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Of dust and hurricanes

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

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

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

"Friday I plan to talk about the November-December record warmth."


well make sure you mention the november freeze here in chicago too...

or will this be a global warming induced post ;)

hey can you make it stay warm here too...i like this 50 degree thing today
When we look back over the last two seasons, both of them were hampered by SAL. The difference between this year and last year was shear, ULL's in the right place at the right time, and slightly lower SST's.
that's right,weatherguy.There were no classic Cape Verde hurricanes like Frances or Ivan in 2005.
Weatherguy03...Completely agree. Shear and dry-air this sesaon were not much different from last season. The main difference, ULL's were close enough this year to interfere with the storm, instead of far enough away to provide good outflow.
Link
Countdown.T-minus 168 days
Flash flood warnings up for Palm Beach and Broward Counties.
Training of cells is occuring.
PBI has 5.39 inches in three hours
3" here in Boynton Beach, now up to 5.66" at PBIA.
"In 2005, 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."

Remember that the QBO and the rainfall over the Sahel was the *magic formula* for the Greycast for some years -- and then stopped being correlated with the activity in the ATL hurricane season. So we are left to wonder if it was a valid correlation.

A factor that was noted both by Steve Gregory in his late July forecast, and, I discovered, by Tom Berg, of Hurricane Alley, was the large size of the tropical waves over Africa (bringing more dust with them). Remember how enormous the wave that spawned Helene was (2000 miles wide -- I called her "Humongous Helen"). Also there was a mid-level 700mb jet that pushed more dust into the ATL and dissipated future Helene's convection, just after that large wave moved off Africa.
I've heard the QBO before.What is it exactly?
If you look at the WU radar, PBC will be saved from widespread flooding by a small break currently just south of Lake Worth and north of Boca Raton.Still lots of rain though.
Quasi-biennial oscillation

From Wikipedia
Is it just me,or does this look like an outflow boundary.I've never actually seen one on the radar.Link
I wonder if any dust is b/c of poor farming practices.
17. Inyo
"oh, it got below freezing in Chicago! this means there is no climate change"
South of Lake Worth and north of Boca is me in Boynton Beach. There is no break, I can tell you.
Read that you've not been well, hope it'll all pass soon and - while here - wish you Very Happy Holidays!
Happy Holidays
Don't ask me why,but it looks to me like there is some sort of convergence point in Broward County creating the thunderstorms.That's why the bottom of the line is staying where it is and the top is moving further north.The storm tracks show this nicely.Link
weatherboykris check your mail!
Looks like the surface low continues to deepen as forcast. Low level clouds are streaming into the center pretty good now.
phone and satellite out here in Lantana SE Palm Beach County, record rainfall. Please be careful when driving, streets cannot be distinguished from canals.
BLIZZARD WARNING IN EFFECT FROM MIDNIGHT TONIGHT TO 10 AM PST
FRIDAY FOR THE SOUTH WASHINGTON CASCADES AND THE NORTH AND CENTRAL
CASCADES OF OREGON
..

THE NATIONAL WEATHER SERVICE IN PORTLAND HAS ISSUED A BLIZZARD
WARNING...WHICH IS IN EFFECT FROM MIDNIGHT TONIGHT TO 10 AM PST
FRIDAY. THE WINTER STORM WATCH IS NO LONGER IN EFFECT.

SOUTH WINDS WILL INCREASE LATER TODAY AND THIS EVENING AS A STRONG
COLD FRONT APPROACHES FROM THE WEST. HOWEVER...SNOW LEVELS WILL BE
HIGH DURING THIS TIME...HOLDING AROUND 8000 FEET. SOUTH WINDS OF
30 TO 50 MPH CAN BE EXPECTED THIS EVENING...WITH GUSTS OF 65 TO 85
MPH.

THE COLD FRONT WILL MOVE ACROSS THE CASCADES BETWEEN MIDNIGHT AND
2 AM. BEHIND THE FRONT...WINDS WILL BECOME WEST TO SOUTHWESTERLY...BUT
REMAIN STRONG. EXPECT WINDS OF 40 TO 60 MPH...WITH GUSTS OF 75 TO
95 MPH. ON THE HIGHER PEAKS AND RIDGES ABOVE 4500 FEET...WIND
GUSTS OF 100 TO 130 MPH ARE EXPECTED
.

