After touring the southern United States for more than a week, the intrepid plume of African dust that has brought haze and poor air quality along the Gulf Coast is finally withdrawing over the Atlantic. The extreme episode has contributed to record heat in South Florida, while draping a dull gray veil over a number of islands from the Caribbean to the Lesser Antilles. Experts heralded the outbreak as one the most prolific measured in the modern era. But more is on the way.
On the heels of the first wave of dust is a second, albeit tamer, cloud of gritty sand again drifting west across the ocean. Its sights are set on the western Gulf of Mexico, the aerosols affecting areas that already experienced dust last week. Air quality and visibility could again decline as the dust cloud closes in.
The dust now
Satellite imagery on Monday afternoon revealed a swath of dust extending from the Bay of Campeche and the Yucatán Peninsula eastward through the Caribbean, with the greatest concentrations of dust particulates west of the Leeward Islands.
Computer models suggested an ebb of that cloud would overspread parts of eastern Mexico, coastal Texas and Louisiana between Tuesday and Wednesday before disintegrating late in the week.
The dust should not be as widespread or as thick as it was during much of last week or over the weekend, though a noticeable tinge to the sky should be evident in the areas affected.
An event to be remembered
Last week’s plume was arguably one of the most, if not the most, extreme events on record from a dust-transport standpoint. Joe Prospero, an atmospheric scientist and professor emeritus at the University of Miami, said the episode will be studied for years to come.
“This event … was the largest event that we have ever seen over 50 years that we have been in Barbados,” said Prospero, who has spearheaded efforts in the Leeward Islands to study Saharan dust. “It’s pretty clear this was a phenomenal event. We know that the aerosol concentrations were the highest we’ve ever measured by a substantial amount.”
He also referenced “optical depth,” a vertically integrated metric measuring how densely the atmosphere is clouded over with a type of aerosol.
“That was higher than everything we’ve seen in 25 years,” Prospero noted. “It was really dramatic.”
A lidar device based at the University of Miami suggested that most of the dust was present between one and 2.5 miles above the ground.
The data he reviewed also indicated a surprising homogeneity, or uniformness, in the size of the sand particles caught up in the air — indicating that most of the dust came from a rather localized area in Africa.
“I’ve never seen anything like that,” Prospero said. “It raises interesting questions about what the sources [of the dust] are, and about the mechanisms for lifting it up.”
What satellite reveals about the approaching dust
Prospero also explained that the dust cloud was strong enough to eradicate cloud cover over an enormous strip of the Atlantic, its parent Saharan Air Layer warm enough to preclude any air pockets near the surface from rising.
“Normally, when you see a Saharan dust outbreak, you have little fractals of cumulus,” Prospero said. “This was unusually cloud-free.”
The same technique of evaluating satellite imagery lends credence to this next event being less significant.
“The second pulse that [is coming] through has a bit more cloud in it,” Prospero noted. “It’s not going to last as long as the other one. But it’s modifying the whole atmosphere up to 12[,000] or 14,000 feet. Sometimes, it’s up to 18[,000] or 20,000 feet off the coast of Africa.”
Last week’s dust plume brought elevated unhealthy air-quality indexes to major metropolitan cities, such as Houston, Galveston and Brownsville in Texas, New Orleans and even Tallahassee. Previously, it had reduced visibility down to as little as three miles in the U.S. Virgin Islands.
Air-quality indexes from dusty particulate matter won’t be as bad with the trailing cloud of dust, but there’s a chance that additional air-quality concerns could arise, particularly in South Texas, as the Saharan Air Layer spreads overhead. The warm air at the mid-levels “caps” the atmosphere, trapping pollutants near the surface.
That means a 24- to 36-hour period of pollution buildup may be possible in some areas where weak winds and little venting could help a few areas climb into the “moderate” category for air quality. A few pockets of air unhealthy for sensitive groups are possible.
Moreover, the hot temperatures — exacerbated by the paltry cloud cover eroded by the Saharan Air Layer — will help brew unhealthy levels of near-surface ozone, a pollutant.
“NASA GEOS-5 modeling indicates that another round of dust is also expected to reach the region by Wednesday morning, with haziest conditions from the dust possible Wednesday and Thursday,” wrote the National Weather Service in Corpus Christi, Tex.
Temperatures near 100 degrees were possible along the South Texas Coastal Plain on Wednesday, the National Weather Service in Brownsville, Tex., advertising that the dust “will contribute to lower air quality.”
The Texas Commission on Environmental Quality is specifically calling for poor air quality over Brownsville and Corpus Christi on Wednesday.
“Overall, depending on the intensity and coverage of the arriving African dust cloud, the daily [particulate matter concentration] is forecast to possibly reach the lower end of the ‘Unhealthy for Sensitive Groups’ range in parts of the Brownsville-McAllen and Corpus Christi areas,” the commission wrote.
Hot temperatures and potentially hazardous heat index values are possible, too, the Saharan Air Layer minimizing the development of afternoon clouds and rain showers that typically cool back coastal areas in the afternoon.
The dust finally looks to diminish by late in the week, but there is a chance that additional waves of dust could propagate across the Atlantic into early July.
washingtonpost.com, 29 June 2020