Dust forecast for the 2009 Atlantic hurricane season
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
Reader Comments
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King TUTT
15N/80W - 25N/50W
T.C.F.A.
NCEP COUPLED GFDL HURRICANE MODEL FORECAST MADE FOR
TROPICAL DEPRESSION TWO 02E
INITIAL TIME 18Z JUN 21
DISCLAIMER ... THIS INFORMATION IS PROVIDED AS GUIDANCE. IT
REQUIRES INTERPRETATION BY HURRICANE SPECIALISTS AND SHOULD
NOT BE CONSIDERED AS A FINAL PRODUCT. PLEASE SEE THE TPC/NHC
OFFICIAL FORECAST.
FORECAST STORM POSITION
HOUR LATITUDE LONGITUDE HEADING/SPEED(KT)
0 14.5 101.5 270./ 8.0
6 15.0 101.5 356./ 4.9
12 15.6 101.9 326./ 6.8
18 15.9 102.1 318./ 3.5
24 16.1 102.2 338./ 2.7
30 16.8 102.3 350./ 6.4
36 17.2 102.7 324./ 5.8
42 17.8 102.9 340./ 6.3
48 18.3 103.8 295./ 9.7
54 19.9 105.6 312./23.1
60 21.3 106.0 342./14.8
66 21.9 106.3 335./ 6.6
72 22.3 107.0 300./ 7.4
78 22.9 107.7 314./ 9.0
84 23.9 108.7 314./13.1
90 24.3 109.1 312./ 6.0
96 24.8 109.9 303./ 8.6
102 25.2 110.5 305./ 7.4
108 25.6 110.7 332./ 3.8
114 25.9 110.8 333./ 2.8
120 25.9 112.4 273./14.4
126 27.1 111.8 27./13.1
...SPECIAL FEATURE...
NEWLY FORMED TROPICAL DEPRESSION TWO-E WAS CENTERED NEAR 14.7N
101.7W OR ABOUT 205 MILES S OF ZIHUATANEJO MEXICO AT 21/2100 UTC
MOVING W 8 KT. ESTIMATED MINIMUM CENTRAL PRESSURE IS 1002 MB.
MAXIMUM SUSTAINED WINDS 30 KT GUSTS 40 KT. SEE LATEST NHC PUBLIC
ADVISORY UNDER AWIPS/WMO HEADERS MIATCPEP/WTPZ32 KNHC AND THE
FULL FORECAST ADVISORY UNDER AWIPS/WMO HEADERS MIATCMEP WTPZ22
KNHC FOR MORE DETAILS. SATELLITE VISIBLE IMAGERY FROM THROUGHOUT
THE DAY HAVE SHOWN THAT THE OVERALL CLOUD STRUCTURE AND
ASSOCIATED CONVECTION HAVE SIGNIFICANTLY BECOME BETTER ORGANIZED
UNDER FAVORABLE UPPER LEVEL CONDITIONS. NUMEROUS STRONG
CONVECTION IS OBSERVED WITHIN 45 NM OF THE CENTER. SCATTERED
STRONG CONVECTION IS ELSEWHERE WITHIN 150 NM OF THE CENTER IN THE
SW QUADRANT...AND WITHIN 60 NM IN THE NE QUADRANT. A RATHER
TIGHT BANDING FEATURE WRAPPING INTO THE CENTER FROM THE W
CONSISTS OF SCATTERED STRONG CONVECTION...ABOUT 90 NM WIDE
...FROM 14.5N105W TO 16.5N103W. SCATTERED MODERATE CONVECTION
IS WITHIN 60 NM OF 13N106W. THE DEPRESSION IS FORECAST TO
STRENGTHEN INTO A TROPICAL STORM OVERNIGHT...AND CONTINUE TO
GRADUALLY THROUGH THE NEXT 48 HOURS PER LATEST NHC FORECAST
GUIDANCE AS IT MOVES IN A GENERAL WESTWARD DIRECTION.
18z HWRF for TD 2-E
This must be wrong... I remember this post very well :)
512. IKE 8:52 PM GMT on June 21, 2009
TD2E looks like a fish-storm.
Now where did that pie go?
I'm sorry..its a slow day and I had to do it :)
WHXX01 KMIA 220045
CHGE77
TROPICAL CYCLONE GUIDANCE MESSAGE
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
0045 UTC MON JUN 22 2009
DISCLAIMER...NUMERICAL MODELS ARE SUBJECT TO LARGE ERRORS.
PLEASE REFER TO NHC OFFICIAL FORECASTS FOR TROPICAL CYCLONE
AND SUBTROPICAL CYCLONE INFORMATION.
EAST PACIFIC OBJECTIVE AIDS FOR
TROPICAL CYCLONE TWO (EP022009) 20090622 0000 UTC
...00 HRS... ...12 HRS... ...24 HRS. .. ...36 HRS...
090622 0000 090622 1200 090623 0000 090623 1200
LAT LON LAT LON LAT LON LAT LON
BAMS 14.7N 101.7W 15.5N 102.0W 16.5N 102.3W 17.6N 103.2W
BAMD 14.7N 101.7W 15.1N 102.8W 15.8N 104.1W 16.5N 105.7W
BAMM 14.7N 101.7W 15.2N 102.7W 16.0N 103.8W 16.8N 105.2W
LBAR 14.7N 101.7W 15.2N 102.7W 16.3N 104.1W 17.6N 105.7W
SHIP 30KTS 38KTS 47KTS 55KTS
DSHP 30KTS 38KTS 47KTS 55KTS
...48 HRS... ...72 HRS... ...96 HRS. .. ..120 HRS...
