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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1394. pottery
Good to be in the same room as you guys this evening.
I am learning some things here.
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This heat reminds me of when I was young.. decades and decades ago. We were staying at hampton beach NH.. There was a wicked heat wave. It just happened to be the week JAWS came out. So we all went to see it. It was late when we got out and EVERYONE was on the beach. It was about 11pm and 100 degress. I could not go in the water past my ankles. I sat there in two inches of water trying to cool off. Of course the night before I would have had no trouble going out over my head! To this day I can not swim in the ocean in the dark.. But luckily I'm poolside tonight.
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Look on the Eastern Side of Andres as that wraps around it has to create shear in the BOC...

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1391. pottery
1387, "Favourable Conditions" nice one. I had not seen that before.
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1390. Levi32
Quoting Weather456:
Levi, you stole my cookie from the cookie jar. lol


LOL and you just stole mine :) We'll call it even.
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1389. Levi32
Quoting TampaSpin:


We are talking about a stronger Andres.....a Stronger Andres will create shear in the BOC in my opinion.


With the upper low over Central Mexico and the deep-layer ridge over the southern US I don't see Andres developing much of a poleward outflow channel no matter how strong he is. I also think this fact is one of the main factors that will limit his intensification.
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Levi, you stole my cookie from the cookie jar. lol
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Quoting TampaSpin:
Weather456 you don't think the outflow of Andres will create any Shear in the BOC?


It can't without a poleward outflow channel and that seems unlikely due the deep layer ridge to Andres' northeast. Favorable conditions remain over the BOC through 48 hrs.
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Quoting Levi32:


Look at the loop. I see no shear. Andres doesn't even have a polar outflow channel to cause shear on its NE side.


We are talking about a stronger Andres.....a Stronger Andres will create shear in the BOC in my opinion.
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1385. Levi32
Quoting TampaSpin:
Weather456 you don't think the outflow of Andres will create any Shear in the BOC?


Look at the loop. I see no shear. Andres doesn't even have a polar outflow channel to cause shear on its NE side.
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Weather456 you don't think the outflow of Andres will create any Shear in the BOC?
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1383. centex
I think the mexican mountain ranges dramatically reduce impact on BOC systems.
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Quoting TampaSpin:


Evening all.......dido.....Too close together...the outflow of Andres...will create shear...


Been lurking,

I agree that time is against this system and that remains the only odd. It is likely to move ashore in the next 36 hrs as a weak low pressure area bringing showers to Oaxaca, Tabasco and Veracruz but I see no shear and Andres is just next to it.

Andres outflow is concentrated within 2 outflow channels, equatorward and zonal, with the latter weakening.

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1381. KEEPEROFTHEGATE (Mod)
looks like hot towers hurricane soon
Member Since: July 15, 2006 Posts: 175 Comments: 54858
Quoting leftovers:
doubt if there is much chance of anything in boc especially with andres strenghtening


Evening all.......dido.....Too close together...the outflow of Andres...will create shear...
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1379. centex
I should have mentioned the graphic was June Climatology.
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1378. pottery
post 1637 Levi.
That worked fine. Thanks.
I will delete my 2.
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That cluster of storms will NOT be affecting Flagler County afterall. They seem to be moving off the coast of St. Augustine.
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hurricane2009,
Severe T-storm watch tonight til 1am interesting night for central Florida wonder how sea breeze will effect that energy in North FL.
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1372. Levi32
High-Resolution visible loop of Andres and the Bay of Campeche

Andres' forward motion appears to have slightly increased towards the NW making it easier to see on satellite loops. I'm seeing hints of an eye-wall structure trying to develop within the recent burst of deep convection, but nothing well-defined yet.
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1371. centex
While looking it's worst, I predict TD2 in BOC in next 36 hours.


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1370. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 175 Comments: 54858
1369. Levi32
Quoting DDR:

Hey! levi
what part of alaska are you from?
Do You have permafrost where you live?


