Dust forecast for the 2009 Atlantic hurricane season

By: Dr. Jeff Masters , 6:58 PM GMT on June 20, 2009

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There will be less African dust than usual over the tropical Atlantic during this year's hurricane season, according to a new experimental dust forecast issued by Dr. Amato Evan of the University of Wisconsin. Dr. Evan used a statistical model that correlated levels of dust activity in past years with rainfall over the Sahel region of Africa and a natural regional wind pattern known as the North Atlantic Oscillation (NAO). He forecasts that dust levels over the Main Development Region (MDR, 8 - 20°N & 15 - 65°W) for Atlantic hurricanes during this year's hurricane season will be similar to last year's below-average levels, thanks in large part to plentiful rains over the Sahel region of Africa during the 2008 rainy season (Figure 1). However, the dust levels expected this year do not approach the record lows seen in 1994 and 2005. Dust forecasts made in May or June are skillful going out five months, with a skill 11 - 16% better than a "no-skill" forecast using climatology.


Figure 1. Rainfall over the Sahel region of Africa was generally 50 - 100 mm (2 - 4 inches) above average during the 2008 rainy season (about 20 - 80% above average). The heavy rains promoted vigorous vegetation growth in 2009, resulting in less bare ground capable of generating dust. Image credit: NOAA/Climate Prediction Center.

The Sahara and the Sahel: significant sources of dust
The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18° - 22° N) and the northwestern Sahel (15° - 18° N) (Figure 2). The dust that originates in the Southwest Sahara stays relatively constant from year to year. However, the dust from the northwestern Sahel varies significantly from year to year, and understanding this variation may be a key factor in improving our forecasts of seasonal hurricane activity in the Atlantic. The amount of dust that gets transported over the Atlantic depends on a mix of three main factors: the large scale and local scale weather patterns (windy weather transports more dust), how wet the current rainy season is (wet weather will wash out dust before it gets transported over the Atlantic), and how dry and drought-damaged the soil is. The level of drought experienced in the northwestern Sahel during the previous year's rainy season (June - October) is the key factor of the three in determining how much dust gets transported over the Atlantic during hurricane season, according to a January 2004 study published in Geophysical Research Letters published by C. Moulin and I. Chiapello. A dry rainy season the previous year will make an expanded area of loose soil which can create dust. It is also possible that the corresponding changes in vegetation can alter the regional weather patterns, causing more dust production.


Figure 2. Map of the mean summer dust optical thickness derived from satellite measurements between 1979 and 2000. Maximum dust amounts originate in the northern Sahel (15° to 18° N) and the Sahara (18° to 22° N). The Bodele depression in Chad is also an active dust source. Image credit: Evidence of the control of summer atmospheric transport of African dust over the Atlantic by Sahel sources from TOMS satellites (1979-2000) by C. Moulin and I. Chiapello, published in January 2004 in Geophysical Research Letters.

How dust suppresses hurricanes
Dust acts as a shield which keeps sunlight from reaching the surface. Thus, large amounts of dust can keep the sea surface temperatures up to 1°C cooler than average in the hurricane Main Development Region (MDR) off the coast of Africa, providing hurricanes with less energy to form and grow. Dust also affects the Saharan Air Layer (SAL), an layer of dry, dusty Saharan air that rides up over the low-level moist air over the tropical Atlantic. At the boundary between the SAL and low-level moist air where the trade winds blow is the trade wind inversion--a region of the atmosphere where the temperature increases with height. Since atmospheric temperature normally decreases with height, this "inversion" acts to but the brakes on any thunderstorms that try to punch through it. This happens because the air in a thunderstorm's updraft suddenly encounters a region where the updraft air is cooler and less buoyant than the surrounding air, and thus will not be able to keep moving upward. The dust in the SAL absorbs solar radiation, which heats the air in the trade wind inversion. This makes the inversion stronger, which inhibits the thunderstorms that power a hurricane. The dust may also act to interfere with the formation of cloud drops and rain drops that these thunderstorms need to grow, but little is known about such effects. It is possible that dust may act to help hurricanes by serving as "condensation nuclei"--centers around which raindrops can form and grow.

For additional reading
Dr. Evan published a study in Science magazine this March showing that 69% of the increase in Atlantic sea surface temperatures over the past 26 years could be attributed to decreases in the amount of dust in the atmosphere.

