A mix of Saharan Dust and La Soufrière’s volcanic ash on a vehicle in Petit Valley, Trinidad following an afternoon shower.
The La Soufrière Volcano‘s eruption has sent copious volumes of volcanic ash and gases into the atmosphere. It began its explosive phase of eruption on April 9th, 2021. Ashfall has been recorded in Grenada and its dependencies and St. Lucia, with severe ashfall across St. Vincent and the Grenadines and Barbados.
Ashfall advisories or warnings are in effect for all of these countries but the question on everyone’s minds in T&T is how will T&T be affected? To understand this, you need to understand how the ash travels, how ash can make it to the surface, and how ash will impact those on the ground.
Low Chance of Ashfall In T&T
La Soufrière’s Ash: At The Mercy Of Winds
Since the start of the La Soufrière Volcano’s explosive eruption, volcanic ash has been dispersing in different directions in the atmosphere. Given in the Lesser Antilles, winds move from the east to west, why is the ash being blown to the east?
Depending on the ash column’s height, winds at different heights of the atmosphere can carry layers of ash in different directions. To determine where the ash may travel, scientists look at numerical weather prediction models to determine wind speeds and directions at different heights.
At low levels of the atmosphere in the Caribbean, generally meaning below 4,000 to 6,000 feet, winds move from east to west, known as our trade winds. At this level of the atmosphere, volcanic ash will generally take a westward path before landing on the ground or the surface of the ocean. It should also be noted that winds at this level are relatively weaker than other levels of the atmosphere in typical conditions, with winds usually between 0 to 30 knots, generally remaining below 15 knots. This relatively slower wind speed means that ash, at low levels of the atmosphere, travels smaller distances.
At the mid- and upper-levels of the atmosphere in the Lesser Antilles, winds are typically from the west (including northwesterly to southwesterly winds). This means that the ash at higher levels of the atmosphere will generally travel eastward, towards the southeast and northeast, depending on prevailing weather features. It is also important to note that winds at these levels are much stronger, ranging between 0-60 knots, at times higher. These faster wind speeds allow ash to be dispersed at much greater distances, high in the atmosphere.
Year-round, T&T and the Lesser Antilles are affected by Saharan Dust surges, which travel from the African coast between 5,000 and 15,000 feet. The natural question is if Saharan Dust can move at these layers, from the east to west, and still make it to the surface, why can’t La Soufrière’s volcanic ash?
How La Soufrière Ash Make It To The Surface
Across St. Vincent and the Grenadines, as well as Barbados, significant concentrations of volcanic ash are traveling eastward as high as 52,000 feet but throughout the entire atmosphere. This is why visibility across both islands has been less than 1 kilometer at times and sunlight can be completely blocked out.
Ash has been making it to the surface in two other ways. The first is ash at low levels of the atmosphere being affected by localized atmospheric features, causing northward shifts in the wind. Volcanic ash, traveling at low levels, does not travel far. For St. Lucia, Grenada, and its dependencies, winds at low to mid-levels of the atmosphere can also have brief directional changes, allowing for volcanic ash to fall across the islands.
The other means of ashfall is through rainfall. As mentioned, La Soufrière’s ash clouds have reached as high as 52,000 feet. Most showers that affect our area through the dry season tend to be capped below 20,000 to 30,000 feet at their strongest. Thunderstorm activity may extend higher into the atmosphere. What shower and thunderstorm activity do in a particle-rich atmosphere is “scrub” the air. This means particles in the air will be brought down in rainfall and accumulate in small amounts on surfaces. These particles usually include run-of-the-mill dust, pollen, Saharan Dust, and now, volcanic ash, which accumulate on vehicles, roofs, or other water catchment means.
#LaSoufriere‘s ashfall has made it to mainland Grenada, with ash being reported at Sauteurs, St. Patrick, Grenada.— TTWeatherCenter (@TTWeatherCenter) April 11, 2021
This follows ashfall overnight on Grenada’s dependencies of Carriacou and Petit Martinique.
Photos: Grenada Broadcasting Networkhttps://t.co/lc2Kv7P4PG pic.twitter.com/Z58rDJ7qRO
Frequently, during dense Saharan Dust events that coincide with showers or isolated thunderstorms, we warn of “muddy rainfall.” This is highly localized to where showers or thunderstorms occur.
This type of accumulation occurred across Grenada and its dependencies overnight Saturday 10th April into Sunday 11th April 2021 and as of Sunday afternoon, parts of Trinidad and Tobago.
Ashfall In Rainfall – What That Means For You
To reiterate, only localized ashfall has occurred in Trinidad and Tobago through rainfall. Widespread ashfall and impacts similar to Barbados and St. Vincent are NOT expected in Trinidad and Tobago.
On the ground, if you capture rainwater, you would notice the water being dark, at times black. On vehicles or other surfaces outdoors, you would notice spots of dust or sand as rainfall dries. Again, this is typical during moderate to significant Saharan Dust events.
What should you do? Follow air quality guidelines, as Saharan Dust through the next several days will be the main contributor to air quality pollution. For those that experience “dirty rainfall,” it is recommended to wash your vehicle off with water initially to remove particles and avoid the abrasive impacts on your property. Ashfall in Trinidad and Tobago will not significantly affect air quality based on the latest air quality models.
Ash clouds are still forecast to continue moving to the east and southeast, away from Trinidad and Tobago so any measurable amount of ashfall is unlikely.
Can T&T See Severe Ashfall?
The quick answer is highly unlikely. For dense ash to travel southward, winds will have to originate from the north and persist. This is a highly unusual circumstance only seen when powerful low-pressure systems such as tropical cyclones move across the region, influencing wind speeds and directions across the Windward Islands.
Through the next 14 days, models indicate mostly easterly winds below the 500 millibar level and mostly westerly to southwesterly winds above the 500 millibar level. Hence the islands most at risk for severe ashfall remain St. Vincent and the Grenadines as well as Barbados.
If winds at the mid to upper levels of the atmosphere shift toward the south-southwest, ashfall will be possible across Grenada’s dependencies, including Carriacou and Petit Martinique, as well as several small, largely uninhabited islands. This occurred overnight from Saturday 10th April 2021 into Sunday 11th April 2021.
Similarly, as ash drifts further and further southeast over time, it is possible areas across both Trinidad and Tobago experience some hazier skies with very, very light ashfall by this week (April 12th-19th). It still remains highly unlikely ash may travel as far south as the island to impact those on the ground materially. If an ash cloud does drift this far south, it will be highly dispersed in the upper atmosphere, remaining mainly an aviation hazard with little to no impacts on the ground. Saharan Dust remains the main impact on air quality.