AT THE SAME TIME...SNOW LEVELS WILL DROP RAPIDLY...FROM 8000 FEET
THIS EVENING TO 2000 FEET BY DAYBREAK. SNOW ACCUMULATIONS OF A
FOOT OR MORE ARE EXPECTED ACROSS THE CASCADE PASSES AND AT THE SKI
RESORTS. IN ADDITION...EXPECT CONSIDERABLE BLOWING AND DRIFTING
SNOW ...WITH WHITEOUT CONDITIONS OCCURRING FREQUENTLY.


A BLIZZARD WARNING MEANS SEVERE WINTER WEATHER CONDITIONS ARE
EXPECTED OR OCCURRING. FALLING AND BLOWING SNOW WITH STRONG WINDS
AND POOR VISIBILITIES ARE LIKELY. THIS WILL LEAD TO WHITEOUT
CONDITIONS...MAKING TRAVEL EXTREMELY DANGEROUS. DO NOT TRAVEL. IF
YOU MUST...HAVE A WINTER SURVIVAL KIT WITH YOU. IF YOU GET
STRANDED...STAY WITH YOUR VEHICLE.

wow the winter storm WARNING that where up for the OR mts have now been update to BLIZZARD WARNING

Does anyone have an update on the storm front that is going to hit the coast of Washington?
In this which is going on about the new study in Geophysical Research Letters, that talks about dust squelshing 'canes, this is also mentioned.

In 2005, the year of Katrina and Rita and the most active hurricane season on record, more Saharan dust arrived in the Caribbean than at any time during the previous 30 years, says Joseph Prospero, director of the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami.

"It goes completely contrary to our argument," Mr. Prospero says, adding that many other factors, like the location of high-pressure systems over the Atlantic and El Nio in the Pacific, affect hurricane formation.


Though from the graph comparision the other day, 2005 had much higher humidity levels across the N Atlantic than 2006.
Rainfall now at 4" here in Boynton Beach, but almost 7" at PBIA.
I'd like to find the actual article that was published..here's another news one about it. Says 2005 was a low dust year. I remembered this being a most dusty year, the other article has it breaking records but this was mentioned.

By doing so (dust hampers storm), however, the dust storms could shift a hurricane's direction further to the west, the researchers say, increasing the likelihood that it would hit the United States and Caribbean Islands.
Ya'll are getting trained on...choo choo.
Rain across this, interesting & a bit off topic.

Abstract
The infrared cloud amount with cloud tops above 180 hPa from the International Satellite Cloud Climatology Project D1 cloud product and the Tropical Rainfall Measurement Mission 3B42 precipitation product are employed to study the impact of the Madden-Julian Oscillation (MJO) on the diurnal cycle of tropical deep convective cloud amount (DCC). Our analysis demonstrates that the diurnal cycle of tropical DCC is enhanced over both land and water during the convectively active phase of the MJO, while it is reduced during the convectively suppressed phase of the MJO. However, the diurnal phase of DCC is not significantly affected by the MJO. The analysis also indicates that the MJO modulation of the diurnal amplitude is much larger over the eastern Indian Ocean (around 50% of the mean diurnal amplitude) than over the western Pacific and the Maritime Continents (around 20% of the mean diurnal amplitude).


Seemed like we had a lack of night time blow ups, with invests, perticularly in the gulf this year. The gulf was in a suppressed phase as far as the MJO a good bit of the season as well. hhhmmm.

I found the abstract

Abstract
It is well known that Atlantic tropical cyclone activity varies strongly over time, and that summertime dust transport over the North Atlantic also varies from year to year, but any connection between tropical cyclone activity and atmospheric dust has been limited to a few case studies. Here we report new results that demonstrate a strong relationship between interannual variations in North Atlantic tropical cyclone activity and atmospheric dust cover as measured by satellite, for the years 19822005. While we cannot conclusively demonstrate a direct causal relationship, there appears to be robust link between tropical cyclone activity and dust transport over the Tropical Atlantic.

Received 3 April 2006; accepted 30 August 2006; published 10 October 2006.


