourself, myself.
An excellent contribution of the understanding of the 'fishbone' trough line.

JohnGaul
NZThS

[Storm Struck]

Regarding the convergence line:


Broadly speaking,


2) All fronts are troughs

From what i believe a front is completly different to a trough.
Fronts often bring more of the bulky stuff whether its a warm or a cold front, with scatterd rain and strong winds ahead of the front.
Troughs usually form in behind fronts and mark a slot of unstable or colder air, bringing heavier showers CB's and thunderstorms.
Cheers
Jason.

[Myself]

Well a trough is an axis of cyclonic curvature of isobars and/or the wind field. You can see it where the isobars bend sharply - that is a trough. Likewise, a ridge is an anticyclonic curvature - they don't bend as sharply as troughs though usually.
A front is a trough, it has to be a trough, but it also has some form of thermal advection associated with it.

What you are saying is not wrong. Say, for example a major SW change comes through. A cold front hits, followed by southwesterlies. There are probably troughs embedded in the SW flow, depending on how cyclonic it is. So the coldest air has already hit (with the front), now all the air is uniformly cold and the troughs just bring brief spells of more "significant weather". Sometimes though they might organise themselves to bring some colder air - in which case they would either be a cold front or a weak cold front...depending on how much temperature gradient there is.


Fronts are always drawn on the warm side of the temperature gradient. So, there is warming AHEAD of a warm front, and cooling BEHIND a cold front.
Once a warm front passes over, the air is then uniformly warm (the warm sector). There is no cooling until the cold front passes.

[Manukau heads obs]

you sometimes see the convergence line drawn in for the easterly surge that affects auckland and northland
minus the barbs