Meteorological conditions and transport pathways during the Transport and Chemical Evolution over the Pacific (TRACE‐P) experiment

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Abstract
The Transport and Chemical Evolution over the Pacific (TRACE‐P) experiment was conducted between February and April 2001. It included extensive chemical sampling by two aircraft based primarily in Hong Kong and Yokota Air Base, Japan. TRACE‐P examined pathways for the outflow of chemically and radiatively important gases and aerosols and their precursors from eastern Asia to the western Pacific and explored the chemical evolution of Asian outflow. This paper describes meteorological conditions and transport pathways over the Pacific Basin during TRACE‐P. Meteorological conditions changed rapidly during the period due to the seasonal winter to spring transition and the decay of prolonged ENSO cold phase (La Nina) conditions. To document these changes, TRACE‐P was divided into two halves, and mean flow patterns during each half are presented and discussed. Important circulation features are the semipermanent Siberian anticyclone and transient middle latitude cyclones that form near eastern Asia and then move eastward over the northern Pacific. Five‐day backward trajectories from the various flight tracks show that air sampled by the aircraft had been transported from a variety of locations. Some parcels remained over the tropical western North Pacific during the entire period, while other important origins were Southeast Asia, Africa, and central Asia. Specifically, lower tropospheric flight segments out of Hong Kong sampled both prefrontal maritime air as well as postfrontal air from the Asian continent. Conversely, low‐level flight segments out of Yokota, Japan mostly sampled postfrontal Asian air. Southern portions of middle and upper tropospheric flight segments from Hong Kong sampled air previously in the deep tropics, while the more northerly flight segments sampled air that originated from the west (e.g., passing over central Africa and India). Most upper level flight segments from Yokota sampled air arriving from the west. Patterns of satellite‐derived precipitation and lightning are described. TRACE‐P occurs during a neutral to weak La Nina period of relatively cold sea surface temperatures in the tropical Pacific. Compared with climatology, the TRACE‐P period exhibits deep convection located west of its typical position; however, tropospheric flow patterns do not exhibit a strong La Nina signal. Circulation patterns during TRACE‐P are found to be generally similar to those during NASA's PEM WEST‐B mission that occurred in the same region during February–March 1994.

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