8/03/04 FCST|Gulf|Hurricane Alex

[Bridget Beddow]

They were talking about how none of the 110mph winds were reaching the ground because there was no convection advecting those winds to the surface.....any chance it was somebody in this forum?

I'm still weather illiterate here.

What do you mean by no convection advecting those winds to the surface?

 

[Zach Bailey]

Let me take a stab at this one: advection is "The horizontal transfer of air mass properties by the velocity field of the atmosphere." Now, what you need is some sort of convection to transfer these air mass properties (i.e. the wind speed, vorticity, etc.) along the ground so that they can be felt. If you're in a hurricane and there's no convection around to transfer the air mass properties (pressure changes leading to strong wind) then you're not going to feel nearly as much wind as in the convection. Hence why the winds in the eyewall are the strongest (lots of convection there moving very quickly to move the air mass), but yet in the eye (where there's no convection) you have no wind.

 

[B Ozanne]

What do you mean by no convection advecting those winds to the surface?

For reasons I wont go into do you accept that it is windier the higher up you go? Ok, and as you get towards the surface friction slows the wind. If you get a thunderstorm the windy air up high will be brought to the surface before friction slows it down. That is also why you get gusts, windier air from aloft is brought to the surface. During hurricanes the highest winds are recorded during the rain squalls. The rain and other vertical movements are "advecting" higher winds aloft to the surface.

 

[Bridget Beddow]

B Ozanne wrote:

If you get a thunderstorm the windy air up high will be brought to the surface before friction slows it down. That is also why you get gusts, windier air from aloft is brought to the surface. During hurricanes the highest winds are recorded during the rain squalls. The rain and other vertical movements are \"advecting\" higher winds aloft to the surface.

Okay, I gotcha now.
Thank you, also, Zach

Chris Rozoff wrote:

The boundary layer turbulence, and ultimately the ground, exerts great drag on the wind, causing the winds to decrease toward the ground from the top of the boundary layer. The winds decrease with height above the boundary layer. Because of turbulence in the boundary layer, higher winds can be mixed down toward the ground, providing wind gust measurements. Winds are not zero in the eye. They increase from 0 (relative to the hurricane motion) at the center to the maximum eye wall wind speeds in a roughly linear fashion.

Winds tend to be concentrated in the convection and convective rainbands because of the very efficient PV production from convection. The radius of maximum winds occurs near the eyewall due to angular momentum conservation principles.

Thank you, too, Chris.

Maybe one of these days, I'll be able to understand enough to be able to give a better description of my hurricane experiences :study:

 

[B Ozanne]

I agree, as you get very high up in a hurricane the winds decrease, but 850-700mb probably have the strongest winds and getting those to the ground is what I am talking about.

 

[Zach Bailey]

Originally posted by Chris Rozoff
Excitingly, if you guys aren't already on top of this, Alex has developed a beautiful eye (I mean it has become visible). Just on visible sat, it looks as though the cat 1 storm is being less sheared. With the visual appearance of the hurricane, it wouldn't be surprising to see it as a cat 2 again, but observations will be a better judge than the naked eye.

AMSU actually shows that the eye has indeed \"improved\" since last night. The precipitation has become fully symmetric about the center, whereas early this morning it was very asymmetric.

Chris

Indeed. Very symmetric, text-book appearance with good outflow. I would agree with you that it appears stronger than when it was glancing the NC coast since to me it appears the eye has contracted a little and has cleared out and according to the IF sat pics, there is an intact eyewall.

However it appears to have come too late as Alex will soon be over less than 26C waters which will mean it loses the tropical moisture to feed off of and will become a cold-core system. Perhaps it will give the brits a good thrashing when it makes it across the pond (check out the extended forcast, days 4 and 5 have it making a beeline for the UK).

 

[Geoff Boyle]

Do you guys think it will have any strength to bother the UK?

I thought once it hit cold water then it would loose most it's strength within 48 hours.

 

[B Ozanne]

Hurricanes make it to the UK all the time as extra-tropical storms. Just your average rain/wind event.

NJ had some amazing swells this mornings. Overhead sets with offshore winds. Tomorrow looks good too with more offshore winds and hopefully more swells coming off Alex.

 

[Glen Romine]

Originally posted by Chris Rozoff
I don't fully buy some of these arguments relating convection and observed winds at the surface. Consider the steady state hurricane. It is a vortex, of course maintained by convective forcing.

Better to say that a hurricane is a heat engine, and a relatively inefficient one at that. The sun warms the ocean surface, and evaporates water, that water vapor having latent heat that is later released as condensed clouds, and these clouds radiate energy back out to space, completing the engine cycle. I think using the term convective forcing is mis-leading - because when you say convection most people think of overturning in the atmosphere - but stratiform rain within a hurricane is much more widespread and likely a larger contributor to the overall heat budget.

