Another Low Pressure system for the weekend 2-3 August

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Manukau heads obs
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by Manukau heads obs »

I remember chasers commenting that the Greensburg tornado was hard to see because there was a distinct lack of lightning.....(i.e less than you would expect for such a severe thunderstorm)
maybe its like the same thing with a hurricane.....the rotation is stopping the vertical charge separation?
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by Wildland »

It wasn't a fizzer here.
Tawa got another 25 mm in the 12 hours from 7:30 last night.
Cracker thunderstorm brought 7 mm just after midnight with a bit of hail being thrown about.
Rain rate peaked at 30.8 mm/hr during the thunderstorm, but eased.

Might be a good day to advertise mud-wrestling on the front lawn... :crazy:
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by NZstorm »

.....the rotation is stopping the vertical charge separation?
Yes, maybe something like that Brian.
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by spwill »

Or the flashing you see in the cloud top (Super Cells) is shut out by thick lower layer cloud when your close to the storm.

I have seen plenty storm clips with regular Cg lightning close to the Tornado
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by NZstorm »

The CG action can quieten down around a tornado but I was making the point that the upper level stuff doesn't.
But then as Spwill says, with some storms it doesn't. The rear flank downdraught which coincides with tornadogenisis introduces drier air into the inflow which must reduce the effective CAPE in the storm updraught, hence less CG's. Thats my quick theory on it but I'm only guessing. Every storm is a bit different.
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by Michael »

Wow! the Sun has come out.For how long its anyones guess.
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by Weather Watcher »

Manukau heads observer wrote:....
maybe its like the same thing with a hurricane.....the rotation is stopping the vertical charge separation?
Rotation within a thunderstorm actually increases the strength of the updraft by adding an additional upward pressure force, which in turn should increase the vertical charge separation.

I did a quick google on the subject and came up with this (from http://findarticles.com/p/articles/mi_m ... 69133/pg_4)....

The rotating updraft produces a localised region of lowered pressure in the mid-levels of the storm, which creates an upward-directed pressure gradient force.... With respect to the three primary ingredients for deep moist convection (i.e., moisture, instability, and upward motion), this pressure gradient force provides enhanced upward motion on a preferred storm flank to initiate new convective development. Numerical modelling studies have shown that this dynamic interaction can contribute around 50% of the total updraft strength....
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by jamie »

Slip in wellington this morning after heavy rain last night. Cant imagine it would be the nicest thing to wake to.

An update on the waikato river flood. Sure is getting high and Taupo is 11cm away from being legally full.

http://www.stuff.co.nz/4641793a11.html
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by Weather Watcher »

Manukau heads observer wrote:re the lightning rotation thing..
it seems to me that mesocylones in the midwest did not have much lightning activity right under the meso....(but instead there was further away with the anvil), but when the meso's all joined up and straightlined in the evening into a bow echo and lost the rotation, then there was lots of lightning....thoughts?
I think you posted that in the wrong thread, so I'll reply in this one....

I'm not an expert on lightning, and I don't really understand why some storms produce lots of lightning and others don't, but I do know that the location of lightning is often a function of updraft strength and vertical wind shear.

In general terms, during the charge separation process, the ice crystals become positively charged and are swept up to the higher levels of the storm while the water droplets become negatively charged and usually fall to the lower part of the storm. I suspect that in the strong updraft part of the storm that's associated with a mesocylcone, both the positively charged ice crystals and the negatively charged water droplets are being swept upwards, with the ice crystals being swept out into the anvil and then discharging (giving the lightning you observe near the anvil). Once the storms weaken (and form a line), the negatively charged water droplets are probably able to fall back down and discharge to the surface (giving all the lightning you observe near the convective line).

BTW, the distribution of positive and negative CG's are a function of vertical wind shear (especially in NZ). In low shear situations, where the storm is quite vertical, only the negatively charged water droplets discharge to the ground as the positively charge ice crystals in the upper parts of the storm can't "see" the ground and are unable to discharge. In storms which are tilted over from moderate or strong vertical wind shear, we see more positive CG's as the positively charged ice crystals can now "see" the ground and are able to discharge.

Interestingly we often see this across the Southern Alps in strong vertical wind shear situations with thunderstorms on the west coast, and spillover rain into Otago and Canterbury. In these situations, we have noticed that most of the CG's on the west coast are negatively charged (from the negatively charged water droplet forming rain) and most of the CG's east of the Alps are positively charged as they are coming from the positively charged ice crystals which have been swept up over the Alps at high levels (and eventually fall as spillover rain).

Hope that all makes sense.
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by Michael »

Heavy Hail here now.
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by jamie »

I went down to Mercer this afternoon and WOW :eek: ! what a lake! I have never seen the river that high ( i only saw huntly hamilton and ngaruawahia in 98). It is level with SH1 but will not get any higher as it is spilling over the stop banks. One raod in the town of mercer is flooded off. The river is still rising. Im hacked off though because i took my camera but it decided after i got there and took 3 photos that it would stop working >_< :banghead: . So i couldnt even get photos of it all. Ill post the pics soon.
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by spwill »

I took this photo of the Waikato River at Mercer on friday while I was down that way, the river will be higher now.
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by jamie »

The 1st picture is from SH1 on a tiny rise a few hundred meters north of mercer. The body of water in the forground is water that has spilt over the stopbanks and the water in the background is the river itself sitting at the top of the about 1.5-2m high stopbank.

The other 2 are up a hill looking down over the town.

3 news just had a quick thing on the river flooding at huntly and mercer too.
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Re: Another Low Pressure system for the weekend 2-3 August

Unread post by jamie »

spwill wrote:I took this photo of the Waikato River at Mercer on friday while I was down that way, the river will be higher now.
Thats the road that i was saying is flooded. Water is well past the speed signs there.
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