Ask a Pro Met

[wx.1028]

Here is an overview with some links... ask after Irene and I can give a better answer.

http://met.nps.edu/~.../marsupial.html

Thanks.

[Wannabehippie]

Why do hurricanes expand as the get further north, even before they start any transition to extra-tropical.

Thanks.

[MJW155]

What exactly is an open eye wall? Is it when severe winds don't surround the eye? Or is it a section of the eye wall being ragged and collasping in on the eye so that it can't maintain it's intensity?

[accatt2204]

Do you think the tilted/enhanced Jet stream will really prevent the storm from significant weakening before landfall or is it yet to be seen or is it just a model run? Why have the winds not picked up even though the pressure dropped significantly?

[phil882]

What exactly is an open eye wall? Is it when severe winds don't surround the eye? Or is it a section of the eye wall being ragged and collasping in on the eye so that it can't maintain it's intensity?

An open eyewall is normally defined when you don't have a full ring of convective activity surrounding the center... which is normally diagnosed through radar.

[Conclue]

Do you think the tilted/enhanced Jet stream will really prevent the storm from significant weakening before landfall or is it yet to be seen or is it just a model run? Why have the winds not picked up even though the pressure dropped significantly?

First question has to do with air flow acting to enhance said systems outflow there for helping to maintain it's upper level structure. The upper level divergence in the NW qaudrant of Irene was very amplified thus resulting in stronger convergence at the surface thanks to divergence associated with the trough. This is why NYS, VT, Mass, etc had such epic ammounts of rain, as convergence was great in this area, thus forcing for ascent. Some locals in NYS got 10' of rain. Albany got it's monthly rain total nearly in one day.

The second one is going to be a thesis for a graduate student at some point. Same thing happened with Ike in 2008 and prolly others that I don't know off the top of my head. In my own opinion I feel as though the larger the system circulation and size the harder it is for the strongest winds to reach the surface. I think it can happen (an Ike or Irene size system reaching cat 5 status with strong winds) but we need pristine perfect conditions to see it happen including the atmospheric enviornment, synoptic set up as well as ample room for the storm to develop.

[Thundersnow12]

You can get big snowfalls from storms with almost no surface low. A good example is "frontal waves" aloft where low amplitude upper tropospheric waves propagate over a region of enhanced baroclinity at some level in the atmosphere above the surface. What you can get is a prolonged period of frontogenesis as the wave passes over the elevated baroclinic zone. If oriented right--you can get a long period of snowfall with heavy banded precip over one location as the front remains parallel to the flow aloft. Sometimes folks call these clippers--but they really don't bear resemblance to a true "clipper".

this happened a few times last year I believe. You said it pretty nicely. You don't need these huge big cyclones/ULL's coming in off the pacific, diving down to baja and coming NE...just gimme a nice mid-level wave aloft rippling along the jet over a tight low-level baroclinic zone and you can get some serious banded snow.

[Ed Lizard]

I'll try here instead...

Well, they say the only stupid question is the one not asked, but Googling around for 3D Var and 4D var and ensemble Kalman filter, I get results like this that don't help much.

http://www.hfip.org/...semble_team.pdf

Maybe a paragrap each, executive summary style description, the average person with some knowledge of math and science but not a met would understand.

[am19psu]

I'll try here instead...

Well, they say the only stupid question is the one not asked, but Googling around for 3D Var and 4D var and ensemble Kalman filter, I get results like this that don't help much.

http://www.hfip.org/...semble_team.pdf

Maybe a paragrap each, executive summary style description, the average person with some knowledge of math and science but not a met would understand.

The only person here that is really qualified to answer these questions is dtk. Hopefully, he gets around to starting that numerical modeling thread (ahem).

[dtk]

The only person here that is really qualified to answer these questions is dtk. Hopefully, he gets around to starting that numerical modeling thread (ahem).

Ha. Noted. I was thinking about this today.

I will try to get something together soon. If u have specific questions you want answered, that would help me get started.

[Ed Lizard]

Ha. Noted. I was thinking about this today.

I will try to get something together soon. If u have specific questions you want answered, that would help me get started.

Initialization schemes, like what 3D and 4D var and ensKf mean as a start. And how spectral models work. I have oil field experience with reservoir models that seem similar to grid block models (mass balance, multi-phase flow in anisotropic annd heterogeneous media) divided by area and depth, and a little hearsay knowledge from grad students about using Monte Carlo simulation or the such to fill in data or run multiple models with unknown data modified) and am well aware of the concept of history matching, as far as oil field production models compared to actual data, but can find little on 'spectral' models or the initialization schemes.

[QVectorman]

Asked this yesterday in the Irene thread but no response. At what latitude and/or forward speed does the coreolis effect have a signifant influence on the path of a tropical cyclone?