So indirectly the convection leads to the winds. But the vortex structure determines the wind profile (through of course the pressure distribution). As tropical cyclones are warm core structures, the winds increase toward the ground (ocean really) to maintain thermal wind balance. The boundary layer turbulence, and ultimately the ground, exerts great drag on the wind, causing the winds to decrease toward the ground from the top of the boundary layer. The winds decrease with height above the boundary layer.

That heat energy is maximized at ~ 700 mb in the typical hurricane, but heating occurs throughout the cloud-bearing layer where there is ascent occuring. Peak rotational winds will occur closer to top of the boundary layer height - and decreasing gradually with increasing height, as you mentioned because of the thermal wind relationship for a warm core system, the horizontal pressure gradient, which is what really drives the winds, is much greater near the surface than aloft. As mentioned, surface friction acts to slow the winds within ~ 500 m of the surface owing to turbulent eddies.

See http://www.nhc.noaa.gov/gifs/JLF_Fig1.gif

Because of turbulence in the boundary layer, higher winds can be mixed down toward the ground, providing wind gust measurements.

Actually, I think there is some clear evidence for convective enhancement of the downward momentum transport - a.k.a. strong gusty winds at the surface accompanying squalls. The precipitation drag aids in the enhancing downdrafts, which are interspersed among bands of ascent. So there is some truth to the notion that active local convection can increase the likelihood of the higher winds at the top of the boundary layer making it to the surface without mixing out towards the mean wind profile.

Winds are not zero in the eye. They increase from 0 (relative to the hurricane motion) at the center to the maximum eye wall wind speeds in a roughly linear fashion. Winds tend to be concentrated in the convection and convective rainbands because of the very efficient PV production from convection. The radius of maximum winds occurs near the eyewall due to angular momentum conservation principles.
Chris

By PV, do you mean potential vorticity? That's probably a bit elevated a subject for here, but since convection causes latent heat release, and the thermal perturbations are a form of potential vorticity anamoly, then you get into a bit of a chicken or the egg argument. Horizontal winds are potentially variant within rainbands, but could be owing to variations in vertical momentum transport, etc... but I don't think PV bands could easily be tied to specific surface wind gusts. The scales are too far separated IMO. Instead, PV bands can enhance the probability of local convective development in the form of squalls/bands, and convective winds wthin this individual convective elements can enhance vertical momentum transport.

Ok, this got really long - sorry - this whole conversation really belongs in weather and forecasting - not the Target thread - but I don't have the authority to move it. Maybe a moderator could step in and do so.

Glen

 

[Zach Bailey]

To bring the forecast thread back on track, let me just add that the latest discussion increased the winds to 90kt and has the system staying AOB hurricane strength for a good stretch across the atlantic. It will be interesting to see if this has any effect on the British Isles.

Get it here:

http://www.nhc.noaa.gov/text/refresh/MIATC...ml/042037.shtml?

 

[Zach Bailey]

Alex has now entered the record books as the strongest storm to form as far north as it did (above 38N latitude).

Alex continues to amaze with its strength and organization. Wind speeds have been increased to an amazing 105kt with the latest advisory and alex still maintains an impressive structure on satellite views.

The storm is still expected to transition to an extratropical system by 48 hours.

Get the latest here: http://www.nhc.noaa.gov/text/refresh/MIATC...ml/050256.shtml?

 

[mark humpage]

Originally posted by Zach Bailey
will be interesting to see if this has any effect on the British Isles.

Get it here:

http://www.nhc.noaa.gov/text/refresh/MIATC...ml/042037.shtml?

I'll keep you posted :wink:

We await in anticipation.

Mark

 

[B Ozanne]

Has the NHC correctly forecasted the wind speeds of this system once?

This should be a real eye opener (no pun intended) on how easily this strong CAT III system could have hit the Northeast US. EVerybody knows strong storms have hit New England in the past but nobody thinks it could happen again. It's just a matter of time when everything comes together and a major hurricane hits New York. I call it the $100 billion storm...and its coming.

On another note storm relative winds are 120 mph with a forward speed of 30mph. That gives max sustained winds of 150mph...nearly a CAT V.

 

[Bridget Beddow]

Chris Rozoff wrote:

It's amazing how this storm fortuitously followed the Gulf Stream to keep it going. While I can't say for sure, the cooler temperatures at the surface might be offset by the cooler temperatures aloft to keep the convection healthy.

Alot of people around this area and along the coastline seem to tune in more to a hurricane/ts once it hits the gulf stream, due to how it seems to affect most storms that cross its path.

It's so amazing how much of an affect the gulf stream has on the way of life. That is the primary spot for fishermen on tournaments as well for the Big Rock Tournament- due to the change in water temperatures. And it has always amazed me how something that seems so small (width-wise) can have such a dramatic affect on something so large as a hurricane.

 

[rdale]

"On another note storm relative winds are 120 mph with a forward speed of 30mph. That gives max sustained winds of 150mph"

NHC's latest says max sustained winds are 120mph - where did you see the 150 report? Are you thinking that NHC doesn't incorporate motion into their wind reports?

- Rob