IOP 15, 19-20 Dec 2001

IOP 15: 19-20 December 2001

Time Period of IOP

2100 UTC 19 Dec-0609 UTC 20 Dec

Overview of IOP

This case differed from all the previous IMPROVE II cases in that a low to the southwest of Oregon produce easterly flow over the study area, so that the eastern slopes of the Cascades became the windward slope of the terrain. Thus, the orographic enhancement occurred on the eastern rather than the western slopes when the easterly flow became predominant. At 500 mb, a well defined short wave trough was moving eastward, intensifying, deepening, and beginning to cut off between 19 December 1200 UTC and 20 December 0000 UTC ( Fig. 1a, Fig. 1b). As the intensifying low moved eastward, the 850 mb flow changed from weak southerly to strong east-southeasterly (Fig. 2a, Fig. 2b). The tongue of high relative humidity at 850 mb reached into the storm from 15 deg south (Fig. 3a, Fig. 3b). The sea-level pressure and temperature pattern showed a warm front moving over the study area during this time period (Fig. 4a, Fig. 4b).

The Convair flew its first mission from 2229 UTC 19 December-0419 UTC 20 December. The infrared satellite data for 19 December 2200 UTC and 20 December 0400 UTC showed the warm-frontal clouds moving over the region during this time period (Fig. 5a, Fig. 5b, Fig. 5c, Fig. 5d). The Convair flew north-south legs both east and west of the crest of the Cascades to see the warm-frontal structure on both sides of the mountains. Before the aircraft reached the study area, the S-Pol showed a layer of precipitation aloft and gradually lowering (indicated by the shrinking circle of no echo around the radar at 1.5 deg elevation, Fig. 6a, Fig. 6b, Fig. 6c). At 2200 UTC, approximately the beginning of the Convair's first flight, the echoes reached the surface (Fig. 6d). At 2300 UTC, the echo appeared as a relatively featureless continuous layer of snow above a well defined intense bright band (at the first range ring in Fig. 6e). By 0000 UTC 20 December, the bright band was weaker and at a higher altitude, indicating general warming with the passage of the warm frontal region (Fig. 6f). By 0100 UTC, the echo was weakening and disappearing to the southwest of the radar, while the echo to the northeast was more continuous (Fig. 6g). From 0200-0300, the echo to the northeast weakened and more cellular echo appeared to the northeast (Fig. 6h, Fig. 6i), as the cold frontal part of the system approached (cf Fig. 5b).

The Convair's second flight was from 0506-0609 UTC 20 December. The infrared satellite data for 0400 UTC-0600 UTC showed that the S-Pol radar was in a gap between warm-frontal clouds to the east of the mountains and cold-frontal clouds moving in from the west across the Oregon coast (Fig. 5d, Fig. 5e). At 0400 UTC, the S-Pol radar showed the warm-frontal precipitation aloft to the northeast of the radar, and convective (warm sector?) echo just west of the radar (Fig. 6j). At 0600 UTC, the warm-frontal echo to the northeast had disappeared, and a fairly continuous echo was moving in from the southwest (Fig. 6k) in association with the cold-frontal cloud band (Fig. 5e). From 0800-1200 UTC, the satellite imagery showed the cold frontal cloud band moving slowly across the radar area (Fig. 5g, Fig. 5h, Fig. 5i). The satellite images suggest that the frontal cloud band had embedded substructure in the form of transverse wavelike bands (esp. Fig. 5h), and the radar showed weak echo with embedded cellular structure (Fig. 6l, Fig. 6m).

Convair-580 Summary

UW Flight Number:

1902

Period of Flight:

2229 UTC 19 December-0419 UTC 20 December 2001

Main Accomplishment of Flight:

North-south oriented vertical profile from 18,000 to 8,000 ft west of Oregon Cascade crest. West-to-east cross-mountain track in southern edge of rainband. Truncated north-south stack in rainband east of Cascade crest. Landed in Eugene, Oregon.