Anyone an AGU member? Not paying $9 to read it.
Windsat of the GOM
i live in palm beach county in Lake Worth. Period of heavy rain and continuos rain has occured throughout the day. I think that at least 3-4in has fell already and as much as 5-6 by the time its all over. This morning the rain sounded like a freight train coming through the building i was in..thats how bad it was here. Although all this rain is occuring at the same time which gives us a chance for flash flodding, we desperatley need the rain considering we are at an 18 in. deficit.
Evironment Canada's forcast for anyone interested.

Evironment Canada's Official Weather Warnings
Warnings
Greater Vancouver
10:08 AM PST Thursday 14 December 2006
Wind warning for
Greater Vancouver continued

Southeast winds of 50 to 70 km/h developing this afternoon veering to west 60 to 90 km/h tonight through Friday morning.

This is a warning that damaging winds are imminent or occurring in these regions. Monitor weather conditions..Listen for updated statements.

A strong pacific storm is moving towards the British Columbia coast today and will cross Vancouver Island tonight. Ahead of the storm this afternoon winds will increase to southeast 50 to 70 km/h in most coastal locations. Winds are expected to shift to west 60 to 90 km/h overnight through early Friday morning across all of the south coast as the low moves inland.

Heavy rainfall is forecast for Howe Sound and the eastern Fraser Valley where rainfall amounts in excess of 50 mm are expected through tonight.

Snow accumulation of 20 to 40 cm is forecast for Whistler through Friday afternoon.

Please refer to the latest public forecasts for further details.

It's possible to see gusts where I am in the Fraser Vally get up to 120-130 kmh tommorow morning with this Forcast.
Gonna blow a good one this time.
NWS out of Miami...

.DISCUSSION...
AREAS OF RAIN AND EMBEDDED THUNDERSTORMS CONTINUE TO AFFECT
PRIMARILY THE EASTERN HALF OF THE SOUTH FLORIDA THIS AFTERNOON...
WITH RECORD-BREAKING RAIN TOTALS REPORTED IN PALM BEACH COUNTY.
THIS ACTIVITY HAS BEEN FOCUSED BY WHAT APPEARS TO BE A WEAK
INVERTED SURFACE TROUGH LOCATED NEAR THE I-95 CORRIDOR. AFTER
INITIAL DEVELOPMENT THIS MORNING...ADDITIONAL CONNECTIVE DEVELOPMENT
OCCURRED AS THE LOW-LEVEL JET FED RICH TROPICAL MOISTURE INTO THE
SYSTEM. MODERATELY STRONG SOUTHERLY FLOW IN THE LOWEST 10 KFT OF
THE TROPOSPHERE HAS ALLOWED FOR ECHO TRAINING...WITH WIDESPREAD
REPORTS OF FLOODING ACROSS BROWARD AND PALM BEACH COUNTIES. AS
WARM ADVECTION CONTINUES TO DRAW VERY MOIST AIR NORTHWARD
OVERNIGHT...EXPECT RAIN TO CONTINUE ACROSS VIRTUALLY THE ENTIRE
FORECAST AREA...AND WILL CARRY CATEGORICAL POPS FOR THURSDAY NIGHT
AND FRIDAY...IN LINE WITH FWC/MAV/MET NUMBERS. ALTHOUGH FLOODING WILL BE
THE GREATEST CONCERN FOR THE REMAINDER OF THIS EVENT...STEEPENING MID-
LEVEL LAPSE RATES AND SIGNIFICANT LOW-LEVEL HODOGRAPH CURVATURE
WILL MAINTAIN A THREAT FOR EMBEDDED THUNDERSTORMS. ALTHOUGH VERY
DIFFICULT TO DETERMINE AT THIS TIME...THE HEAVY RAINFALL THREAT
MAY BE GREATEST NEAR INVERTED SURFACE TROUGH ACROSS EASTERN HALF
OF THE PENINSULA.
And for Washington State......

STATE FORECAST FOR WASHINGTON
NATIONAL WEATHER SERVICE SEATTLE WA
100 PM PST THU DEC 14 2006


WAZ001-503>519-020>022-039-040-142330-
STATE FORECAST FOR WESTERN WASHINGTON
330 AM PST THU DEC 14 2006




..HIGH WIND WARNING FOR MOST OF WESTERN WASHINGTON FOR THIS AFTERNOON THROUGH FRIDAY MORNING...
...HEAVY SNOW WARNING FOR THE CASCADES FOR NOON TODAY THROUGH NOON FRIDAY...

