Flight summary, DC-8 Hong Kong local 4  -  03/13/2001.  

Title: Aged Asian outflow  

Objectives: (1)  to sample aged Asian outflow slowly transported southeastward in the lower troposphere over the western Pacific around a weak high pressure system, (2) to sample strongly subsiding air with stratospheric influence capping this Asian outflow, (3) to sample upper tropospheric outflow from biomass burning in southeast Asia.  

Execution:  We flew an extended wall from HK east to (22N, 136E) with extensive low-altitude sampling on the eastern half of the wall .  On the return we extended the wall south to (18N, 114E), with focus on high altitudes, in order to sample the biomass burning outflow.  

Results: The flight was a success.  Aged Asian pollution was consistently observed below 5Kft on the extended wall; this pollution was characterized by CO up to 190 ppbv, O3 up to 70 ppbv, high C2Cl4, high PAN, high sulfate (but low SO2), moderately high  HCHO.  Above 5 Kft the aircraft sampled strongly subsiding air with occasional evidence of stratospheric influence and some aged pollution layers at altitudes below 15 Kft (though not as strong as below 5 Kft).  On the return the aircraft sampled combustion plumes in the upper troposphere (31-39 Kft) with high and correlated CO (up to 140 ppbv), CO2, O3, and aerosol (including a large non-volatile component); these plumes were probably of biomass burning origin although no confirmation of this was available from the flight data.  

Meteorological Summary DC-8 Hong Kong Local # 4

Relevant Flow Features
         Surface-A high pressure area was centered over South Korea.  No fronts were located within the flight area.  Most of the flight track experienced easterly or northeasterly low-level flow.

         Middle troposphere-Westerly flow dominated the flight track by the 500 mb (~18,000 ft) level.  A short wave trough was located just east of Japan, and a closed anticyclone was located near Southeast Asia.

         Upper troposphere-A major subtropical anticyclone was located near 15N, 155W.  A short wave trough was located near Japan. The polar jet stream was better defined than on most previous missions-it extended over northern Asia.  The subtropical jet stream stretched across southern Asia. The two jets merged east of Japan.  The DC-8 traversed the southern portion of the jet streak east of Japan.  Westerly flow covered the entire flight area.

Relevant Cloud Patterns
         Abundant low level clouds blanketed the flight track.  They typically had bases near 3000 ft and tops near 5,000 ft. Middle and upper level clouds were virtually absent over the area.

         Southeast Asia had been experiencing scattered deep convection over the past several days, and this continued today.  The Indonesia area also exhibited convection.  Trajectories indicated that upper level outflow from these storms had been transported toward the DC-8 flight track.

         Water vapor imagery denoted an area of dry middle tropospheric air over the northeast portion of the flight area.  This dry air probably was associated with subsidence due to the jet streak and the surface anticyclone in the area.  In situ data showed that the dry air extended as low as 5400 ft.  Other portions of the flight track were relatively humid in the middle levels.