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 1445 UTC and will recover back at Barbados at 2045 UTC. The flight track will take the G-IV east through a recent Saharan Air Layer (SAL) outbreak, westward through an AEW axis (~47°W), and finally southwestward through the remnants of an older SAL outbreak. The flight track and 23 dropwindsonde points are shown in Fig. 1.

Fig. 1: Flight track (green line) for SALEX mission 20050928N1. The GPS dropsonde points (23 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). The previous day's mission (20050927n1) sampled two distinct SAL airmasses and the relatively moist environment of an AEW positioned between them. The 20050928n1 mission presented an opportunity to sample changes in these SAL airmasses and the environment of the AEW. It also provided an opportunity to sample a convective area (associated with the AEW) that was likely being enhanced along the leading edge of the more recent SAL 2 outbreak (moist tropical air being lifted by relatively denser, dry SAL air).

Fig. 2: Meteosat-8 SAL imagery (21-28 September 2005) showing two large SAL outbreaks (SAL 1 & 2) separated by an AEW (AEW 1). A region of dry polar air (Polar 1) is also indicted in the bottom two panels. 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 20050928n1 mission, the AEW axis was located at ~47°W and fairly deep convection (-60 to -70°C cloudtops) was oriented north-south along this longitude (Fig. 2d). A 'newer' SAL outbreak (SAL 2) was positioned just east of this convective area and was likely enhancing the local convection. By takeoff time, the cloudtops in this area had warmed significantly (~-50°C), as anticipated with the passing of the diurnal maximum. The flight track was therefore drawn to pass directly over the top of this convective area. b) Mission Specifics The flight pattern was designed to investigate several specific areas (Fig. 3):

• The moist environment associated with the AEW axis (AEW 1);
• An area of SAL air (SAL 2) behind (east of) the AEW of interest (AEW 1);
• An area of convection associated with AEW 1 that was located along the leading edge of the 'newer' SAL outbreak (SAL 2). This convection was likely being enhanced by relatively denser, dry SAL air uplifting the moist tropical air out ahead of it.
• An area of 'older' SAL air (SAL 1) out ahead (west) of the AEW of interest (AEW 1) that appeared to be moderating (moistening) fairly quickly in recent days, even since yesterday's 20050927n1 mission;

Fig. 3: Mosaic of total precipitable water (TPW) from the constellation of SSM/I satellites (1800 UTC 28 September 2005). Regions where TPW values of <45 mm (dotted lines) indicate dry air in the low to mid-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 AEW (AEW 1) are indicated in the imagery. The G-IV flight track and dropsonde points are overlaid on the imagery. Imagery courtesy of NRL-Monterey.

Takeoff was at 1445 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 long west-east leg from Barbados across the axis of an AEW (AEW 1). Dropwindsondes across this leg (drops 1-4) showed that the middle to upper troposphere (~200-500 hPa) was quite dry (~5-30% RH). However, the ~500 hPa to surface layer was quite moist. The next few dropsondes (drops 5-7) sampled the transitional environment across the leading edge of a more recent SAL outbreak (SAL 2) and showed that the lower to middle troposphere was becoming quite dry as the G-IV was crossing into the environment of SAL 2. The next several dropsondes (drops 7-11) indicated that:

• SAL 2 contained extremely low values of RH (~5-25%) in a deep layer from ~200-800 hPa.
• SAL 2 was associat ed with strong temperature inversions (~1-3°C) that capped its top (~500 hPa) and base (~800 hPa).
• SAL 2 was also associated with additional temperature inversions (~1-3°C) at multiple levels (e.g. 650 & 700 hPa). The source of these inversions is no t known and will have to be investigated further.

Fig. 4 GFS model analysis of 700 hPa RH (%) for 1800 UTC 28 September 2005. The G-IV flight track and 23 dropwindsonde points are overlaid on the analysis.

The final sequence of dropwindsondes (drops 19-23) sampled the environment out ahead (west) of the AEW. Satellite imagery from GOES and TPW imagery from the constellation of SSM/I satellites indicated that this area contained remnants of an 'older' SAL outbreak (SAL 1) that was modifying (moistening) fairly quickly in recent days. However, several dropsondes (e.g. drops 22 & 23) showed that this air still contained dry air in the lower to middle levels (~5-30% RH from 550-750 hPa).

It is interesting to note that the GFS model was analyzing considerably higher 700 hPa RH values than were observed by dropwindsondes launched during this mission (Fig. 4):

• The GFS indicated 700 hPa RH values of ~45-50% in the SAL 2 environment (drops 13-18), while observed values ranged from ~10-35%.
• The GFS indicated that the 700 hPa RH was >70% beneath the area of AEW convection that was sampled along the flight track. However, dropwindsondes in this region recorded RH values of ~20-50%.
• The GFS indicated that the area along the flight track from drops 19-23 contained 700 hPa RHs of ~45-50%. However, dropwindsondes showed that this area of the 'older' SAL outbreak (SAL 1) contained considerably drier air (~15-30% RH).

Problems :

There were no major problems related to this flight. The vintage 2000 GPS dropsondes that were used did, however, have a moderate failure rate (~18% or 5 failed dropwindsondes out of 28 deployed).

Jason Dunion
Principal Investigator