...HIGH SURF ADVISORY AND COASTAL FLOOD WATCH FOR THE COAST...

...FLOOD WARNING FOR THE SKOKOMISH RIVER...

TODAY...RAIN DEVELOPING. SNOW IN THE MOUNTAINS. BECOMING VERY WINDY THIS AFTERNOON...WITH LOCALLY DAMAGING WIND 40 MPH WITH GUSTS TO 65 MPH. HIGHS IN THE MID 40S TO LOWER 50S.

TONIGHT...RAIN. SNOW IN THE MOUNTAINS. VERY WINDY...WITH WIDESPREAD WIND 40 MPH WITH GUSTS TO 65 MPH...AND LOCAL WIND 50 TO 60 MPH WITH GUSTS AS HIGH AS 90 MPH MAINLY ALONG THE COAST...STRAIT OF JUAN DE FUCA...AND ADMIRALTY INLET. LOWS IN THE LOWER TO MID 40S.

FRIDAY...SHOWERS IN THE MORNING...THEN MOSTLY CLOUDY WITH SCATTERED SHOWERS IN THE AFTERNOON. VERY WINDY EARLY...EASING DURING THE LATE MORNING AND AFTERNOON. HIGHS IN THE 40S.
That windsat you posted skye appears to be showing a little love, almost heart shaped lol.
Are those 45 and 50kt vectors correct?
A very significant rainfall event is occurring across Florida at this time. Read more on

Florida Weather

or my blog.
The rain here is like a tropical depression is mvoing through.
12/14/2006

A significant rain event is occurring over South and Central Florida at this time. Many areas across the South East Florida metro areas have received well over an inch of rain with 6+ inches in Palm Beach. Alot more rain is still downstream however, as indicated by radar, making the possibility for continued flooding very real. Anyone in a low lying area should stay tuned to flood advisories, and take the necessary precautions. The rain will begin to taper off sometime around Saturday afternoon as the bulk of moisture and possibly a surface low, ejects out to sea. After that, a slightly cooler and tranquil pattern will take hold.

South Florida

South Florida continues to be in a seasonal pattern with highs in the upper 70s to low 80s and lows in the 50s to 60s, with 70s in the Keys. This pattern will continue in regards to temperatures, with perhaps some cooler air past Saturday. The precipitation forecast is for rain, heavy at times to continue into Saturday, and then taper off. Models indicate significant rainfall with this system as mentioned above.


Blog party on ryang's blog tonight!
Lots of rain in South Florida today. Training cells. Take that over a hurricane every day and twice on Sunday:)
PP~ those vectors is what's happening in the upper atmosphere ~400-0mbs.
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 . . .
Anything that help reduce storms is a plus for us here in the Keys!!


Florida Keys Fishing
Now we're getting the rain here in Volusia County. We're going to have to invert our trough.
Goodbye Key West!
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.
..........perseverance.........its good.
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.


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........
Hi Pottery.Long time no see.LOL
So, this is what's left of Utor:

utor
Everyone!!! Come to ryang's blog party!!!
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........
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!
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.
clone, I don't seem to be able to zoom in from that link. What are we looking at?
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.
To see what I mean about the dry air:
Link
*sigh* Has to be a dull year in the Atlantic when we get excited over blobs and swirlies.
Could it be?

Nah
I know what that is; models have been showing a warm-core storm (not necessarially tropical though) developing in the NE Atlantic, like this one:



SSTs are rather marginal, similar to the NE Pacific "thingamabobbercane":

CB...I'd hate to see what kind of apparatus you have to keep the Christmas tree watered....
LOL!!!!

SSTs ARE NOT EVERYTHING!!!

You even said so a while ago! According to this, we can still get Cat 5 super hurricanes (like certain storms last year), but why don't we?

To those who await the release of my blog with much anticipation,(sarcasm).It will be delayed until tomorrow.Goodnight
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.
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. . . .
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.
This may be the new "Thingamabobbercane" that MichaelSTL was talking about:

thingamabobbercane

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