Instrument Problems:

PMS 1-D cloud probe (?).

Flight Scientist:

Nick Bond

Approximate UTC Timeline (Local time= UTC-8 hours) Activity for UW Flight 1902:

2229	Engines on.
2231	Out of blocks.
2241	Takeoff.
2253	No TANS/winds.
2257	2-DC & HVPS working.
2302	Winds now working 160˚ @ 5 m s^–1 @ 576 mb.
2322 Lat. 45˚45'	. Winds 218˚ @ 20-25 m s^–1. Possible significance of warm front aloft.
2336	Decide to ascend to 18,000 ft.
2340	Starting first southbound (18,000-16,000 ft). Wind 215˚ @ 22 m s^-1, T ~–21˚C. Solid cloud.
2348	Conversation with S-Pol radar operator indicates that legs should be shorten because of back edge of system approaching.
0008	CPI not working; open ground visible below.
0010	End of leg 1.
0012	Start of leg 2 (north bound at 16,000 ft).
0020	End of leg 2 (whole leg in precipitation).
0023	Start of leg 3 (south bound 16,000-14,000 ft). Winds 205˚ @ 20 m s^-1, T ~–14˚C, solid cloud.
0027	End of leg 3 (steep descent).
0029	Start of leg 4 (north bound @ 14,000 ft).
0033	Heavier precipitation; bigger crystals. Wind 210˚ @ 22 m s^-1. LWC 0.04 à 0.1 g m^-3.
0038	End of leg 4.
0041	Start of leg 5 (south bound 14,000-12,000 ft).
0049	End of leg 5.
0050	Possible computer problem; displays not updating.
0056	Data displays back.
0058	End of leg 6.
0100	Start of leg 7 (south bound 12,000 ft).
0107	End of leg 7; descent at turn.
0108	Start of leg 8 (north bound, 10,000 ft). Wind 212˚ @ 18 m s^-1. T ~–5˚C.
0116	End of leg 8.
0120	Start of leg 9 (south bound 10,000-8,000 ft).
0124	2-DC temp. down.
0127	End of leg 9.
0128	Start of leg 10 (north at 8,000 ft). Wind 185˚ @ 15 m s^-1 but noisy. T ~–2˚ to –4˚C. Low LWC (0.05-0.1 g m^-3).
0132	Some chop
0136	End of leg 10 (last north-south). Ferry to Pt. M.
0141	Descending to 7,000 ft.
0145	Moderate chop; near freezing level.
0150	Near cloud base.
0156	At Pt. M; lining up to cross mountain
0202	Climb to 8,000 ft.
0208	Begin climb to 10,000 ft.
~0213	Turn and climb for north-south stack.
0228	Start first run south bound (18,000-16,000 ft).
0231	Wind 200 @ 14 m s^–1. T ~–20˚C. Still in light precipitation.
0236	Lighter precipitation in south end.
0239	Start of second leg north (16,000 ft). Wind 210˚ @ 23 m s^-1. T ~–17˚C.
0251	End of second leg.
0254	Start of third leg south (16,000-14,000 ft). Wind 212˚ @ 16 m s^–1.
0305	End of third leg.
0307	Start of fourth north (14,000 ft).
0311	Break in precipitation.
0320	End of fourth.
0322	Start of fifth south (14,000-11,000 ft). Wind 210˚ @ 17 m s^–1.
0326	Relatively heavy precipitation near 45˚10'.
0332	End of fifth leg.
0334	Heading north (descending to min ~8,000 ft).
0340	Winds 125˚ @ 22 m s^–1 at 8,000 ft.
0341	End of descent to 8,000 ft.
0345	Quasi-isothermal in climb from 8,000 to 11,000 ft; veering wind.
0349	End of south bound climb. Ferry to Salem.
0401	Mod. chop and precipitation. Over crest and foothills.
0414	Landed at Eugene, Oregon
0419	Engines off.