Jeff Masters

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Quoting WAHA:

I don't think this storm will make it. By the way it looks right now, it looks like it's already gone. I would give it a 30% chance of it surviving the night.
Agreed
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1442. WAHA
Quoting OnTheFlats:
Tough call, same pattern setting up as last night though. Our local forecasters seem to think so somewhere down here. Storms keep popping up to my east/northeast and moving southeast and they keep popping up ever so slightly more toward the coast and I might get one or two before the night's over. class='blogquote'>Quoting OnTheFlats:
Tough call, same pattern setting up as last night though. Our local forecasters seem to think so somewhere down here. Storms keep popping up to my east/northeast and moving southeast and they keep popping up ever so slightly more toward the coast and I might get one or two before the night's over.

I don't think this storm will make it. By the way it looks right now, it looks like it's already gone. I would give it a 30% chance of it surviving the night.
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1440. Levi32
Quoting hurricane2009:


As I was saying earlier, if that surface low becomes dominant it will give the BOC area more time to possible develop. Either way we will see what strides it makes overnight and tomorrow.


Yeah, and furthermore the motion is not and won't be WNW as said by the NHC. The mid-upper-level flow is clearly going to take this NW to NNW, and this is confirmed by satellite loops for both the mid-level and low-level circulations.

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1438. RTLSNK
Quoting pottery:
Just spoke to my wife who is in Ft.Lauderdale.
She says that she cannot wait to get out of the heat there, and return here to the balmy temps of 88F LOL


Message on the Ark for you.
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1426. If we don't get hit today, tomorrow is a good bet!
Member Since: July 7, 2005 Posts: 0 Comments: 5199
I'm telling you, I've been living here my whole life (Shhhh 40 years this August)and I can't remember a hotter day than today. I've been surfing and fishing for 25 years and following the weather almost as long and it was one of the strangest heats as well. Kind of reminded of Las Vegas in a weird way because there was a very dry portion to the day relatively speaking, for South Florida at least.
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Off topic...Jon and Kate are officially broke up. :(
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1432. Levi32
Andres-Feria comparison:

Tropical Storm Andres:



Tropical Depression Feria:



Both are in proximity to land with similar structure and no poleward outflow channels to speak of.
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Partial eye wall

Member Since: July 24, 2005 Posts: 407 Comments: 19076
Quoting TampaSpin:


NOt saying any shear exist yet...i'm saying as Andres gets stronger and it will....the outflow will inhibit anything in the BOC.....ITS JUST MY FREAKING OPINION...ONLY WHEN ANDRES gets STRONGER!.......Seen it too many times.....We will see if anything Develops in the BOC....Looking very good now huh......LOL



sorry TS the huricane's not big enough to have its outflow effect the surface low exiting the coast in the BOC area,if it was a larger circulation,than maybe,IMO....I would be suprised if we did not have a new invest in the BOC by tommorow night!!!!
Member Since: October 5, 2007 Posts: 20 Comments: 4970
Quoting Ossqss:


Will it make it ???

Tough call, same pattern setting up as last night though. Our local forecasters seem to think so somewhere down here. Storms keep popping up to my east/northeast and moving southeast and they keep popping up ever so slightly more toward the coast and I might get one or two before the night's over.
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1427. pottery
Just spoke to my wife who is in Ft.Lauderdale.
She says that she cannot wait to get out of the heat there, and return here to the balmy temps of 88F LOL
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1425. Levi32
Switching to shortwave IR for the night
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1424. Ossqss
Quoting OnTheFlats:
OssGss (Quote 1420) I hope so, man I'm in Deerield Beach at the Broward/Palm Beach border and at my house right now it's 93 deg out and feels more humid than at 5:00pm today. We need the cool down from some evening T-storms.


Will it make it ???

Member Since: June 12, 2005 Posts: 6 Comments: 8185
OssGss (Quote 1420) I hope so, man I'm in Deerield Beach at the Broward/Palm Beach border and at my house right now it's 93 deg out and feels more humid than at 5:00pm today. We need the cool down from some evening T-storms.
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Quoting Levi32:


Well that's not outflow. That's upper and mid-level cirrus debris drifting north within the weak flow aloft between Andres, the upper low over central Mexico, and the high over the southern states.


Caused by the Atmospheric Turning of Andres.....OK!
Member Since: September 2, 2007 Posts: 178 Comments: 20439
1421. KEEPEROFTHEGATE (Mod)


i believe we are at or very near CAT 1 HURRICANE STATUS with our epac system
Member Since: July 15, 2006 Posts: 169 Comments: 53308
1420. Ossqss
East FL is going to get strafed.

