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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SEVERE THUNDERSTORM WATCH NUMBER 495
A Severe Thunderstorm Watch is issued when weather conditions are favorable for the development of severe thunderstorms, not nessarilly thunderstom presesnt. A watch must not be confused with a severe thunderstorm warning.
All a severe thunderstorm watch does is indicate that conditions are right for storms to form. It does not indicate that a severe thunderstorm WILL form, nor does it indicate that your area will get rain.
Considering how much that we don't know about what causes a storm to be severe vs. just strong, I would think that a watch is warranted. Nothing weather-wise can be predicted with 100% certainty, and that includes severe thunderstorms.
Since you seem to be pretty perturbed by the lack of severe storms... may I ask what difference it makes whether the NWS called a severe thunderstorm watch or not, since a storm didn't impact you? Wouldn't you rather be advised to be wary, since conditions are ripe, or would you rather to suddenly have a warning issued when a storm does pop?
I wasnt arguing the point, i was asking, Really? and then providing my own info. I dont know for sure what was forcast and i sure dont know for sure what occurred anywhere but here.
uno memento!!!!,they'll start firing near I-75 to the north of the bay and move SSE between 25-40mph,as the line that forms moves slowly SW....give it a hour or 2 things w/be very active in our area,IMO(I'm down on siesta key)
I know, but I do not see conditions being all that favorable at the moment.
Have a good Tuesday!
Houston did get rain yesterday evening. A good steamy soaking downtown and points northwest. First good dousing in a month.
I'm in Clear Lake... not a drop
pics or it didn't happen.
The entire Houston area needs 3-4 days of a steady rain to break the drought. A single storm won't do it, since a lot of that runs off, and doesn't get absorbed.
That said.... I'm not expecting steady rain until October-ish.
Thought you moved to Katy?
I can't remember the last time we we're under a severe t-storm watch in these parts, it's got my attention. We get severe thunderstorms everyday?
Yeah, they don't issue watches for Florida too often so its interesting to see.
Severe Weather Bulletin Number FOUR
Tropical Cyclone Warning: Tropical Storm "FERIA" {NANGKA}
Issued at 10:45 p.m., Tuesday, 23 June 2009
Tropical Storm "FERIA" has crossed Northern Samar and is now in the vicinity of Masbate.
Location of Center:
(as of 10:00 p.m.) in the vicinity of Masbate
Coordinates: 12.5°N, 123.6°E
Strength: Maximum sustained winds of 75 kph near the center and
Gustiness of up to 90 kph
Movement: West Northwest at 22 kph
Forecast Positions/Outlook:
Wednesday evening:
90 kms West of Iba, Zambales
Thursday evening:
290 kms Northwest of Laoag City
Friday evening:
420 kms North Northwest of Basco, Batanes
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
800 AM EDT TUE JUN 23 2009
FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...
1. A TROUGH OF LOW PRESSURE IN THE SOUTHWESTERN GULF OF MEXICO
CONTINUES TO PRODUCE DISORGANIZED SHOWERS AND THUNDERSTORMS. THIS
ACTIVITY IS EXPECTED TO MOVE SLOWLY NORTHWESTWARD...BRINGING
LOCALLY HEAVY RAINFALL TO PORTIONS OF EAST-CENTRAL AND NORTHEASTERN
MEXICO OVER THE NEXT COUPLE OF DAYS. SIGNIFICANT DEVELOPMENT IS NOT
EXPECTED...WITH A LOW CHANCE...LESS THAN 30 PERCENT...OF THIS
SYSTEM BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS.
ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.
$$
FORECASTER FRANKLIN
Just wondering where you found that particularimage. I'm out here on Bee Ridge Rd. 2 miles from I-75, due east of Siesta Key.
Link to that image Link
Look whats coming off the coast of Africa
Thanks. Nice loop.
Look at this. Link
Isnt the W coast of Mexico closer to you then florida anyway? Though can't say I blame you for the monetary (and safety) risk... not exactly stable down there.
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