Bomb Low: Jul 25-29th 2008

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spwill
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Re: Bomb Low: Jul 25-29th

Unread post by spwill »

Good footage there David.

In contrast to bunters some people on ZB talkback radio yesterday thought the warnings for this storm where overstated considering the weather they got at their place, forecasters will never please everyone.

Some were waiting in fear for the arrival of the storms eye at midnight. :lol:
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Re: Bomb Low: Jul 25-29th

Unread post by Nev »

spwill wrote:Some were waiting in fear for the arrival of the storms eye at midnight. :lol:
Yes, I sincerely hope the term 'eye' hasn't now entered the media's vernacular to describe the centre of any depression/low other than that that normally occurs in a severe tropical cyclone. The often flippant past use of the term 'weather-bomb', along with the obviously demeaning term 'mini-tornado', is bad enough!

I see the media have also misconstrued last weekend's event as being the "worst storm in decade", most intense maybe, but hardly the 'worst'?

Just my thoughts
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Re: Bomb Low: Jul 25-29th

Unread post by Manukau heads obs »

yes,the problem with trying to compare to an eye of a hurricane is that in a hurricane the strongest winds are in the eye wall
but with a depression like we had, that is not the case at all, in fact the wind would decrease steadily as the low center came over

this is what happened in the auckland area a few years back when we did get a bomb low center pass right over
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Vertigo
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Re: Bomb Low: Jul 25-29th

Unread post by Vertigo »

are you sure about that, brian? i would have thought that winds in a deep depression would act a lot like a tropical cyclone, only weaker. i would expect the winds to die down near the center, but this happens in cyclones as well.
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Re: Bomb Low: Jul 25-29th

Unread post by Manukau heads obs »

would expect the winds to die down near the center
isnt that what I said?
i.e
in fact the wind would decrease steadily as the low center came over
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Michael
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Re: Bomb Low: Jul 25-29th

Unread post by Michael »

The depression probably started to shallow as it moved closer lessening the wind/keeping it constant.
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Re: Bomb Low: Jul 25-29th

Unread post by Vertigo »

Manukau heads observer wrote: isnt that what I said?
i mean that in the dead center. the winds around the center should be the strongest, just like a tropical cyclone in my opinion. is there any reason this shouldnt be the case?
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Re: Bomb Low: Jul 25-29th

Unread post by Manukau heads obs »

no, its not the case...Paul Malinson explained that in a post he made the other day
its due to losses due to the turning motion (angular motion) around the tight center
also with a hurricane, its a different structure, whereby the largest pressure gradient drop is in the eye wall, due to thats where the powerfull thunderstorms are
and so thats where the strongest winds are (and that pressure drop is so large it overcomes the problem of the wind looses energy having to go around a tight circle
also there is very little pressure drop from just inside the eyewall to the center of the eye

in a normal depression, there is not those intense thunderstorms around the center, and just sprialing frontal arms

its a different structure

sure the low on the sat image just before it got to cape reinga had concentric clouds around the low center, but they were just a band of shower cloud that had managed to loop the loop right around the center, and did not have the power of thunderstorms driven by 27oc water like a hurricane has (and which results in the very strong pressure drop in the eye wall of a hurricane)
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Re: Bomb Low: Jul 25-29th

Unread post by Nev »

Agree with Brian re the winds. This would probably be one of the biggest differences between a cold-cored mid-latitude depression, which draws its energy from contrasts in temperature with latitude, and their warm-cored cousins, which draw energy from vertical contrasts in temperature. I think apart from the trough line extending to the tropics, this depression looked non frontal, i.e. having a more symmetrical appearance similar to that of a tropical cyclone.
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Re: Bomb Low: Jul 25-29th

Unread post by spwill »

i.e. having a more symmetrical appearance similar to that of a tropical cyclone.
But not the band of intense Thunderstorms around the eye that you find with Hurricane/Tropical Cyclone style of storm.
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Re: Bomb Low: Jul 25-29th

Unread post by Nev »

spwill wrote:But not the band of intense Thunderstorms around the eye that you find with Hurricane/Tropical Cyclone style of storm.
And a more gradual decrease of winds towards its centre, which was painly evident on Saturday night.
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Re: Bomb Low: Jul 25-29th

Unread post by Vertigo »

thanks for clearing that up guys.
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Re: Bomb Low: Jul 25-29th

Unread post by NZstorm »

Yes, the term 'eye' is a scientific word in meteorology and is part of the structure of a Tropical Cyclone, but not a mid lattitude depression like we had on Saturday.

Interestingly, you do not get thunder activity within a Tropical cyclone, only on the convective bands outside the TC. The reason for this is the strong horizontal movement of the air reduces the vertical motion of the convection to a point where it reduces the electrification process. Something like that anyway. It was explained on stormtrack once. 8o
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Re: Bomb Low: Jul 25-29th

Unread post by Manukau heads obs »

i guess there is not enough charge separation occuring ?
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Re: Bomb Low: Jul 25-29th

Unread post by NZstorm »

Yes. I had noticed in the past the lack of lightning activity in hurricanes in the USA region and had wondered.
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Re: Bomb Low: Jul 25-29th

Unread post by spwill »

Yes, I had noticed a lack of TStorm activity within Tropical Cyclones in the past but surely you would expect some lightning within the deep cloud band around the eye.
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Re: Bomb Low: Jul 25-29th

Unread post by Paul Mallinson »

Manukau heads observer wrote:yes,the problem with trying to compare to an eye of a hurricane is that in a hurricane the strongest winds are in the eye wall
but with a depression like we had, that is not the case at all, in fact the wind would decrease steadily as the low center came over

this is what happened in the auckland area a few years back when we did get a bomb low center pass right over
Interesting discussion about winds you guys. I tend to agree with what Brian has said.
What we often observe is that as the big cloud shield on the eastern side of the developing low approaches from the north or northwest you get a strengthening easterly wind ahead of the warm front. It is directly to the south of the warm front that we often get the strongest winds - this happened on the weekend and it is forecast to happen again with this next deep low. The stability of the airmass, the relatively straight (gently curving isobars), the warmer less dense air aloft coupled with the strong ascent through a deep layer over the warm front all play a part to create the tight pressure gradient to the south of the front.
Then as the wave pulls out of the low centre and occludes away to the east, you see the occlusion wrap around the low centre as we did on the weekend. If the low deepens far enough, this can spiral around and into the low a number of times. The strong winds continue to be located approximately within the width of that spiral cloud band, but on the side away from the low centre. Hence not only do you get a spiral of cloud around the low centre, but in effect there is also a spiral of stronger winds. From memory, I think this was why we saw winds jump up again at Cape Reinga late Saturday night. When the low starts to fill, the pressure gradient near the centre flattens out and gets bigger. This means the winds get lighter there, but stronger winds remain on the outside of what is left of the decaying occlusion. This is the general idea.
I remember there is a rule of thumb that works in a general way for Atlantic storms. You count the number of half (hemisphere) spirals out of the centre and for every hemisphere you subtract 10 hPa from 1000 hPa to get the approximate pressure at the centre of the low. So for instance 3 hemipheres equals 3x10= 30hPa suggest a centre pressure of 970hPa. Try it!

Cheers.

Paul
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Re: Bomb Low: Jul 25-29th

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Re: Bomb Low: Jul 25-29th

Unread post by Manukau heads obs »

sorry to re open this thread
here is a photo I took this arvo of a shed that had been blown off its foundations by last weekends easterly severe gales....looks like the only thing that stopped it from blowing away was the power cable!
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