Member Since: June 12, 2005 Posts: 6 Comments: 8185
1419. Levi32
Quoting TampaSpin:
Maybe you can see the Low Level outflot from here............LOL

WV Link


Well that's not outflow. That's upper and mid-level cirrus debris drifting north within the weak flow aloft between Andres, the upper low over central Mexico, and the high over the southern states. Outflow over a tropical cyclone spreads out while curving anticyclonicly due to the area of high pressure in the upper atmosphere that forms above the storm. These clouds are not moving that way.
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1418. WAHA
ow my foot is asleep! lol
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A shear reading of between 5-20kts in the BOC

With shear dropping
Member Since: September 30, 2007 Posts: 9 Comments: 15932
Hey Tampa - can you post that link next time there is a storm in the gulf? I think it could make an awsome desktop pic.-- goosebumps (an that is not easy sitting here on my patio with a heat index of 98)
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1415. WAHA
The BOC has some of it's northeast side growing. Also, this might seem offtopic, but, I just had DeJaVu.
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1414. Ossqss
Lots of moving energy !

Link
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1413. KEEPEROFTHEGATE (Mod)
Quoting hurricane2009:


Pacman CDO? LOL
lol
hurricane pacman
Member Since: July 15, 2006 Posts: 169 Comments: 53308
Maybe you can see the Low Level outflot from here............LOL

WV Link
Member Since: September 2, 2007 Posts: 178 Comments: 20439
1410. pottery
Yeah RMM. But I never go swimming at night. Used too though LOL
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1409. Levi32
Quoting hurricane2009:


Pacman CDO? LOL


LOL
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Tampa bay radar tonight is strange.. it's like the storms are trying to come in but are being repelled.. Link
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Look at the Low Level cloud cover moving into the BOC from the South......IM out.....!
Member Since: September 2, 2007 Posts: 178 Comments: 20439
LOL Pottery.. that must be difficult don't you live on an island??
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1404. KEEPEROFTHEGATE (Mod)
LATEST PASSIVE MICROWAVE IMAGERY AND VISIBLE
SATELLITE IMAGERY INDICATE WHAT MAY BE AN EYE FEATURE FORMING
WITHIN THE DENSE CIRRUS OVERCAST OVER THE CENTER. THIS IS
SUGGESTING FURTHER STRENGTHENING...AND THE CYCLONE MAY BECOME A
HURRICANE SOONER
THAN THE OFFICIAL NHC GUIDANCE FORECAST SHOWS.
A GROWING LARGE CLUSTER OF VERY DEEP CONVECTION IS OBSERVED OVER
AND AROUND THE CENTER. THIS CONVECTION IS MARKED BY NUMEROUS
STRONG CONVECTION WITHIN 100 NM OF THE CENTER IN THE SE
QUADRANT. SCATTERED STRONG CONVECTION IS ELSEWHERE WITHIN 120 NM
OF THE CENTER OVER THE S AND SW QUADRANTS AND WITHIN 60 NM OF
THE CENTER IN THE N SEMICIRCLE. SCATTERED MODERATE TO STRONG
CONVECTION IS OCCURRING FROM 14N-19N BETWEEN 104W-107W...AND E
OF THE CYCLONE FROM 11N-16N BETWEEN 94W-98W.
Member Since: July 15, 2006 Posts: 169 Comments: 53308
1403. Levi32
Last visible image of the night....deep convection continues to fire and re-fire within the CDO:

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You all have a good evening.......
Member Since: September 2, 2007 Posts: 178 Comments: 20439
1400. Ossqss
I could not find anything on Crossfire Hurricanes, that was not rolling stones related :)
Member Since: June 12, 2005 Posts: 6 Comments: 8185
Quoting hurricane2009:
Im not seeing the shear either, the BOC system just isnt well organized


NOt saying any shear exist yet...i'm saying as Andres gets stronger and it will....the outflow will inhibit anything in the BOC.....ITS JUST MY FREAKING OPINION...ONLY WHEN ANDRES gets STRONGER!.......Seen it too many times.....We will see if anything Develops in the BOC....Looking very good now huh......LOL
Member Since: September 2, 2007 Posts: 178 Comments: 20439
1398. pottery
RMM, you must be the same age as me.
I STILL have problems with that. Wherever there is water.
Even going to the toilet in the dark, I hear that sound track LOL
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1395. Levi32
Quoting TampaSpin:
Look on the Eastern Side of Andres as that wraps around it has to create shear in the BOC...



What are you refering to that is "wrapping around"? If you're talking about outflow that's the wrong word to use lol. There is no outflow to his NE. The outflow directly to his east is a zonal outflow channel that was being enhanced by an upper trough in the western Caribbean but is starting to weaken due to Andres moving west, and that's not going to "wrap around" to the north.
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1394. pottery
Good to be in the same room as you guys this evening.
I am learning some things here.
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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