Using CALIPSO to Study the Influence of the
Saharan Air Layer (SAL) on the Intensity of Tropical Cyclones
Christopher Spells and M. Patrick McCormick, Hampton University
What is the Saharan Air Layer?
The Saharan Air Layer (SAL) is a layer of warm, dry, dusty air which normally overlays the cooler more humid surface air of the Atlantic
Ocean. Over the Sahara Desert from late spring to early fall, air moving across the desert becomes warm and dry forming a deep mixed
layer in the troposphere. This layer can extend from 1,500-1,600m in the atmosphere, be traced as far west as the Gulf of Mexico, and is
characterized by mineral dust, dry air, and strong winds. The Saharan Air Layer has been shown to help increase vertical wind shear
and allow for the entrainment of dry air into a tropical wave, which aids in weakening tropical disturbances.
CALIPSO
Launched April 28, 2006, CALIPSO (Cloud-Aerosol Lidar and
Infrared Pathfinder Spaceborne Observations) flies in formation with
AQUA, CloudSat, PARASOL, and Aura around the earth to form the
“A-Train” (Afternoon-Train). CALIPSO incorporates its own LIDAR
(light detection ranging), which uses short pulses of light to probe
through the atmosphere instead of radio waves. CALIPSO is
providing global, vertically- resolved measurements of aerosol and
cloud distribution, as well as their physical properties. CALIPSO is
expected to have a lifetime of three years, CALIPSO is a joint
project between Hampton University, NASA, and the French
Government.
Current Monitoring of the Saharan Air Layer
Geostationary satellites such as the GOES instruments and Meteosat-8
are helpful for tracking the position and movement of dry air masses such
as the Saharan Air Layer. Images created by GOES and Meteosat are
valuable tools in helping understand where dry, dusty air comes from.
Shown below is a product of Meteosat-8, with weaker SAL depicted by
the yellows and stronger SAL depicted by the red coloring.
Some CALIPSO Objectives
CALIPSO will play a vital role in obtaining better information on the
altitude and thickness of clouds and aerosol layers. This information
may be used to help improve weather prediction and climate models.
CALIPSO may also help provide new insight as to why some tropical
cyclones are suppressed more by the SAL than others. Below is an
example of how CALIPSO data looks. The first data was collected on
June 7, 2006.
GOES Saharan Air Layer Tracking
The image below displays GOES tracking the SAL and Hurricane Erin in September
2001.This is an example of the SAL’s influence on tropical cyclones. As depicted in the
image, when the hurricane was surrounded by the SAL, formation and
intensification was suppressed. Once out of the SAL, intensity increased. I would
like to track tropical cyclones moving across the Atlantic using CALIPSO data while
possibly integrating the data with new versions of McIDAS (Man computer Interactive
Data Access System) to display data.
Goal of Project
The goal of this project is to use the vertically resolved CALIPSO data to
research how the structure of tropical cyclones are effected when they interact
with the SAL. The hope is to track tropical disturbances across the Atlantic,
especially those forming off the coast of Africa, from formation to disintegration
while monitoring the location of the SAL and dynamic conditions of the area
such as vertical wind shear.
Summary
While the ideas for my research are still in the initial stages, the Saharan Air
Layer undoubtedly plays a role in the development and intensification of
tropical cyclones. Hopefully with a successful CALIPSO mission,
understanding the SAL’s dynamic effects on tropical cyclones can be
improved by providing vertically resolved images to be studied. Also,
CALIPSO observations could be used to improve forecast models.
References
Dunion, J.P. and C.S. Velden, 2004: The Impact of the Saharan Air Layer on
Atlantic Tropical Cyclone Activity. Bull. Amer. Meteor. Soc. Vol. 85 no. 3
353-365