Flight Details - AGASP IV

Flight Details - AGASP IV

Excerpts for Flights 402 and 407 from

Herbert, G.A., et al., Analysis of meteorological conditions during AGASP-IV: March 30-April 23, 1992, NOAA Tech. Memo. ERL CMDL-5, 118 pp., NOAA/ERL/Climate Monitoring and Diagnostics Lab., Boulder, Colo., 1993.

3. FLIGHT 402, APRIL 10-11,1992

3.1 Objective

On April 5 the replacement engine was successfully flight tested and the WP-3D was certified ready to continue the sampling program. For the next few days conditions north of the Brooks Range were less than favorable for monitoring Arctic haze. For the period April 6-10, BRW reported broken to overcast conditions with westerly winds, changing to clear to scattered conditions early on April 7. Later in the day (1700Z) the wind shifted to easterly and the cloud cover returned. By 1000Z April 8, BRW was reporting light snow. Light snow and fog continued until 0004Z April 10. On the basis of a forecast for clearing and a shift to northerly winds of 5-10 m s -1 in the surfacelayer and northeasterlies of 8-15 m s-1 aloft during the next 24 hours in the BRW region, an AGASP flight was scheduled for April 10-11.

The WP-3D took off at 1715Z April 10 and followed a flight plan taking it over Fairbanks, at which point the plane turned toward the north, following latitude 148°W to 72°N. The aircraft reached flight altitude (6.1 km, 465 mb) 20 minutes after takeoff. During the period 1757 to l8l5Z, the aircraft climbed to 7.3 km in search of the tropopause. At 1834Z the aircraft left that altitude, climbing to 7.95 km (1839Z). The WP-3D remained at that altitude until l951Z when the descent profile was begun.

Horizontal projection of the aircraft flight track on a latitude-longitude grid, April 10-11,1992.


The portion of the flight path over the Beaufort Sea consisted of three segments, starting with a vertical profile 220 km upwind, northeast of BRW, followed by a low-level traverse on a north-south heading to sample the plumes from the Prudhoe Bay region. The third leg consisted of a cross-wind traverse to the north of BRW at the top of the planetary boundary layer. The profile began with a westerly segment from 1942 to 1952Z, to provide a clear view of the sun for turbidity measurements. The aircraft descended at an average rate of about 200 m min-1 (16 mb min-1). At 2022Z, the aircraft conducted a second westerly segment for turbidity measurements. The descent was continued at 2033Z, reaching the lower sampling level of 1015 mb (0.15 km) at 2057Z. A third radiation segment was flown from 2107 to 2119Z.

The second portion of the flight consisted of a level segment along 150°W from 72°N to 70°N, at 150 m altitude, and back to 71°N, at 330 m altitude (991 mb). This segment started at 2119Z, reaching the southernmost point at 2202Z and ending at 2224Z. From 71°N, l50°W the aircraft took a heading of 290 to sample a cross section of air upwind of BRW. At 156.61°W, the longitude of BRW, the WP-3D passed about 13 km to the north of the station. The first half of this segment was flown at a height of 150 m (1018 mb), the second part at 90 m (1029 mb). At l59°W (2330Z) the aircraft turned toward the east and began a gradual climb to a maximum altitude of 9.5 km. (0040Z), after which the airplane began a gradual descent into Anchorage via Fairbanks. The WP-3D landed at 0242Z.

3.3 Synoptic Situation

Two days preceding this flight, the 500 mb flow over northern Alaska was determined by a ridge over the Bering Sea and eastern Siberia, with a 516 decameters closed high off the southwest coast, and a small low north of BRW at 75°N. During the next 36 hours the ridge intensified over eastern Siberia and the East Siberian Sea while the closed central portion moved to a position over the Aleutians at 53°N. The low drifted slowly to the southeast to a position over the Beaufort Sea north of Barter Island. With the high forecast to continue to increase inintensity, and the low forecast to move slowly to the south, the synoptic conditions were favorable for trans-Arctic transport for the first time in a week.

At the time of the analysis, the low was located over north central Alaska, with a central height of 511 decameters. The ridge to the west has split into two segments, a dominant center over central Siberia and a weaker high center south of the Aleutians. A minor ridge over the Beaufort Sea followed the low as it moved to the south.

