Abstract
Abstract. We derive the tropical modal age of air from an analysis of the water vapor tape recorder. We combine the observationally derived modal age with mean age of air from CO2 and SF6 to create diagnostics for the independent evaluation of the vertical transport rate and horizontal recirculation into the tropics between 16–32 km. These diagnostics are applied to two Global Modeling Initiative (GMI) chemistry and transport model (CTM) age tracer simulations to give new insights into the tropical transport characteristics of the meteorological fields from the GEOS4-GCM and the GEOS4-DAS. Both simulations are found to have modal ages that are in reasonable agreement with the empirically derived age (i.e., transit times) over the entire altitude range. Both simulations show too little horizontal recirculation into the tropics above 22 km, with the GEOS4-DAS fields having greater recirculation. Using CH4 as a proxy for mean age, comparisons between HALOE and model CH4 in the Antarctic demonstrate how the strength of tropical recirculation affects polar composition in both CTM experiments. Better tropical recirculation tends to improve the CH4 simulation in the Antarctic. However, mean age in the Antarctic lower stratosphere can be compromised by poor representation of tropical ascent, tropical recirculation, or vortex barrier strength. The connection between polar and tropical composition shown in this study demonstrates the importance of diagnosing each of these processes separately in order to verify the adequate representation of the processes contributing to polar composition in models.
Highlights
An age spectrum is the distribution of transit times since a stratospheric air parcel last made contact with the troposphere
These empirically derived modal ages are combined with tropical mean age profiles to evaluate vertical transport and tropical recirculation in Global Modeling Initiative (GMI) chemistry and transport model (CTM) simulations using meteorological fields from the Goddard Earth Observing System Version 4 (GEOS4) general circulation model (GCM) and the GEOS4 data assimilation system (DAS)
We show that tropical vertical transport in the GEOS4-GCM is well simulated and that its mean age deficiency stems from too much tropical isolation
Summary
An age spectrum is the distribution of transit times since a stratospheric air parcel last made contact with the troposphere. In this paper we calculate modal ages in the tropical stratosphere (16–32 km) using the vertical velocities calculated from the 15-year water vapor data set in Schoeberl et al (2008) These empirically derived modal ages are combined with tropical mean age profiles to evaluate vertical transport and tropical recirculation in Global Modeling Initiative (GMI) chemistry and transport model (CTM) simulations using meteorological fields from the Goddard Earth Observing System Version 4 (GEOS4) general circulation model (GCM) and the GEOS4 data assimilation system (DAS). The GMI-DAS simulation is integrated for 20 years by recycling 1 July 2004–30 June 2005 meteorological fields; the GMIGCM recycles the 5-year sequence of wind fields four times to complete a 20-yr integration Both versions of the CTM are integrated at 2◦ latitude by 2.5◦ longitude resolution with 42 vertical levels and a model lid at 0.01 hPa. Spatial resolution in the upper troposphere/lower stratosphere is 1 km or less. Details of the GMI Combo CTM and the credibility of its lower stratospheric transport can be found in Strahan et al (2007) and Duncan et al (2007)
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