Extended Abstract (240K)13A.3
The Saharan Air Layer—Insights from the 2002 and 2003 Atlantic hurricane seasons
Jason P. Dunion, NOAA/AOML/HRD and Univ. of Miami, Miami, FL; and C. S. Velden, J. D. Hawkins, and J. R. Parrish
Recently developed GOES split window IR satellite imagery has allowed for tracking of the Saharan Air Layer (SAL) and its interactions with Atlantic tropical cyclones (TCs) and African easterly waves since the 2001 hurricane season. Since that time, research has been focused on various aspects of the SAL’s role in influencing TCs and lower tropospheric moisture in the tropical North Atlantic. These efforts include developing a SAL detection algorithm for the new GOES-12 satellite, using the constellation of SSM/I satellites to create basin wide total precipitable water mosaics to help supplement SAL detection, re-examining the Jordan mean tropical sounding and how the SAL may have influenced the results of this 1958 study, and utilizing GPS sondes launched from NOAA aircraft to investigate the SAL’s low humidity and how effectively this dry air is being represented in the global models.
In the spring of 2002, GOES-12 replaced GOES-8 as the operational GOES East satellite. Among other changes in its configuration, GOES-12 no longer has the 12.0 mm longwave IR channel. The original SAL tracking imagery developed with GOES-8 used the 10.7 and 12.0 mm channels to detect the dry, dusty air contained in the SAL. Therefore, a new algorithm was developed for GOES-12 that uses the 3.9 and 10.7 mm channels to track the SAL. This algorithm relies on the same principals for tracking the SAL, but is limited to nighttime use because of the affects of solar reflectance in the 3.9 mm channel. The details of this new algorithm will be discussed. To supplement SAL detection, NRL has developed a mosaic of total precipitable water imagery derived from the constellation of SSM/I satellites to track the SAL’s low humidity air. Details of this satellite imagery, as well as comparisons with both the GOES SAL imagery and GPS sondes will be discussed.
The original work by Jordan in 1958 included the calculation of a climatological sounding for the West Indies during the “hurricane season” (July-October). In light, of recent advances in our understanding of the SAL and its ability to advect extremely low humidity as far west as the western Caribbean Sea (~7,000 km from its source over northwest Africa), the Jordan sounding may need to be revisited. Jordan’s study was replicated for the 2002 “hurricane season” using data from 4 West Indies and Caribbean raob stations. GOES SAL imagery was used to target raobs taken in SAL versus non-SAL environments. The results of this study and the likely existence of a bi-modal climatological moisture sounding for the tropical Atlantic will be discussed.
2003 Hurricanes Fabian and Isabel marked the first ever attempts to target SAL regions in the TC environment with GPS dropsondes launched from the NOAA G-IV reconnaissance jet. The results of comparisons between these first ever targeted SAL drops and the GOES and SSM/I satellite imagery, as well as disparities between the GPS soundings and initial fields from the GFS and NOGAPS models will be discussed.
Session 13A, Tropical Cyclone Intensity Change II: Environmental Factors
Thursday, 6 May 2004, 10:15 AM-11:45 AM, Le Jardin Room
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