Mission Plan :

NOAA 49RF will fly a combination TDR mission, Rapid Intensity (RI) Experiment and Saharan Air Layer Experiment (SALEX) around Hurricane Bill. The G-IV will leave Barbados at 0749 UTC and will recover back at Barbados at 1407 UTC. The flight track will take the G-IV on a counterclockwise circumnavigation of the TC and is shown in Fig. 1, along with the 24 GPS dropsonde points.

Fig. 1: Flight track (black line) for TDR/RI/SALEX mission 20090820n1 overlaid on GOES visible imagery from 1045 UTC. The GPS dropsonde points (24 total) are indicated by black circles and hash marks. Imagery courtesy of NRL-Monterey

Mission Summary :

a) Synoptic Situation
On 20 August 1200 UTC, Hurricane Bill was a moderate category 3 hurricane (110 kt) located at ~22.1°N 61.0°W (Fig. 2) and positioned between two large Saharan Air Layer (SAL) air masses. Figure 2 (bottom) also shows that the SAL air mass west of Hurricane Bill contained a significant amount of dust from the near-surface up to ~4.5-5 km. The storm was tracking 305 degrees at 16 kt through a weakness in a deep layer ridge north of the storm [Fig. 3 (left)]. A trough was continuing to amplify over the central U.S. at this time. Bill was located in a region of increasing westerly shear (15-20+ kt, Fig. 3, right). Interestingly, the region of 20+ kt vertical shear being analyzed SE of the storm appears to be a bonafide localized shear maximum and was also indicated in the dropsonde data.

Fig. 2: (Top) TPW imagery (20 Aug 0600 UTC) showing Hurricane Bill and two large SAL outbreaks (green to blue shading; ≤45 mm TPW) located east, south and west of the storm. The black line labeled "A" and "B" shows a coincident CALIPSO satellite overpass. (Bottom) Cross section of aerosols detected by a 0707-0729 UTC CALIPSO overpass along the transect labeled "A" and "B" in the TPW image.

Dropsonde #3 was launched in this area SE of the storm and showed the following winds: 200 mb (6.4 kt @240 deg), 700 mb (42 kt @160 deg), and 850 mb (37 kt @173 deg). The winds in this dropsonde profile equate to the following deep layer wind shear values: 200-850 mb (35 kt @353 deg) and 200-700 mb (41 kt @331 deg). Though there may be some vortex influence (particularly with the 200 mb winds) on these shear values calculated from dropsonde #3, the strong low to mid-level flow was clearly driving the high shear values. These winds were not related to the storm circulation, but were in fact associated with a low to mid-level jet collocated with a dry SAL intrusion in this region of the storm (see Mission Specifics section below).

Fig. 3: Plots of (left) 250-850 mb deep layer mean steering [magnitude (direction) of the steering flow is indicated by colored shading (white streamlines] and (right) vertical wind shear for 20 August 1200 UTC. Images courtesy of UW-CIMSS.

GOES low-level cloud drift winds indicated strong (25-35 kt) winds NW and west of the storm (Fig. 4, left). GOES water vapor winds also indicated that the upper-level outflow pattern was continuing to become less favorable. Upper-level winds were now cyclonic around most of the storm except in the NW quadrant. Upper-level outflow was concentrated on the north side of the storm with winds of 40-65 kt (Fig. 4, right). TPW imagery indicated that a SAL intrusion was continuing to wrap into the storm from the SW quadrant and had penetrated around to the SE and NE quadrants (Fig. 2, top). GPS dropsonde data indicated that the dry air had even reached the north side of the storm. This dry air intrusion also appears to have triggered a significant arc cloud event on the SW and N/NW sides of the storm later in the day (Fig. 5).

Fig. 4: Plots of (left) GOES low to mid-level cloud drift winds and (right) mid to upper-level water vapor winds for 20 August 1200 UTC. Images courtesy of UW-CIMSS.

b) Mission Specifics
The flight plan was designed to investigate Hurricane Bill with targeted sampling of SAL air masses positioned around the storm, a dry air intrusion wrapping around the storm, and the moist tropical environment associated with the storm (Figs. 2 & 6). TPW values of ≤45 mm (>45 mm) were used to discern the SAL (moist tropical) environments around the storm. The G-IV star pattern that was flown was a new flight pattern that was designed to sample the environment surrounding the storm and the possible penetration of that environment in toward the "inner core" region. Extra GPS dropsonde sampling along the "radial" legs of the starfish pattern were designed to observe this transitional environment and improve our understanding of how the surrounding environment may or may not be impacting the "inner core" of the storm.

Fig. 5: Hurricane Bill GOES visible imagery on 20 August 2009 at (left) 1415 UTC, (middle) 1715 UTC, and (right) 2015 UTC). Arc cloud events in the SW and NW quadrants of the storm are indicated in the images.

Takeoff was at 0749 UTC from Barbados. The flight plan called for a counter-clockwise sampling of the storm at an optimal flight level of 41,000-45,000 ft. It was anticipated that the dry air intrusion that had penetrated to the northern semicircle might trigger an arc cloud event on the north and NW sides of the storm. Therefore, it was desirable to reach these areas during daylight hours and a counter-clockwise flight pattern would ensure that. GOES water vapor winds indicated that given the largely upper-level cyclonic flow around the storm, the G-IV would actually encounter more tail winds by flying an anti-cyclonic pattern.

Fig. 6: TPW imagery showing Hurricane Bill on 20 August 1200 UTC. The SAL's dry air is indicated by values of ≤45 mm (green to blue shading) in the TPW image. The G-IV flight track (black curve) and dropsonde points (black circlesand hash marks) are overlaid for reference. Imagery courtesy of UW-CIMSS.

Main targets included the SAL 1 air mass NW, SW and SE of the storm, the SAL 2 air mass NE of the storm, the SAL intrusions that was advecting into the SW and SE quadrants, the moist tropical atmosphere close to the "inner core" region and the transitional environment between the SAL and the "inner core" environments (Figs. 2 and 6). Dropsondes launched from points 3 to 6b captured unusual thermodynamic and wind patterns associated with the dry SAL intrusion. Many of the profiles indicated dry air (10-40% RH) in the middle levels (~500-800 mb) that was collocated with ~45-85 kt jets that appeared to follow the inflow of the dry intrusion. Though it is not clear exactly what mechanism was driving these jets, they appear to have been highly correlated to the vertical placement of the dry air and represent an extremely efficient means of advecting that dry air in toward the storm. Dropsondes just east and NE of the center showed near-surface RH values of 40-78% and suggest that strong convectively driven downdrafts may have been occurring as the dry SAL air reached these precipitating areas of Hurricane Bill. It is also likely that the dry air intrusion and evidence of strong downdrafts that were observed were the precursors to the formation of the significant arc cloud events that occurred near the end of the G-IV mission (Fig. 5).

Problems :

There were no major problems related to this flight. Twenty-six GPS dropsondes were deployed during the mission, three of which were fastfalls.

Drop log

Page last updated October 13, 2009
Return to Mission page.