Deriving a Transport Diagnostic for the Convectively Dominated Tropical Western Pacific Upper Troposphere based on Airborne Trace Gas Measurements

We present a newly developed tropospheric transport diagnostic based on airborne trace gas measurements, which builds on the concept of the Age Spectrum, widely used in diagnosing stratospheric transport and circulation. Transport time scales from the boundary layer (BL) to the upper troposphere (UT) over the convective transport dominated Tropical Western Pacific (TWP) are characterized using a set of transit time distributions (TTDs). These TTDs are derived using 42 volatile organic compounds (VOCs) observed during the Convective Transport of Active Species in the Tropics (CONTRAST) experiment by the AWAS and TOGA instruments aboard the NCAR Gulfstream V research aircraft. Lifetimes of the VOCs span a few hours/days (e.g. DMS, acetaldehyde), to intermediate lifetimes of a few weeks (e.g. bromoform), to long-lived species ranging years to decades (e.g. CFCs, carbon tetrachloride). The chemical lifetimes of the measurements provide information on a wide range of transport pathways. The resulting TTDs for the convective outflow layer over the TWP in the boreal winter season show a modal transit time of 2-3 days. The metric has also been applied to evaluating convective transport in a kinematic trajectory model analysis, and is in the process of being applied to chemical climate models. Our analysis also found that the airborne samples contain sufficient information to identify transport variability in different dynamical environments. Variability in transport information is captured by variations in trace gas mixing ratios in the UT, especially those with lifetimes of less than ~1 year. The resulting modal transit times are found to range from less than a day to several days/weeks, around the ensemble average of a 12.7 day mean and 3.3 day mode. We will also show initial results of the VOC based TTDs using modeled species from the Community Atmospheric Model with Chemistry (CAMchem). This method has the potential to become a valuable diagnostic tool to assess transport using the wealth of airborne campaign data, and provide insight to model representations of chemical transport.