Anchorage was reporting easterly winds at 5 m s-1 and overcast skies, consisting of altostratus clouds with tops below the 500 mb level, at takeoff. Also, according to the flight log, the undercast extended to just south of Fairbanks, where at 1800Z scattered clouds were reported. Skies were generally clear aloft in this region. Approaching the Brooks Range the lowlevel cloudiness increased to an undercast, and scattered high cirrus was encountered at flight altitude. At the time the WP-3D passed to the west of Prudhoe Bay, Deadhorse was reporting a cirrus overcast with northeasterly winds of 1-2 m s-1. No obstruction to visibility was indicated. One hour later while the aircraft passed Barrow, the observatory there was measuring a wind of 8 m s-1 from the north. The observer was reporting scattered clouds, and no obstruction to visibility was indicated.

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8. FLIGHT 407, APRIL 21-22, 1992

8.1 Objective

The seventh flight in the series was dedicated to aerosol and gas sampling in the Barrow region and to providing a comparison of measurements with those from a Russian aircraft sampling the Siberian Arctic at the same time. A secondary interest was in sampling the plume from a large Palynya, to the west of BRW.

The WP-3D took off from Anchorage at 1742Z and climbed to a cruising altitude of 6.1 km (466mb). This altitude was maintained from 1800Z, at 62.18°N, 149.62°W, to 1934Z, at 68.92°N, 153.28°W. The aircraft followed the same track as in previous flights as far as Fairbanks, after which it turned toward Barrow. By 1929Z the aircraft had climbed to 6.3 km (427 mb) and remained at this altitude until the beginning of the descent profile. The slow descent began at 2021Z at 71.45°N, 158.90°W, and concluded at 2223Z, 71.21°N, 157.63°W, and at an altitude of 16 m (1018 mb). Level segments of 10-15 min duration were interspersed in the sounding at 3.0 km (702 mb), 1.5 km (848 mb), and 0.15 km (999 mb). At the conclusion of the sounding the aircraft was about 34 km west of Barrow. For the next 40 minutes the WP-3D conducted along- and cross-wind sampling segments at 16 m. A brief sounding from the surface to 5.9 km was begun at 2304Z, at 71.19°N, 157.60°W, after which at 2358Z, at 71.93°N, 165.90°W, the WP-3D descended to an altitude of 0.1 km to rendezvous with the Russian plane.


Horizontal projection of the aircraft flight track on a latitude-longitude grid, April 21-22, 1992.

Following the sounding, the aircraft turned to the south and then southeast to return to Anchorage. The ascent to cruise altitude began at 0004Z, at 72.25°N, 165.92°W. Cruise altitude of 7.6 km (375 mb) was attained after only 14 minutes, a climb rate of 0.5 km min-1 (45 mb min-1). After 18 minutes at 7.6 km the aircraft climbed to 9.5 km (286 mb). The aircraft maintained this altitude from 0055Z, at 71.06°N, 156.53°W, to 0138Z, at 68.06°N, 154.02°W. After a brief climb to 10.1 km (261 mb), and a 14 min segment at that level, the aircraft began the descent to Anchorage at 0204Z (66.18°N 152.77°N). The aircraft landed at about 0329Z.

8.3 Synoptic Situation

The dominant synoptic features at 500 mb at the time of this flight were a highpressure ridge over the Bering Sea and an accompanying low-pressure region over the Gulf of Alaska. The ridge had developed as a result of a sign)ficant low to the west. This low was the remnant of the low at 60°N, 150°W at the time of the preceding flight. The central pressure of the low was now 526 decameters at that time. The remnant of the high pressure ridge that was over northern Alaska during the preceding flight had become a weak high with a central height of 536 decameters. As a result of the decrease in the height gradient over the state, the steady southeasterly geostrophic gradient present during the preceding two flights was gone. Winds at 500 mb were generally light and variable.

At the surface, low pressure centered to the south and east of Anchorage, in alignment with the low at 500 mb, continued to advect low-level moisture into southern Alaska. This low and a secondary low in western Canada coupled with a large high-pressure cell in the vicinity of the pole resulted in a northeasterly geostrophic flow over most of the state. A minor trough was analyzed in the BRW region at the time of the flight. Barrow was reporting light snow with winds from the southeast at 3 m s-l.

Overcast and broken sky conditions were reported at Anchorage and Fairbanks at 1800Z. While some regions between Fairbanks and the Brooks Range were reporting clear or broken cloud conditions, the region north of the Brooks Range had extensive cloud cover.

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