Publication Date

2010-01-01

Availability

Open access

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2010-06-08

First Committee Member

Chidong Zhang - Committee Chair

Second Committee Member

Sharanya Majumdar - Committee Member

Third Committee Member

Jason Dunion - Outside Committee Member

Abstract

The vertical structures of tropospheric temperature and moisture over the oceans have not been well observed to date. The Atmospheric Infrared Sounder (AIRS) aboard NASA?s Aqua satellite offers the opportunity to provide observed soundings of these variables. This thesis focuses on the validation and application of AIRS soundings in the tropical troposphere over the Atlantic Ocean, with emphasis on the Saharan Air Layer (SAL). SAL outbreaks occur every few days, producing a warm air mass that is particularly dry at the middle levels. These westward-propagating plumes inhibit convection and are thereby thought to possess a detrimental effect on African easterly waves and tropical cyclones (TCs). First, AIRS soundings are compared with concurrent Global Positioning System (GPS) dropwindsonde data released from NOAA?s Gulfstream-IV jet aircraft, for three TC cases. In SAL environments, temperature soundings from both instruments are usually consistent. Additionally, AIRS is able to capture the very dry air in the middle levels, but it generally underestimates the moisture in the boundary layer and often misses the sharp vertical moisture gradient at the SAL base (~850 hPa). In the moist tropical boundary layer, AIRS also exhibits a dry bias. Cloud cover also prevents AIRS from accurately sampling the low-level moisture. Next, total precipitable water is derived from AIRS soundings and averaged over daily, monthly and seasonal timescales. The significant monthly and interannual variability of the moisture distribution is found to be consistent with expectations. A peak in the probability density function of mixing ratio corresponding to dry air is observed in the lower-mid troposphere in early summer, consistent with the increased frequency of SAL outbreaks during this period. Finally, the relationship between dry air derived from AIRS and TC intensity is explored. As the amount of dry air increases, particularly in the southeast and northeast quadrants of the TC, the TC becomes more likely to weaken. In the presence of high wind shear or low sea surface temperature, the likelihood of weakening increases further. While these results highlight some shortcomings of the AIRS data, their importance and uniqueness are emphasized via new applications of AIRS soundings over data sparse regions.

Keywords

SAL; Saharan Air Layer; Tropical Cyclone Environments

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