The Saharan Air Layer and Concept of the Optical Depth (credit: Meteorologist Brian Matilla and UW-CIMSS)
DISCUSSION: A bright blue sky is typically associated with clear sunshine and an overall calm weather day. But why is it at times that, in the tropics, this familiar bright blue backdrop becomes more milky white and opaque? The culprit is a feature known as the Saharan Air Layer (SAL): a warm, relatively dry, and dusty air mass that extends for several hundred miles across and resides 1-3 miles above the surface. An enormous amount of dust particles are transported westward with the tropical trade winds from the African Sahara to much of the Caribbean and Florida during the late spring and early summer months. SAL plays a role in hampering the development of convection and thunderstorms over much of the tropical belt and is notorious for hindering the organization of tropical storms and hurricanes. However, the true phenomenon with SAL lies in the theoretical understanding of how it affects the atmosphere’s aerosol optical depth (AOD).
To not be too verbose, the AOD is just a measurement of the ratio of light received versus light transmitted through a medium (e.g., SAL dust particles) and is crucial to understanding how light is scattered in the atmosphere. AOD ranges from 0 to 1, although the value hardly exceeds 0.3 without the presence of SAL or other foreign particulates. In SAL events, previous research has shown that the presence of SAL in the atmosphere tends to affect the amount of sunlight that is absorbed by the lower atmosphere, and the AOD during these SAL intrusions increases considerably.
Furthermore, the Angström exponent is another useful quantity that relates the AOD of particles between two wavelengths (observed and reference). In principle, the Angström exponent has an inverse relationship with the particles in the aerosol such that small particles have a high Angström exponent and vice versa. Similarly, a high Angström exponent is correlated to a higher optical depth. Hence, high values for AOD and the Angström exponent signify the presence of foreign particles in the atmosphere.
The picture at the top of this article was taken en route from San Juan, PR to Fort Lauderdale, FL earlier this month as SAL intrusion occurred over the western Atlantic Ocean waters surrounding the Bahamas. From above, hazy areas appear closer to the surface beneath the fair weather cumulus clouds. Closer to Florida and the Bahamas, the concentration of SAL is considerably less compared to the islands in the eastern Caribbean Sea. Still, the appearance of SAL at least once or twice every summer brings about a shift from the typical weather patterns.
To learn more about other high-impact weather and weather-related events occurring across the Atlantic Ocean and the Caribbean Sea, be sure to click here!
© 2018 Meteorologist Brian Matilla