090624 0000 090625 0000 090626 0000 090627 0000
LAT LON LAT LON LAT LON LAT LON
BAMS 18.8N 104.3W 20.2N 107.3W 21.1N 111.1W 22.0N 115.4W
BAMD 17.4N 107.5W 18.8N 111.7W 19.9N 116.2W 20.3N 120.9W
BAMM 17.8N 106.9W 19.2N 110.5W 20.4N 115.1W 20.9N 119.8W
LBAR 19.3N 107.4W 22.5N 110.8W 25.2N 113.6W 27.0N 114.6W
SHIP 55KTS 51KTS 41KTS 34KTS
DSHP 55KTS 51KTS 41KTS 34KTS
...INITIAL CONDITIONS...
LATCUR = 14.7N LONCUR = 101.7W DIRCUR = 280DEG SPDCUR = 5KT
LATM12 = 14.6N LONM12 = 100.5W DIRM12 = 270DEG SPDM12 = 7KT
LATM24 = 14.7N LONM24 = 99.3W
WNDCUR = 30KT RMAXWD = 45NM WNDM12 = 25KT
CENPRS = 999MB OUTPRS = 1004MB OUTRAD = 125NM SDEPTH = M
RD34NE = 0NM RD34SE = 0NM RD34SW = 0NM RD34NW = 0NM
has any one evere thing a 999mb 30kt TD befor???
Agreed but TD1-E and TD1-L all looked like they should have been named so I think I'll get named tomorrow. The Name would be Andres by the way.
Official pressure is 1002mb per the Latest NHC Advisory. In the Atlantic that would usually always give a storm TS-force winds, but storms like TD 2 that are close to mexico are in a region of sub-1008mb pressures associated with the Mexican thermal low. That makes TD 2's central pressure higher relative to its environment, meaning it has to achieve a lower pressure relative to its environment to atain a certain wind speed.
That said, it does look very close if not already a TS.
ture but not with the new INFO that i posted the nhc will lower the the mb to 999mb at the next update
NO GARBAGE
NO POOLS
NO PARKS
NO DAYCARE
NO COMM.CENTRES
NO CITY SERVICES
by friday in this heat and humity it may smell real nice
welcome to toronto
I think that's only a model initialization but I don't know if the NHC directly inputs current information into them. It could be the 0z pressure reading but it could also just be a model initialization that's not always accurate.
ok
Maybe so...looking pretty good.
This convergence map started showing the feature about the time it got started...might prove to be useful.
http://www.cpc.ncep.noaa.gov/products/hurricane/
Lee from 05 never had a central pressure below 1006mb. So why are storms that are stronger not tropical storms too?
It is probably due to its magnitude or its sustained winds.
I beileve the initial conditions box is what the NHC puts into the model thus at the next advisory the pressure will most likely be 999mb.
Do they fly invests into EPAC storms?
Action: Quote | Ignore User
WEATHER RECONNAISSANCE FLIGHTS
CARCAH, NATIONAL HURRICANE CENTER, MIAMI, FL.
1030 AM EDT SUN 21 JUNE 2009
SUBJECT: TROPICAL CYCLONE PLAN OF THE DAY (TCPOD)
VALID 22/1100Z TO 23/1100Z JUNE 2009
TCPOD NUMBER.....09-024
I. ATLANTIC REQUIREMENTS
1. NEGATIVE RECONNAISSANCE REQUIREMENTS.
2. OUTLOOK FOR SUCCEEDING DAY.....NEGATIVE.
II. PACIFIC REQUIREMENTS
1. NEGATIVE RECONNAISSANCE REQUIREMENTS.
2. SUCCEEDING DAY OUTLOOK.....NEGATIVE.
SEF
The pressure deficit relative to surrounding ambient pressure is what really determines the intensity of a tropical system...not that this reply necessarily answers your point/question.
EDIT: I see I was not the first to put this solution out there...
This is from what I told Taz when he asked:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In the Atlantic that would usually always give a storm TS-force winds, but storms like TD 2 that are close to mexico are in a region of sub-1008mb pressures associated with the Mexican thermal low. That makes TD 2's central pressure higher relative to its environment, meaning it has to achieve a lower pressure relative to its environment to atain a certain wind speed.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In the Atlantic the normal pressures are around 1012 to 1016ish. That makes Lee's pressure 6-10mb lower than its environment. TD-2 is in an environment of sub-1008mb pressures, so its central pressure is 4-6mb lower than its environment. So despite having a lower pressure than Lee, TD-2 has a higher pressure relative to its environment, making its winds slower.
That is a tenable explanation. Lower surrounding pressures would only weaken the pressure gradient force. The more substantial the pressure gradient, the higher the wind will blow. The more steep a mountain, the faster a river stream for example will flow to the surface.
Which is exactly what determines a storm's ability to generate sustained wind velocities.
I can't remember the storms but several years ago we had a tropical depression at 1014 or 1015mb off the southeast coast. Made landfall before getting stronger. It was in an area where the average pressure at the time was 1020-1025mb. I also remember a tropical storm out over the central atlantic having a pressure of 1010mb before. Depending on the surrounding atmosphere you can get some weird pressures for storms.
Anyway it's dinner time. Be back in a bit.
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