Take-off time 8:56 am local (1856Z); flight duration 10.3 hours  

Title: Subsidence over East Pacific and MOPITT transect  Objectives:  (1) To characterize aged continental outflow subsiding around the East Pacific High; (2) to conduct a MOPITT underpass spiral as well as an extensive transect along the Terra orbit track to validate horizontal gradients seen by MOPITT.  (It was unfortunately not possible on this flight to sample Asian outflow traveling in the jet stream, because of a persistent blocking ridge over the central Pacific that deflected the jet stream north of 50N).  

Execution: We flew from Kona (20N, 156W) straight E to (20N, 140W) for a MOPITT underpass spiral at (21N, 140W).  From there we flew NNE to (37N, 136W), remaining in the MOPITT orbit track throughout and with extensive vertical profiling up to 28Kft.  We then headed straight back to Kona along a SW track, again with extensive vertical profiling, and flew a series of low-altitude legs near Kona to characterize the chemistry of oxygenated organics in the MBL.  

Results: The objectives were met.  The MOPITT spiral (from 33Kft down) was conducted over a solid stratus deck with a few breaks and cloud tops at 6Kft.  There were no higher-altitude clouds.  Above cloud tops the CO concentrations were relatively uniform at 120 ppbv with a moderate pollution layer at 18-22Kft (CO up to 150 ppbv).  In the MBL below cloud top was aged polluted air (150 ppbv CO, 1890 ppbv CH₄, high C₂Cl₄, etc). Remarkably high concentrations of carbonyls and methylhydroperoxide were observed in the MBL.  Heading NNE on the MOPITT transect we sampled a variety of air masses with a fair degree of layering and latitudinal structure (CO ranged from 80 to 210 ppbv).  Solid stratus (tops 4-6Kft) with a few breaks and no higher-altitude clouds persisted for the duration of the transect.    The CO/CH₄/CO₂ ratios varied quite a bit between pollution layers, and CH₄/CO ratios were low in the higher-altitude layers, perhaps providing an indication of European vs. Asian origin. Heading back SW we encountered a few more aged pollution layers against a clean background, and persistently high concentrations of CO below 5Kft (typically 150 ppbv).  Low-altitude legs at the end of the flight sampled air below, within, and above the stratus cloud deck.

Relevant Flow Features:
         Surface-A weak subtropical low was located near 30N 165W.  This system had been moving slowly northward during the previous several days, showing little change in intensity.  A major area of high pressure was near 45N 150W.  Its axis was oriented northwest to southeast.  A weaker high was centered just south of Hawaii. The intertropical convergence zone was becoming better defined south of Hawaii along ~5N.
         Middle troposphere-The weak subtropical low was almost vertically stacked, extending through all flight levels.  The major high northeast of Hawaii extended through 500 mb and then became a ridge line at higher levels.  This feature brought northerly winds to the eastern leg of the flight track.  The subtropical high south of Hawaii was evident at all altitudes.
         Upper troposphere-Westerly flow dominated the flight track in the upper levels.  The closed subtropical low northwest of Hawaii and the ridge line east of the Islands caused some directional changes along the northern part of the track.  The primary jet stream was far north of the flight track.  However, a wind maximum just east of Hawaii brought flight level winds to ~95 kt in that area.  Elsewhere, winds generally were less than 50 kt-sometimes much less.

Cloud Patterns and Other Goodies:
         The subtropical low produced a broad area of low, middle, and high clouds.  This area had a north-south axis, extending as far east as the central Hawaiin Islands.  The wind maximum south and east of Hawaii produced a band of cirrus.  Much of the flight track was covered by stratocumulus clouds that often reached broken to overcast conditions.
         The MOPITT spiral point (21N 140W) had good cloud conditions. Only overcast stratocumulus were present, with occasional small breaks. Bases were near 4800 ft and tops near 6300 ft.  A few stratocumulus were at ~3,000 ft.  The trade wind inversion was near 6300 ft, separating moist air below from very dry air aloft. The ensuing boundary layer run beginning near 2031Z had a few, very light rain showers. During the climbout, cloud tops were somewhat lower than below--~5100 ft.
         The second boundary layer run (near 2211Z) had broken cumulus clouds, with bases near 2900 ft and tops near 3500 ft.  The trade wind inversion was well defined near 3500 ft.  This height was considerably lower than during the first boundary run. The temperature at 500 ft (~12.8C) was cooler than at the first boundary run (17.3C).  During he climbout beginning near 2241Z, the trade wind inversion was not well defined, with relatively humid ir extending into the middle troposphere.
         The initial part of the third boundary layer run (near 0028Z) had a broken layer of clouds with bases of 3100 ft and tops of 5100 ft.  A scattered layer was at 2500 ft.  The clouds became less common during the run.  Winds initially were from the north, but changed to a more northeasterly direction as the leg continued.
         The fourth boundary layer run (beginning at 0213Z) included segments within the clouds and above the clouds.  The clouds in the area had more vertical development than during previous runs.  A scattered stratus deck was between ~6500 - 7500 ft. A broken layer of cumulus had bases of ~2000 ft and maximum tops of ~7500 ft.  Some of the taller clouds produced light rainshowers  The "above-cloud" segment did pass through some clouds.