Will this SAL cap the tropics?

[EDR1222]

Just wondering if this dust storm will cause the tropics to slow down a little? A local news station out of Orlando was discussing it with a meteorologist who said it can stabilize the atmosphere and supress tropical storm development. I guess the bigger question is how long is it expected to be around?

Thoughts and comments welcome

[wxmann_91]

No not really. Look at last year and Ivan. Ivan encountered the SAL but still strengthened to a Cat 5.

And just because there's a lot of Sahara dust now doesn't mean it will last throughout the entire season. Don't know how long it will last, possibly a few more weeks?

[Swimdude]

I'm fairly sure it's expected to last about 2 weeks from now.

[mike18xx]

No not really. Look at last year and Ivan. Ivan encountered the SAL but still strengthened to a Cat 5.

Dust = condensation-nuclei.

[baygirl_1]

No not really. Look at last year and Ivan. Ivan encountered the SAL but still strengthened to a Cat 5.

Dust = condensation-nuclei.

okay, i smell a learning moment here...
mike: do you mean that the dust offers a surface to which moisture can adhere? and, therefore, as long as a system doesn't take in much dry air, the sal will not hinder a system? is that right? oh... in that case could the sal actually help a developing system???

[wxmann_91]

No not really. Look at last year and Ivan. Ivan encountered the SAL but still strengthened to a Cat 5.

Dust = condensation-nuclei.

okay, i smell a learning moment here...
mike: do you mean that the dust offers a surface to which moisture can adhere? and, therefore, as long as a system doesn't take in much dry air, the sal will not hinder a system? is that right? oh... in that case could the sal actually help a developing system???

Let me answer that question for him. Yes, the dust offers a surface that moisture condenses on to. But, IMHO, the SAL can still harm a system by allowing too many water droplets to form and therefore disrupting the inner core of a system. This almost happened with Ivan, where the eye dissapeared for about a day as the SAL had an effect on it. But, as a result, Ivan's circulation got bigger, and once uncontaminated by African dust, it strengthened.

This was the idea for Project Stormfury, an attempt to weaken hurricanes by pouring particles of silver iodide into a hurricane's eyewall.

Then again, according to this article, cloud seeding doesn't necessarily affect the amount of precip from a cloud. Instead, it does make the clouds in a hurricane less dense and disperse. But it also says that seeding could enhance the precip, so the effect on the amount of precip is inconclusive.

http://en.wikipedia.org/wiki/Cloud_seeding

OK, I don't even know what I'm saying. But the point is that one can make a parallel between foreign dust particles entering a hurricane and cloud seeding inside a hurricane. And I think that the SAL isn't as important as people think. After all, Project Stormfury wasn't successful, and SAL usually affects those TC's trying to develop, not a mature hurricane. So natural cloud seeding is a bit overrated IMO.

[mike18xx]

No not really. Look at last year and Ivan. Ivan encountered the SAL but still strengthened to a Cat 5.

Dust = condensation-nuclei.

okay, i smell a learning moment here...mike: do you mean that the dust offers a surface to which moisture can adhere? and, therefore, as long as a system doesn't take in much dry air, the sal will not hinder a system? is that right? oh... in that case could the sal actually help a developing system???

Provided other factors aren't hostile, "dirty air" makes for a stronger storm....or at least a rainier one.

[wxcrazytwo]

No not really. Look at last year and Ivan. Ivan encountered the SAL but still strengthened to a Cat 5.

Dust = condensation-nuclei.

okay, i smell a learning moment here...mike: do you mean that the dust offers a surface to which moisture can adhere? and, therefore, as long as a system doesn't take in much dry air, the sal will not hinder a system? is that right? oh... in that case could the sal actually help a developing system???

Provided other factors aren't hostile, "dirty air" makes for a stronger storm....or at least a rainier one.

hmm, interesting..

[wxmann_91]

Now that I think of it, perhaps I overthought. The dry air that usually comes with the SAL will limit moisture that condenses on to the dust particles.

