Of dust and hurricanes

By: Dr. Jeff Masters , 3:40 PM GMT on December 14, 2006

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The Saharan Air Layer (SAL) is 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.


Figure 1. 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.

The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18 - 22 N) and the northwestern Sahel (15 - 18 N) (Figure 1). 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 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. In 2005 (Figure 2), precipitation across the northwestern Sahel averaged near normal, so I'm a bit surprised we saw so much dust over the Atlantic. So far in 2006, precipitation in the northwestern Sahel has been lower than in 2005. If the research cited above is any indication, we should have at least as much dust over the Atlantic during the 2007 hurricane season as the 2006 hurricane season had, which should act to hamper hurricane formation in the region between Africa and the Lesser Antilles Islands.



Figure 2. Departure of precipitation from normal for the African rainy season. Precipiation was near normal averaged across the northwestern Sahel region. Image credit: NOAA.


Jeff Masters

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94. philliesrock
10:45 PM GMT on December 15, 2006
This may be the new "Thingamabobbercane" that MichaelSTL was talking about:

thingamabobbercane
Member Since: June 29, 2006 Posts: 65 Comments: 3197
92. BigBake
2:14 PM GMT on December 15, 2006
Cyclone shut the hell up and quit spamming the blog with your junk. You have your own blog so use it, no one cares for your propaganda. Nothing you have posted so far in this blog pertains to the Sahara, and you will not reverse thousands of years of desert. If it was remotely possible you would put the mid latitudes back into ice age in which mass extinction would occur.
88. BahaHurican
11:41 AM GMT on December 15, 2006
Morning everyone,

Looks like Nassau and most of the NW Bahamas is set to get that line of Tstorms coming through sometime today. It's already pretty overcast here, and based on the NASA viewer, it looks like we will be seeing some rain by 9 - 10 a. m.

As I said earlier, this seems a remarkable amount of rain for the Bahamas in December. Usually Dec - Feb are quite dry for us. . . .
Member Since: October 25, 2005 Posts: 19 Comments: 20686
86. Skyepony (Mod)
4:59 AM GMT on December 15, 2006
Australians flee to beach to escape bushfire

Illinois Set Tornado Record In 2006
Member Since: August 10, 2005 Posts: 156 Comments: 36062
84. Skyepony (Mod)
4:51 AM GMT on December 15, 2006
Alot of NOAA news this week

NOAA REPORTS 2006 MARKED BY SEVERE HEAT WAVES, WIDESPREAD DROUGHT, WILDFIRES

Dec. 14, 2006 — The average annual temperature for the contiguous U.S. will likely be the third warmest on record in 2006, according to scientists at the NOAA National Climatic Data Center in Asheville, N.C. The year is noted for widespread drought and record wildfires, as well as heavy precipitation and flooding in some parts of the country. Following the warmest year on record for the globe in 2005, the annual global temperature for 2006 is expected to be sixth warmest since recordkeeping began in 1880.

Including 2006, six of the seven warmest years on record have occurred since 2001 and the ten warmest years have occurred since 1995. The global average surface temperature has risen between 0.6 degrees C and 0.7 degrees C since the start of the 20th Century, and the rate of increase since 1976 has been approximately three times faster than the century-scale trend.



Which one of ya'll gonna go out there & make us proud?
NOAA SEEKS APPLICANTS FOR THE 2007 ERNEST F. HOLLINGS UNDERGRADUATE SCHOLARSHIP PROGRAM
Up to $29,000 Available to Each Student for Studies and Internships

Dec. 13, 2006 — NOAA is accepting applications for a scholarship program in honor of retired South Carolina senator Ernest F. Hollings, who promoted ocean and atmospheric study and research throughout his career. This is the third year the scholarship is being made available to students interested in pursuing degrees in ocean and atmospheric sciences and education.


NOAA TRACKING SPACE WEATHER EVENT
so this explains the flickering lights.

Combs added, “With the Space Shuttle in orbit and astronauts working on continued construction of the International Space Station, NASA is watching the activity with great interest. NASA officials said they did take precautions to avoid the effects of the radiation storm from the solar flare by having the astronauts aboard the International Space Station and shuttle Discovery sleep in protected areas of their respective spacecraft overnight.”
Member Since: August 10, 2005 Posts: 156 Comments: 36062
82. weatherboykris
4:18 AM GMT on December 15, 2006
To those who await the release of my blog with much anticipation,(sarcasm).It will be delayed until tomorrow.Goodnight
Member Since: December 9, 2006 Posts: 125 Comments: 11346
78. aquak9
4:12 AM GMT on December 15, 2006
CB...I'd hate to see what kind of apparatus you have to keep the Christmas tree watered....
Member Since: August 13, 2005 Posts: 163 Comments: 25493
73. HIEXPRESS
4:02 AM GMT on December 15, 2006
Could it be?