Sorry a little late on this, thought I would help out a little...TC's can not form closer than 5N or 5S of the equator. The CF is not strong enough between 5N or 5S to produce rotation on the flow. From this, to answer your question, this means the CF starts to influence the path of a TC from its inception which is any region above 5N or 5S

[Paweatherguy1]

Here is my question for a pro met.

Why is it that deep winter storms almost never track up the spine of the appalachians? They either Go OTS, hug the coast, or cut into the lakes?

[phil882]

Here is my question for a pro met.

Why is it that deep winter storms almost never track up the spine of the appalachians? They either Go OTS, hug the coast, or cut into the lakes?

Well you actually do see some lows that track up the spine of the Appalachians if they are strong enough, but perhaps what are you noticing is that the Appalachian mountains tend to weaken the circulation of surface cyclones since they often bump up into the topography and as it travels through the Appalachians. In this case what happens is that the upper level forcing that started the cyclone continues to move over the Apps without losing strength forcing the surface cyclone to ascend over the mountains. This causes vortex squashing which weakens the overall cyclone since the total column of air is smaller, and in order to maintain continuity the cyclonic circulation must expand outward in the horizontal and weaken in magnitude. In some cases the first cyclone does not survive this trip across and dissipates to the west of the mountains. However, as the upper level features move east of the Appalachians, the upper level forcing will then generate a "lee cyclone" since the column once again extends downward in height which forces the cyclone to tighten up through continuity in the low levels. The added upper level divergence to the right of the upper level forcing also aids in cyclogenesis which allows the surface cyclone to form directly east of the Appalachians or along the coast that could either travel up the coast or out to sea.

I don't know if this answers your question directly... but that might explain why you see stronger cyclones on either side of the Appalachians.

[am19psu]

Ha. Noted. I was thinking about this today.

I will try to get something together soon. If u have specific questions you want answered, that would help me get started.

What is 3DVAR? What is 4DVAR? Why is 4DVAR considered superior to 3DVAR? Where does ensemble Kalman filtering fit into all of this?

Is getting the initial conditions right the biggest hold up for better models? What other things would make models better? Can you explain why there are so many parameterizations?

For gridded models, can you explain how finite differencing is used for each time step? How does that compare with the time step calculations for spectral models?

Why do spectral models have their resolution expressed in numbers of triangles instead of distances?

That ought to get you started...

[wkd]

I want to thank all of the mets who responded to my question about the coreolis effect on the path of a TC. What might be interesting is to plot what path Katia would take given her present heading and say a forward speed of say 12 knots without any other synoptic influences, One could then maybe better understand the potential influences of ridges, troughs, prevailing upper winds, etc. Is there a site where this can be done?

[Ed Lizard]

What is 3DVAR? What is 4DVAR? Why is 4DVAR considered superior to 3DVAR? Where does ensemble Kalman filtering fit into all of this?

Is getting the initial conditions right the biggest hold up for better models? What other things would make models better? Can you explain why there are so many parameterizations?

For gridded models, can you explain how finite differencing is used for each time step? How does that compare with the time step calculations for spectral models?

Why do spectral models have their resolution expressed in numbers of triangles instead of distances?

That ought to get you started...

Bump.

[QVectorman]

I want to thank all of the mets who responded to my question about the coreolis effect on the path of a TC. What might be interesting is to plot what path Katia would take given her present heading and say a forward speed of say 12 knots without any other synoptic influences, One could then maybe better understand the potential influences of ridges, troughs, prevailing upper winds, etc. Is there a site where this can be done?

I believe if you look at the XTRP model, it will give you the path you desire. I may be wrong.

[ice1972]

Well you actually do see some lows that track up the spine of the Appalachians if they are strong enough, but perhaps what are you noticing is that the Appalachian mountains tend to weaken the circulation of surface cyclones since they often bump up into the topography and as it travels through the Appalachians. In this case what happens is that the upper level forcing that started the cyclone continues to move over the Apps without losing strength forcing the surface cyclone to ascend over the mountains. This causes vortex squashing which weakens the overall cyclone since the total column of air is smaller, and in order to maintain continuity the cyclonic circulation must expand outward in the horizontal and weaken in magnitude. In some cases the first cyclone does not survive this trip across and dissipates to the west of the mountains. However, as the upper level features move east of the Appalachians, the upper level forcing will then generate a "lee cyclone" since the column once again extends downward in height which forces the cyclone to tighten up through continuity in the low levels. The added upper level divergence to the right of the upper level forcing also aids in cyclogenesis which allows the surface cyclone to form directly east of the Appalachians or along the coast that could either travel up the coast or out to sea.

I don't know if this answers your question directly... but that might explain why you see stronger cyclones on either side of the Appalachians.