But the moisture near a hurricane would be enhanced, so a hurricane or any TC would increase in size. But as explained in the above post, the effect of particles on enhancing rainfall is inconclusive.

Nevertheless, the end result is that clouds become less dense and shrivel up. A hurricane's cloud tops would warm and the inner core would weaken. Look at recent examples such as Emily and Ivan and you will see that happened. But as soon they got away from the SAL they strengthened back up. The SAL's effects on mature TC's cannot last long because though the inner core weakens, dry air often lacks the power to penetrate through the moist air around a TC, and the inner core, though weakened, would remain intact.

[mike18xx]

Yes, the dust offers a surface that moisture condenses on to. But, IMHO, the SAL can still harm a system by allowing too many water droplets to form and therefore disrupting the inner core of a system.

Dust particles are normally small and fairly hydrophobic (they don't readily become condensation-nuclei until convected up into supercooled regions -- it's fat, hydophilic air-borne salt crystals, otoh, that enable cumulus-congestus outer-band squalls to rain like Kansas thunderheads. Dust particles should, theoretically, preferentially supercharge CDO convection by wringing the last moisture out of supercooled air before it is exhausted out.

This almost happened with Ivan, where the eye dissapeared for about a day as the SAL had an effect on it.

If you're talking about the period prior to hitting Grenada, IIRC, Ivan was moving extremely fast, and the undercutting effect of an easterly-surge (associated with the SAL, one presumes) was what temporarily sapped the system, not the dust.

This was the idea for Project Stormfury, an attempt to weaken hurricanes by pouring particles of silver iodide into a hurricane's eyewall..... But the point is that one can make a parallel between foreign dust particles entering a hurricane and cloud seeding inside a hurricane.

Silver-iodide crystals are extremely hydrophilic -- the exact opposite of dust particles.

[wxmann_91]

If you're talking about the period prior to hitting Grenada, IIRC, Ivan was moving extremely fast, and the undercutting effect of an easterly-surge (associated with the SAL, one presumes) was what temporarily sapped the system, not the dust.

According to the NHC, it was dust and dry air that weakened it. And I'm pretty sure that easterly surges aren't powerful enough to weaken a mature category 4 hurricane that much, maybe a tropical storm, but not a major hurricane, as the convection around the eye of a major hurricane would protect it from any easterly surge.

From Hurricane Ivan Discussion 19:

A NOAA JET IN IVAN THIS EVENING HAS FOUND THE EXISTENCE OF A SAHARAN AIR LAYER IN THE NORTHERN QUADRANTS OF THE STORM. THIS MAY HELP EXPLAIN THE WEAKENING OF IVAN THROUGHOUT THE DAY TODAY.

From Hurricane Ivan TC Report:

...almost as quickly as Ivan strengthened it also weakened -- as much 20 kt over the following 24 h. Conventional and microwave satellite data indicated the probable cause of the rapid weakening was due to mid-level dry air that got wrapped into the center of the hurricane and eroded the eyewall convection.

[ericinmia]

I don't have time to read all of the above... BUT, the SAL is always around to one degree or another.

It is the strenght and amount and proximity to a storm which causes the storm to not form, or die.

Here is where you can track the SAL...

http://cimss.ssec.wisc.edu/tropic/real- ... split.html





Hope that helps some
-Eric

[TS Zack]

Looking at WV loop, they have dry air but it is not completely covering all the Tropical Atlantic. In other words, it isn't enough to stop development if something is there to develop.

[baygirl_1]

thanks wxmann_91 and mike. very informative!

[EDR1222]

Ditto. Thanks for all the good information everyone!

[mike18xx]

Here is where you can track the SAL...

Caveat: One should not make the mistake of automatically assuming that any "red" in the Atlantic sub-tropics on that pic is "saharan air"; all that's certain is that it is DRY. (One can observe the same reds/subsiding-high-pressure off the west coast of the US.)

Dry air is dry air, and hurricanes don't like it whether it came out of Libya or subsided from 40,000 feet up.

[WeatherEmperor]

my only question is, is the SAL normal for this time of year in the eastern atlantic?

<RICKY>