Thursday, August 1, 2013

Saharan Dust

Saharan Dust heading to Texas? Believe it or not, it's possible! Matter of fact, it has already happened a couple of times this summer. Two days ago NASA's MODIS satellite photographed a massive dust storm pushing east off of the northwest coast of Africa with origins in the western Sahara Desert. Strong winds in that region this time of year frequently pick up large amounts of fine dust particles and strong easterly trade winds frequently carry that large cloud of dust and dry air across the Atlantic and into North America. This layer of dry, dusty air suppresses tropical storms and hurricanes from developing. Therefore, expect a quiet couple of weeks ahead in terms of tropical activity in the Atlantic. This massive cloud of dust and dry desert air is known as the SAL (Saharan Air Layer).

Massive Dust Storm moving off the coast of Africa (Image taken: July 30, 2013...MODIS)

Incredible animation of the dusty dry air moving across the Atlantic. The yellows, reds, and pinks indicate that dry dusty air. It is amazing to see just how much dust is crossing the Atlantic and at how fast it is traveling.

Information below was taken from Dr. Jeff Masters' WunderBlog
How dust affects hurricanes
Saharan dust can affect hurricane activity in several ways:

1) 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) from the coast of Africa to the Caribbean, providing hurricanes with less energy to form and grow. Ocean temperatures in the MDR are currently 0.7°F above average, and this anomaly should cool this week as the dust blocks sunlight.

2) The Saharan Air Layer (SAL) is a 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.

3) Dust may also act to produce more clouds, but this effect needs much more study. If the dust particles are of the right size to serve as "condensation nuclei"--centers around which raindrops can form and grow--the dust can act to make more clouds. Thus, dust could potentially aid in the formation and intensification of hurricanes. However, if the dust acts to make more low-level clouds over the tropical Atlantic, this will reduce the amount of sunlight reaching the ocean, cooling the sea surface temperatures and discouraging hurricane formation (Kaufman et al., 2005.)

Incredible forecast model showing where the dust layer should be over the coming days provided by NOAA's Visualization Laboratory using the NOAA NGAC Aerosol Model. Some of that dust is likely to make it to Texas sometime next week! When that happens our skies turn a milky, hazy color instead of blue and sunsets take on a characteristic red glow. Of course, the dust in the air will put air quality levels in the moderate category. Therefore, those that are sensitive to dust and allergens will need to limit time spent outdoors or just avoid outdoor activity altogether.

As for our weather here in central Texas over the next 7-10 days, expect more of the same. With high pressure directly overhead air is forced to sink. When air sinks it compresses and heats up. That sinking air combined with our dry ground is the perfect combination to give us day after day after day of triple digit heat. You can think of high pressure as a big shield over Texas keeping rain out.

New drought report just put out today shows that 97% of the state of Texas is in drought. Unfortunately new forecasts out today also show our drought persisting into the fall.

Don't get excited yet, because it is August and it is a long way's out, however, long range forecast models are showing a cool front pushing into Texas late next week or next weekend that may help to push highs back into the lower to middle 90s for a couple of days. However, because of that front we may see extremely HOT temps in the 104-108°F range by the time we get into the middle to end of next week ahead of the front. 


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