Cloud draft structure and trace gas transport

Abstract

Field observations obtained during the second NASA Amazon Boundary Layer Experiment (ABLE 2B), and two‐dimensional moist cloud model simulations are used to determine the dominant transport pathways within a continental tropical squall line. A surface‐based network triangle provided the focus for a multi‐instrumental sampling of the May 6, 1987, squall line which propagated through the central Amazon basin at a rate of 40–50 km h−1. Extensive use is made of the vertical distribution of specific trace gases that are representative of the prestorm and poststorm environment. One‐dimensional photochemical model results suggest the observed poststorm changes in ozone concentration can be attributed to convective transports rather than photochemical production. Two‐dimensional cloud model results detail the dynamic and thermodynamic attributes of the simulated squall convection. The well‐mixed moist troposphere in which the observed squall system developed may have hindered strong downdraft development. Parcel trajectory analyses are conducted to investigate the flow patterns of convective transports. A significant proportion (> 50%) of the air transported to the anvil region originated at or above 6 km, not from the boundary layer via undilute cores. The presence of a midlevel inflow and a strong melting layer at 5.5 km reduced the vertical development of the core updraft and aided in the maintenance of a rotor circulation. The predicted absence of more than one active cell in the model cloud field, the lack of a well‐organized downdraft in the presence of model estimated net upward mass flux, and the initial wind profile suggest the May 6 squall line was unicell in character.