Dust Forecast For The 2009 Atlantic Hurricane Season

[Cookie]

talk about learning something new every day, I didn't realize dust had an effect on hurricanes and that their was even a dust forecast!

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1243

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.

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.

[Somerset Squall]

Indeed Cookie, Saharan dust can be the primary problem for tropical waves rolling off the African coastline, and when it is significantly above average it can really surpress the formation of Cape Verde hurricanes. 97L is a good example- it has managed to battle it's way through the Saharan dust but the main reason for the invest not having much convection until now is the dust supressing it, as other peramiters susch as shear and sea temps have been favourable thus far.

[Cookie]

thanks SS

its probably something I should have already know, but what be the fun then if their wasn't new things to learn

[Somerset Squall]

thanks SS

its probably something I should have already know, but what be the fun then if their wasn't new things to learn

There'a always new stuff to learn in Meteorology, and I agree, that's the fun of it.

[sammie]

Thanks for this, I always assumed from that I did last semester in uni that lots of dust would have been good ie lots of condensation nuclei! It's surprising to me, but completely logical how too much stunts development. Makes you wonder where that tipping point between conductive to development and detrimental is.