Mission Plan :

NOAA 49RF will fly a Saharan Air Layer Experiment through an African easterly wave (AEW) and two SAL airmasses 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 east through an area of 'older' Saharan Air Layer (SAL) air, an AEW axis at ~47°W, and finally through a 'newer' SAL outbreak. These areas and the moist tropical air south of these features will be sampled on the return east-west leg. The flight track and 24 dropwindsonde points are shown in Fig. 1.

Fig. 1: G-IV light track (green line) for SALEX mission 050927N. The GPS dropsonde points (24 total) are indicated by green circles.

Mission Summary :

a) Synoptic Situation A large SAL outbreak moved off the African continent on 20 September and by the next day, had pushed as far west as ~30°W (Fig. 2a, SAL 1). An AEW closely followed this SAL outbreak, emerging from the coast of North Africa on 22 September (Fig. 2b, AEW 1). As AEW 1 moved across the basin over the next several days, its northwest heading brought it into the SAL and it struggled to intensify (Fig. 2c & d). Early on 26 September, it was determined that the series of SAL outbreaks and the embedded AEW presented a good case for SALEX and the G-IV was deployed to Barbados.

Fig. 2: Meteosat-8 SAL imagery (21-27 September 2005) showing two large SAL outbreaks (SAL 1 & 2) separated by an AEW (AEW 1). Yellow to red shading indicates areas of dry (and possibly dusty) air in the lower to middle troposphere (~600-925 hPa).

On the morning of the mission, the AEW axis was located at ~47°W) and the Meteosat-8 SAL imagery indicated that the environment around the wave was fairly moist in the lower to middle levels (Fig. 2d). Although there was some convection associated with this AEW, there were no areas of concentrated deep convection. However, 850 hPa vorticity analyses from UW-CIMSS did show that there was a broad area of relative vorticity associated with this AEW from ~10-20°N and ~40-50°W (Fig. 3).

Fig. 3: 850 hPa relative vorticity analysis for 1500 UTC 27 September 2005. The area of vorticity from ~10-20°N and ~40-50°W is associated with the AEW (AEW 1) that was investigated during the SALEX mission. Analysis courtesy of UW-CIMSS.

b) Mission Specifics The flight pattern was designed to investigate several specific areas (Fig. 4):

• An area of SAL air (SAL 1) out ahead (west) of the AEW of interest (AEW 1);
• The moist environment associated with the AEW ( AEW 1);
• An area of SAL air (SAL 2) behind (east of) the AEW of interest (AEW 1);
• A region of convection along the southwestern edge of the second SAL outbreak (SAL 2). This convective area does not appear to have been linked to a propagating AEW and was more likely being enhanced by uplift along the leading edge of SAL 2. TD 19 formed in this area a few days later (30 September).

Fig. 4: Meteosat-8 SAL tracking imagery for 1200 UTC 27 September 2005. Red to yellow shading indicates dry, dusty air (associated with the SAL) in the lower to middle levels of the atmosphere (~600-925 hPa). Two distinct areas of dry SAL air (SAL 1 & 2), one area of dry polar air (Polar 1), and an African easterly wave (AEW 1) are indicated in the imagery. The G-IV flight track and dropsonde points are overlaid on the image. The AEW axis was located at ~47°W at this time. Imagery courtesy of UW-CIMSS.

Takeoff was at 1450 UTC from Barbados. The flight plan called 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. The initial flight pattern involved a short northeast leg from Barbados to the IP at ~14°N 58°W. The initial drops targeted the leading area of dry SAL air (SAL 1) ahead of AEW 1 (drops 1-5) and the moisture gradients along its boundaries (drops 5-7). The next several drops sampled the moist environment of the AEW (drops 6-11) and the transitional environment between it and the next SAL outbreak [(SAL 2), drops 11-13]. Drops 13-17 sampled SAL 2 and the northern edge of the convective area that stretched along ~10°N along the southwest leading edge of SAL 2. The remainder of the drops (drops 18-24) sampled the moist tropical environment south of SAL 1, AEW 1, and SAL 2. This last series of drops showed that although the lower to middle troposphere was quite moist along this transect, the middle to upper troposphere (~400 hPaÐflight level) was very dry. The environment of SAL 2 was noted to be extremely dry and dusty, with strong temperature inversions capping the top and bottom layers of the SAL. Dropwindsondes in SAL 2 (drops 13-17) indicated the following:

• RH values of ~0-30% in the typical SAL layer of ~500-850 hPa
• The presence of dry air that was also present in the ~200-500 hPa layer
• Strong temperature inversions (~1-3oC) in these soundings. These inversions seemed to correlate with the base and top of the SAL, with a third inversion often seen at ~300-450 hPa. This third, higher altitude inversion may have been associated with subsidence above the SAL layer.

Fig. 5: Photos taken from the NOAA G-IV jet from ~45,000 ft. Both photos were taken at ~1820 UTC at ~14°N 35°W just south of drop point #15 during the SALEX mission and are looking west on a north-south leg. Saharan mineral dust can be seen as a milky white

Problems :

There were no major problems related to this flight. The vintage 2000 GPS dropsondes that were used did, however, have a fairly high failure rate (~30% or 10 failed dropwindsondes out of 33 deployed).

Jason Dunion
Principal Investigator

Page last updated January 24, 2006
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