This is a fantastic explanation.......is that what is happening when a met talks about the cyclone "transferring its energy to the coast".....Miller B style I think? Like in the case of an Alberta Clipper that dies before the Apps but then reforems out off Delmarva or something? I could be totally off here.....

[Wannabehippie]

Why do weaker tropical systems tend to more more westerly but more powerful ones tend to be influence by ridges and troughs to recurve in the mid atlantic?

[phil882]

This is a fantastic explanation.......is that what is happening when a met talks about the cyclone "transferring its energy to the coast".....Miller B style I think? Like in the case of an Alberta Clipper that dies before the Apps but then reforems out off Delmarva or something? I could be totally off here.....

More or less... the 500mb energy is not affected generally by the topography of the mountains, but in the low levels the cyclone is impeded by Apps. and most low level cyclones redevelop to some degree on the other side. However, most of the time you see the surface cyclone develop near the coastline because of the strong baroclinicity that sets up along the coast thanks to the strong temperature gradient that sets with very cold land compared to the warmer moister air that is observed over the Gulf Stream.

Why do weaker tropical systems tend to more more westerly but more powerful ones tend to be influence by ridges and troughs to recurve in the mid atlantic?

It all has to do with how vertically deep the storm is. The stronger a system is, the stronger of a reflection it will have typically in the mid to upper levels, which then makes it more likely to be affected by the steering currents in the mid to upper levels. When a storm gets sheared, the mid and upper level circulations of the storm are displaced from the low level circulation, and that gives the system the opportunity to be steered more by the low level steering currents.

So when you know the system is expected to be weaker than expected... focus on the 850mb and 700mb steering flow and heights, which could have a stronger influence on the storm motion. However, when a storm becomes a formidable hurricane, the 500mb flow is normally the best steering guide. There are cases where the low level flow allows a cyclone to move more easterly than the low level flow (Emily as it moved away from FL was a good example) so its not always a "weaker storm = further west" rule, but more a "weaker storm = lower level heights for steering" rule.

[Analog96]

Why do weaker tropical systems tend to more more westerly but more powerful ones tend to be influence by ridges and troughs to recurve in the mid atlantic?

Weaker systems basically are more shallow, so upper level steering currents don't effect them as much, so they can just scoot along the prevailing Westerlies in the tropics. When a storm gets deeper and cloud tops get higher, they tend to get influenced more by upper level flow created by troughs, etc.

[k***]

phil's answer was way better

[BTRWx]

If convergence and instability (lifting) creates low pressure systems, can strong stability (sinking) cause surface winds? If so, would this effect be classified as a high pressure system? Since high pressure systems are known to generally create calm conditions, how would strong stability relate to that?

[isohume]

If convergence and instability (lifting) creates low pressure systems, can strong stability (sinking) cause surface winds? If so, would this effect be classified as a high pressure system? Since high pressure systems are known to generally create calm conditions, how would strong stability relate to that?

The maximum wind speeds around a sfc high and anticyclonic flow are limited based on the gradient wind equation.

It's too complicated to get into here, but suffice to say the radius of curvature around a high is low and the radius of curvature is directly related to the rate of change of pressure.

So...the isobars must get further apart the closer you get to the center of a high (where the radius of curvature goes to zero). This of course leads to a weak pressure gradient force (PGF) and hence low winds. On the other hand and theoretically, there is no limit to the PGF with cyclones.

[AtticaFanatica]

Does polarimetric radar provide any potential utility in nowcasting where mesoscale snow bands are going to form within synoptic-scale systems?

[isohume]

Does polarimetric radar provide any potential utility in nowcasting where mesoscale snow bands are going to form within synoptic-scale systems?

Not really...dual pol will help in identifying the freezing layer and any possible re-freezing layer...hence helping to locate winter precip types better. As far as locating areas of deformation etc, the base/rpg products will still be the best tools for metwatching.

[AtticaFanatica]

Not really...dual pol will help in identifying the freezing layer and any possible re-freezing layer...hence helping to locate winter precip types better. As far as locating areas of deformation etc, the base/rpg products will still be the best tools for metwatching.

Thanks.

[Maximum lawman]

A stupid hydrology question (you did say I could ask anything). We had a wet spring, then from May 20 to August 1 (10 weeks) we had less than 2" of rain IMBY. Since then, we've had over 20 inches in several events over several weeks. 20 inches in five weeks. My ground is still pretty much rock hard. Why isn't it soup?

[isohume]

A stupid hydrology question (you did say I could ask anything). We had a wet spring, then from May 20 to August 1 (10 weeks) we had less than 2" of rain IMBY. Since then, we've had over 20 inches in several events over several weeks. 20 inches in five weeks. My ground is still pretty much rock hard. Why isn't it soup?

You have good drainage soil and the ground water tables must have been pretty low. Your area is most likely not in a high end runoff area either. 20 inches over several weeks isn't extraordinary for dry areas with good drainage.