I live in Homer on the southern end of the Kenai Peninsula in southcentral Alaska. Actually no I don't think we have permafrost here this far south, but they have it further north.
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1368. KEEPEROFTHEGATE (Mod)


TROPICAL STORM ANDRES 18:00UTC 22June2009
UW-CIMSS Experimental Vertical Shear and TC Intensity Trend Estimates

Current Conditions (from TPC) :
Latitude : 16:17:15 N
Longitude : 102:10:23 W
Intensity (MSLP) : 1000.0 hPa

Max Pot Int (MPI,from Emanuel) : 890.0 hPa
MPI differential (MSLP-MPI) : 110.0 hPa

CIMSS Vertical Shear Magnitude : 2.3 m/s
Direction : 85.1 deg

Outlook for TC Intensification Based on Current
Env. Shear Values and MPI Differential
Forecast Interval : 6hr 12hr 18hr 24hr
F VF VF VF

Legend : VF-Very Favorable F-Favorable N-Neutral
U-Unfavorable VU-Very Unfavorable

-- Mean Intensity Trend (negative indicates TC deepening) --
6hr 12hr 18hr 24hr
VF <-3.0mb/ 6hr <-6.0mb/12hr <-9.0mb/18hr <-12.0mb/24hr
F -3.0 - -1.0 -6.0 - -2.0 -9.0 - -3.0 -12.0 - -4.0
N -1.0 - +1.0 -2.0 - +2.0 -3.0 - +3.0 -4.0 - +4.0
U +1.0 - +3.0 +2.0 - +6.0 +3.0 - +9.0 +4.0 -+12.0
VU >+3.0 >+6.0 >+9.0 >+12.0

-
Member Since: July 15, 2006 Posts: 175 Comments: 54858
1367. Levi32
Quoting pottery:
Help Needed--
Can someone post the link to the SAL Analysis page ( cimss etc).
I have 2 bookmarks for that, and both show a page of script when I try to open.
All other links are fine. Maybe my links for that are corrupted? A new one might work ?
Many thanks.


Here
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1366. pottery
Help Needed--
Can someone post the link to the SAL Analysis page ( cimss etc).
I have 2 bookmarks for that, and both show a page of script when I try to open.
All other links are fine. Maybe my links for that are corrupted? A new one might work ?
Many thanks.
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1365. pottery
From Trinidad--
Looking at the Central Atl. Infrared Loops, it looks like most of the heavy rains have dissipated east of us, and are now flaring up to the south west ( visible lightening over Venezuela right now)
Still hoping for some more showers overnight and tomorrow.
had 3/4" today. Need more....
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1364. KEEPEROFTHEGATE (Mod)

latest ir floater 2345
Member Since: July 15, 2006 Posts: 175 Comments: 54858
1363. HadesGodWyvern (Mod)
611
TCNA21 RJTD 230000
CCAA 23000 47644 NAMELESS 05111 11268 1429/ 220//=

0:00 AM UTC June 23
NAMELESS
11.1N 126.8E
Dvorak Intensity: T2.0


---
Japan Meteorological Agency
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Quoting aquak9:
here's the watch zone, Reedzone.



Yep I'm just north of Daytona Beach, I have a cluster of severe storms heading south from Jax.
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1361. HadesGodWyvern (Mod)
there is even a tropical disturbance summary in the Atlantic ;-)
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Quoting hurricane2009:


Yea its been that way for awhile, remember the area in the SW Caribbean at one point was labeled invest, but it was never officially labeled an invest.


Even if it were officially labeled an invest, I'm not expecting anything to come of it. This thing is going to run into land very quickly, though locally heavy rains will be possible across mountainous areas of northeastern Mexico over the next couple of days.

On another note, the NHC has just dropped the area of disturbed weather several hundred miles southwest of Baja California that had been lingering in the area for days. I did not expect that it would develop, since not only did they never think very highly of it, all models were pointing toward very hostile upper-level winds for at least the next five days over the disturbance. The system may be able to find a more favorable upper-level environment about a week from now, but by then, it may have already dissipated.
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Also, I just noticed that the NHC has a floater up over the BoC disturbance, and has it labeled an invest.
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Quoting WPBHurricane05:
...SUMMARY OF 500 PM PDT INFORMATION...
LOCATION...16.4N 102.8W
MAXIMUM SUSTAINED WINDS...60 MPH
PRESENT MOVEMENT...NORTHWEST OR 315 DEGREES AT 6 MPH
MINIMUM CENTRAL PRESSURE...995 MB

Not even a nudge stronger.


I've been looking for hints of an eye on visible satellite imagery, but no luck yet. Perhaps microwave data will pick up something soon enough.