Nah
Member Since: October 13, 2005 Posts: 4 Comments: 2155
72. Caffinehog
4:00 AM GMT on December 15, 2006
*sigh* Has to be a dull year in the Atlantic when we get excited over blobs and swirlies.
Member Since: June 5, 2003 Posts: 0 Comments: 40
71. Caffinehog
3:56 AM GMT on December 15, 2006
To see what I mean about the dry air:
Link
Member Since: June 5, 2003 Posts: 0 Comments: 40
70. Caffinehog
3:55 AM GMT on December 15, 2006
Cyclone... might be slightly subtropical. The quickscat shows a sharp wind shift on the surface, and it has good mid-level circulation. Lots of dry air around it, though, and not much in the way of upper level divergence. It might technically become a storm, but I wouldn't bother naming it.
Member Since: June 5, 2003 Posts: 0 Comments: 40
68. HIEXPRESS
3:06 AM GMT on December 15, 2006
clone, I don't seem to be able to zoom in from that link. What are we looking at?
Member Since: October 13, 2005 Posts: 4 Comments: 2155
66. hurricane23
2:25 AM GMT on December 15, 2006
882mb right now it looks like light rain over the area.But lots of clouds and on and off rain showers over the next 24 hours before this mess moves out of here.Weekened looks ok with sunday looking like the best day.
Member Since: May 14, 2006 Posts: 8 Comments: 13597
65. 882MB
2:19 AM GMT on December 15, 2006
Hey everyone if you look closely on the MIAMI RADAR you can see a line of THUNDERSTORMS developing offshore paralleling the coast line training right by the coast and if you see closely you can see it trying to drift TOWARDS THE COAST, if this happens we can see some training right over MIAMI in the next hours!
Member Since: September 29, 2006 Posts: 0 Comments: 348
64. pottery
2:10 AM GMT on December 15, 2006
Hi Ryang, I'm over at your house partying. What are you doing here??? I'm trying to find where the ice is, I brought a bottle........
Member Since: October 24, 2005 Posts: 0 Comments: 23080
63. 1900hurricane
2:08 AM GMT on December 15, 2006
Everyone!!! Come to ryang's blog party!!!
Member Since: August 2, 2006 Posts: 45 Comments: 11553
62. philliesrock
2:02 AM GMT on December 15, 2006
So, this is what's left of Utor:

utor
Member Since: June 29, 2006 Posts: 65 Comments: 3197
61. ryang
2:00 AM GMT on December 15, 2006
Hi Pottery.Long time no see.LOL
Member Since: August 25, 2006 Posts: 329 Comments: 12397
59. pottery
1:58 AM GMT on December 15, 2006
I never said it was a bad idea. And listen, when that guy said the world was round noboddy took him on, or the one who claimed the Earth goes round the sun, so dont take it on. Keep up the fight, lets see what happens next........
Member Since: October 24, 2005 Posts: 0 Comments: 23080
58. 1900hurricane
1:57 AM GMT on December 15, 2006
COME HERE!!!
Member Since: August 2, 2006 Posts: 45 Comments: 11553
57. hurricane23
1:55 AM GMT on December 15, 2006
Radar out of key west shows some more rain on the way for south florida in the over night hours threw tommorow morning as things should start to somewhat clear by late afternoon.


Member Since: May 14, 2006 Posts: 8 Comments: 13597
55. pottery
1:50 AM GMT on December 15, 2006
..........perseverance.........its good.
Member Since: October 24, 2005 Posts: 0 Comments: 23080
51. pottery
1:43 AM GMT on December 15, 2006
Greetings. I was out all day and am sorry to have missed participating in the discussion on the SAL etc. There do seem to be conflicting views on the effect of the dust on the storms, and more info will surely be coming up now that the issue is on the front burner. I look forward to that. Thank you Dr. M. , for all the explanations.
Member Since: October 24, 2005 Posts: 0 Comments: 23080
49. Caffinehog
1:36 AM GMT on December 15, 2006
Goodbye Key West!
Member Since: June 5, 2003 Posts: 0 Comments: 40
48. HIEXPRESS
1:28 AM GMT on December 15, 2006
Now we're getting the rain here in Volusia County. We're going to have to invert our trough.
Member Since: October 13, 2005 Posts: 4 Comments: 2155
46. MarcKeys
12:40 AM GMT on December 15, 2006
Anything that help reduce storms is a plus for us here in the Keys!!


Florida Keys Fishing
45. BahaHurican
12:06 AM GMT on December 15, 2006
Meanwhile it's absolutely dry here right now. We had some miserable weather last night - constant light rain just a bit heavier than a mist - that prolly gave us a full inch of precip. before the night was over. I must say it seems unseasonably wet here this month. Normally it's much drier in Dec. than it has been this year . . .
Member Since: October 25, 2005 Posts: 19 Comments: 20686
44. Skyepony (Mod)
11:44 PM GMT on December 14, 2006
PP~ those vectors is what's happening in the upper atmosphere ~400-0mbs.
Member Since: August 10, 2005 Posts: 156 Comments: 36062

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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.