Transcript Influence of the Saharan Air Layer on Tropical

The Influence of the Saharan Air Layer on
Tropical Cyclones During the
2011 Hurricane Season
Katelyn Barber (SUNY Oswego) and Advisor Dr. Humberto Barbosa
Objective
Introduction
The Saharan Air Layer is an elevated dry, warm air mass containing West African
dust particles that migrates across the North Atlantic Basin during early spring to
late fall. The SAL is characterized by humidity values less than 50% of tropical air
and winds stronger than 15 knots. The Saharan Air Layer impacts the development
of tropical cyclogenesis in productive and destructive ways making it unclear
whether or not the Saharan Air Layer is responsible for a general increase in
tropical cyclones over the last decade.
In this study we will provide a preliminary investigation of the 2011 Hurricane Season and
the influence the Saharan Air Layer had on three tropical storms that developed near the
Saharan Air Layer. Using Meteosat satellite infrared images we were able to track the
Saharan Air Layer from June to October, by taking the difference between 12.0
micrometers and 10.8 micrometers infrared images using McIDAS-V.
Theory and Methods
Exactly how the Saharan Air Layer affects the conditions for tropical cyclogenesis is still debated by meteorologists.
According to Karyampudi and Carlson (1988), the SAL contributes to easterly wave growth, which in turn contributes
to tropical cyclogenesis. There are documented storms intensifying within the vicinity of the SAL, one of these is
Hurricane Helena. On 16 September 2006, Helena was surrounded by air with relative humidity values around 20%
to the north, yet later that day it intensified and by 18 September 2006 Helena was a category 3 hurricane (Braun
and Shie 2008). On the other hand, scientists have also concluded that the SAL inhibits tropical cyclogenesis for
three main reasons. First, the temperature inversion created by the radiative warming of dust over the ocean water
stunts convective development. Secondly, the vertical wind shear within the SAL removes heat and moisture,
decreasing instability. Lastly, the dry SAL air intruding into tropical cyclones create cold downdrafts, and lower
convective potential energy available for the cyclone (Emanuel 1989).
The 2011 hurricane season observed twenty tropical depressions, nineteen tropical storms, and seven hurricanes
(four being major hurricanes). Statistically, 2011 was an above average year for tropical storms but an average year
for hurricanes. The Saharan Air Layer may have been a reason as to why so many tropical storms formed, but never
developed into hurricanes.
Figure 1. Schematic of the Saharan Air Layer, Courtesy of METED
Dry Air Tracking Product images were created from the difference of Meteosat Second Generation Satellite infrared
12.0 micrometers images and 10.8 micrometers images from 1 August to 6 August 2011 of Tropical Storm Emily, 29
August to 6 September 2011 of Hurricane Katia, and 6 September to 10 September 2011 of Hurricane Maria with
McIDAS-V. In these images the color red is dry air, and green air is moist air. The Cooperative Institute for
Meteorological Satellite Studies archived SAL/Dry Air Product, the Mid-Low Level Wind Product, and Mid Level Water
Vapor Enhanced for Dry Air Tracking Product at 7.3 micrometers for the North Atlantic Basin were used as qualitative
data and a comparison to the created images. Investigating the data, we were looking for obvious influences the SAL
had on tropical cyclone development.
.
Figure 2: Hurricane Season 2011 tracks , Courtesy of the National
Hurricane Center
Results
The Saharan Air Layer delayed Emily’s organization and later
in her life span eroded the northwest portion of the cloud
formation and exposed the core to stable air. It was not until
Emily was completely removed from the dry environment did
she gain strength and return to tropical storm status.
The overall effect the Saharan Air Layer had on Katia was
minimal. Due to the weak phase of the SAL, Katia had
optimal conditions to form; warm waters, little wind shear,
and instability. The concentrated Saharan Air Layer in the
south may have played a key role in the strength of the
tropical wave that Katia developed from.
Conclusion
The presence of dry air near the West African coast
increases the frequency of easterly waves because of the
vertical wind shear and vorticity. Easterly waves increase
the occurrence of tropical disturbances. However, the
Saharan Air Layer inhibits tropical cyclogenesis once a
low has developed. From a preliminary investigation this
is a possibility because all major hurricanes during the
2011 season developed once away from the Saharan Air
Layer.
References
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Similarly to Hurricane Katia, Hurricane Maria formed to the north of
an area of concentrated dry air and the Saharan Air Layer was still in
a weakened state. However, during the development of Maria there
was a significant amount of vertical shear that inhibited the growth of
the storm. But once removed from the dry air strengthened.
Acknowledgments