I still think this thing will become a hurricane, though it would not surprise me if it did not, due to land interaction.
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Quoting fire831rescue:
It's cool. We all have fun trying to see what mother nature's going to throw at us each year. Sometimes, we're spot on. Other times, we're off by a long shot. But, all in all, it's fun.
This might be off topic but judging by your avatar are you by any chance a firefighter and if so, where ?
Member Since: October 9, 2008 Posts: 0 Comments: 8427
...SUMMARY OF 500 PM PDT INFORMATION...
LOCATION...16.4N 102.8W
MAXIMUM SUSTAINED WINDS...60 MPH
PRESENT MOVEMENT...NORTHWEST OR 315 DEGREES AT 6 MPH
MINIMUM CENTRAL PRESSURE...995 MB

Not even a nudge stronger.
Member Since: July 31, 2006 Posts: 56 Comments: 8112
Quoting pottery:
post 1349, you left an awful lot unsaid, LOL

Yea sorry. I fixed it. My baby daughter had her hand on the enter button when I was grabbing the mouse to post haha.
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1353. aquak9
here's the watch zone, Reedzone.

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All I am asking for is that this High breaks down enough so we can get some sea breeze TS. We need the rain to cool off the temps here in southern MS. Everything is capped right now. No chance for any afternoon TS down here.
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1351. pottery
post 1349, you left an awful lot unsaid, LOL

Ah, you fixed it .
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1350. Ossqss
I hope I did not provide the wrong impression on the Met item. They are paid pro's and educated and field trained to do what they do, just as any other professional. That's all I meant.
Member Since: June 12, 2005 Posts: 6 Comments: 8186
Quoting pottery:
Haboobs, quite a few people on this blog, consider this "their" blog. I do not include 831 in this category, or for that matter StormW.
The ones I refer to, are quick to jump on any comment that goes contrary to their thinking. It is often not worth the effort, to try to justify your thinking with them. It always ends in acrimony.
Just a thought......

I can not disagree with this at all. I have to coordinate with a lot of weather folks each day and we shot each other in the foot by jumping on each other when someone disagrees with you. Hopefully if they see something different they can just explain what they see and move on.

On a side note I really do enjoy reading what most write. A lot of great Meteorologist on this blog even if I dont always agree with what they say :)
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Quoting reedzone:


Thats the mid level circulation, the low level circulation is SSE from that, has a very small burst of convection.. Tonight will be interesting, and for me cause there is a Severe Thunderstorm watch for my county in NE Florida.


Its a very disorganized system. Dont expect much from this.
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1347. 10Speed
Quoting sporteguy03:

Energy is diving South from GA as well could get quite active in CFL later
As it stands at the moment later might just be tomorrow or some other day. Maybe as it cools off just a bit something will start popping up but I'm not counting on it. It's dropped to 93 F here in Winter Haven at 7:30 PM. My high today was 99.
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1346. HadesGodWyvern (Mod)
Philippines Atmospherical Geophysical Astronomical Services and Administration

Tropical Cyclone Bulletin #1
===========================
At 5:00 AM PhST, Tropical Depression Feria located at 10.9°N 127.3°E or 210 kms east southeast of Borongan, Samar has 10 minutes sustained winds of 55 km/h (30 knots).

Signal Warnings
===============

Signal Warning #1 (30-60 kph winds)

Luzon Region
-----------
1. None

Visayas Region
-------------
1.Samar Provinces
2.Leyte Provinces
3.Camotes Is.
4.Biliran Is.
5.Northern Cebu
6.Bohol

Mindanao Region
----------------
1.Dinagat Is.
2.Siargao
3.Surigao del Norte

Additional Information
========================
Residents living in low lying areas and mountanous areas under signal #1 are alerted against possible flashloods and landslides.

The public and the disaster coordinating councils concerned are advised to take appropriate actions and watch for the next bulletin to be issued at 11 A.M. today.
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1345. pottery
Haboobs, quite a few people on this blog, consider this "their" blog. I do not include 831 in this category, or for that matter StormW.
The ones I refer to, are quick to jump on any comment that goes contrary to their thinking. It is often not worth the effort, to try to justify your thinking with them. It always ends in acrimony.
Just a thought......
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It's cool. We all have fun trying to see what mother nature's going to throw at us each year. Sometimes, we're spot on. Other times, we're off by a long shot. But, all in all, it's fun.
Member Since: Posts: Comments:

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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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