NASA’s Hurricane and Severe Storm Sentinel (HS3) field program is currently flying an unmanned Global Hawk aircraft to sample an enormous Saharan dust storm emerging from the coast of Africa and a weak tropical disturbance to its south. These periodic dust storms, known as Saharan Air Layer (SAL) outbreaks contain extremely dry air and strong winds and can be the size of the continental U.S. as they cross the North Atlantic and reach areas as far west as the Caribbean, Florida, and even the Gulf of Mexico. Scientists from the Hurricane Research Division at AOML have hypothesized that the SAL’s hot, dusty, dry air and strong winds can inhibit tropical disturbances, the seedlings for Atlantic hurricanes. You can read more about it here. During today’s mission these same scientists are helping the HS3 program process data from GPS dropsondes being launched from the Global Hawk as it flies 60,000 ft over the ocean surface. These parachuted instruments collect temperature, moisture, pressure, and wind data every 1/2 second as they fall through the atmosphere and relay that information back to the Global Hawk and to scientists and modeling centers around the world. Tonight’s mission has found that the SAL was moving across the Atlantic in a 2 mile thick layer elevated about 1 mile above the ocean surface. The layer contained extremely dry air and powerful winds as strong as 45-50 mile per hour, creating an environment hostile to hurricane formation.
Infrared satellite imagery showing the SAL (yellow to red shading) and the NASA Global Hawk flight track (red line) as it sampled the area late on Saturday August 24th 2013. The coast of western African can be seen on the right side of the image.