HS3 Hurricane Intensity Change and the Storm Environment

Hurricane intensity change and storm environment
A major scientific question to be addressed by HS3 is: To what extent do environmental factors such as the Saharan Air Layer (SAL) determine whether a tropical storm will intensify into a major hurricane? The impact of the SAL on the development and intensification of tropical cyclones has garnered significant attention in recent years. Early studies (e.g., Karyampudi and Carlson 1988; Karyampudi and Pierce 2002) suggested a potential positive influence of the SAL on tropical cyclogenesis via influences on African Easterly Wave (AEW) growth and support of convection at its leading and southern borders (where cyclonic shear is largest). The SAL air, characterized by very warm low-level air, is intimately linked to the location and strength of the mean African Easterly Jet (AEJ) and growth of AEWs (e.g. Thorncroft and Hoskins 1994a&b, Thorncroft and Blackburn 1999). In contrast to Karyampudi and Carlson (1988), Dunion and Velden (2004) focused on mechanisms that generally inhibit tropical cyclone genesis and intensification. They suggested that the SAL negatively impacts tropical cyclones in the following ways: 1) The enhanced low-level temperature inversion suppresses convective development, 2) vertical wind shear associated with the AEJ inhibits tropical cyclone intensification based upon studies that have shown that shear tends to weaken storms, and 3) intrusions of dry SAL air into tropical cyclones foster enhanced cold downdrafts and lower the convective available potential energy within tropical cyclones. The disagreement between the results of Karyampudi and collaborators and Dunion and Velden (2004) and other studies raises questions regarding the role of the SAL in the genesis and intensity change of individual storms. A recent study by Braun (2010), focused on the role of the SAL, examined composites from both strongly intensifying storms (in the 2-4 day period following genesis) and weakening storms. The results suggested that the SAL is an integral part of the Atlantic hurricane environment, but is not a determining factor in intensity change following tropical storm formation.

HS3 will provide high-resolution temperature and humidity profiles coincident with vertical profiles of Saharan dust. When combined with wind profiles from the TWiLiTE wind lidar and dropsondes, the GH with the environmental payload will provide the most comprehensive data set on SAL structure and evolution in tandem with measurements of upper-tropospheric flow that will map out the winds associated with large-scale troughs and interactions with storm outflow.


An example of environmental interaction from Reimer and Montgomery (2010). This schematic diagram shows the potential pathway of unfavorable environmental air into a storm. The dashed curves denote the outermost streamlines for which environmental air reaches the rainband region. The source region of air that interacts with the tropical cyclone is indicated to the northwest. The area shaded in blue indicates a hypothetical region of very dry environmental air that would be unable to interact with the storm.

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–––––––, and H.F. Pierce, 2002: Synoptic-scale influence of the Saharan air layer on tropical cyclogen- esis over the Eastern Atlantic. Mon. Wea. Rev., 130, 3100-3128.
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Thorncroft C. D., and M. Blackburn, 1999: Maintenance of the African easterly jet. Quart. J. Roy. Meteor. Soc., 125, 763–786.