SALEX mission summary

Mission Plan :

NOAA 49RF will fly a Saharan Air Layer Experiment around Tropical Depression #9 as part of IFEX. The G-IV will leave Barbados at 1450 UTC and will recover back at Barbados at 2200 UTC. The flight track will take the G-IV on a clockwise circumnavigation of the TC and is shown in Fig. 1, along with the 26 dropwindsonde points.

Mission Summary :

a) Synoptic Situation
TD #9 emerged from the coast of North Africa as a disorganized AEW on 01 August (Fig. 2). Figure 2 also shows that a very large Saharan Air Layer (SAL) outbreak was located to the north and west of this system at this time. As the system moved across the basin over the next several days, its northwest heading brought it into the SAL and it struggled to intensify. On 05 August, several of the forecast models were suggesting that the system might recurve out to sea before it got into range for SALEX. However, by early morning hours of 06 August, it was clear it would be in range for SALEX and the G-IV was deployed to Barbados.

On the morning of the mission, TD #9 (located at ~20.5°N 45.5°W) was upgraded to Tropical Storm Irene (35 kt)- the earliest forming 'I' storm on record. Most of Irene's deeper convection was located well east of its low-level center and it was on the southern fringes of 20-30 kt of northwesterly shear (Fig. 3, left). This strong shear was being enhanced by an upper-level cold low that was located to the north of Irene (~29.0N 41.0W) and was easily discernable using mid to upper-level water vapor winds from UW-CIMSS (Fig. 3, right).

b) Mission Specifics
The flight plan was designed to investigate several areas of SAL air around Irene, including 'older' Saharan air to its west (Fig. 4, SAL 2) and a 'new' SAL outbreak that was impinging on the inner core region from the east (Fig. 4, SAL 3). The more recent SAL outbreak (SAL 3) had substantial dust associated with it, and was evident in GOES visible imagery (Fig. 5, left). The flight plan called for a clockwise circumnavigation of the storm at an optimal flight level of 41,000-45,000 ft and required that all dropwindsondes be transmitted in real-time, so that the humidity data from the sondes could be included in parallel runs of the NOAA GFS model.

Takeoff was at 1450 UTC from Barbados and it quickly became apparent that the region west and southwest of Irene contained SAL air (Fig. 4, SAL 2). The low to mid-level moisture for drops 1-2 and 6-8 were ~10-30% RH from ~600-850 hPa and not until drop 9 did these levels of the atmosphere moisten-up significantly. The areas of low humidity corresponded extremely well with low values (<45 mm, green to blue shading) of total precipitable water (TPW) from the constellation of SSM/I satellites (Fig. 4 and Fig. 5, right).

A series of drops was then made to capture the moisture gradient along the leading edge of the SAL outbreak that was impinging on Irene from the east (Fig. 4, SAL 3; Fig. 5, right). Drops 10-16 exhibited increasing dryness from northwest to southeast. The easternmost drops in the series (drops 16-19) depicted extremely dry air in the ~450-900 hPa layer and strong winds (25-30 kt) at the 700 hPa level (likely associated with the SAL's mid-level easterly jet).

The final series of drops (20-26) crossed back into the region of the 'old' SAL outbreak (Fig. 4, SAL 2) and indicated that the region was quite dry in the low to middle levels and TPW imagery indicated that this dry SAL air extended as far west as ~68°W.

There were no major problems related to this flight. Even though vintage 2000 GPS dropsondes were used, only 4 sondes failed out of the 30 that were dropped. This flight would definitely have been more manageable with two scientists onboard. As it was, there were probably too many tasks to be effectively handled by one scientist. These included assessing satellite imagery using the G-IV's high speed internet to alter several of the GPS dropsonde points on the fly and working up/transmitting all of the dropsondes in real-time.

Jason Dunion
Principal Investigator