The Convective Transport of Active Species in the Tropics (CONTRAST) experiment was an aircraft-based field campaign conducted from Guam (14o N, 145o E) during January-February 2014. Aircraft measurements included over 80 vertical profiles from the boundary layer to the upper troposphere (~15 km), and a large fraction of these profiles revealed layered structures with very low water vapor (relative humidity < 20%) and enhanced ozone, primarily in the lower-middle troposphere (~3-9 km). Comparing CONTRAST water vapor measurements with co-located profiles from NCEP Global Forecast System (GFS) analyses showed overall good agreement for dry layers, including profile-by-profile comparisons and statistical behavior. The GFS data provide a global perspective to the campaign measurements, and allow analysis of the climatological spatial structure and frequency of subtropical dry air. GFS data show that dry layers occur ~50-80% of the time in the subtropical troposphere, maximizing on the equatorward side of the subtropical jets in the winter hemisphere (see Fig. 1). Subtropical dry layers occur most frequently over isentropic levels ~320-340 K, which extend into the extratropical upper troposphere-lower stratosphere (UTLS). Similar statistical behavior of dry, ozone-rich layers is found in long-term balloon measurements from Reunion Island (21o S, 56o E). The climatologically frequent occurrence of dry, ozone-rich layers, plus their vertical and spatial structure linked to the subtropical jets, all suggest that dry layers are linked to quasi-isentropic transport from the extratropical UTLS. These results demonstrate a ubiquitous UTLS influence on the subtropical troposphere.
Randel, W.J., L. Rivoire, L. Pan and S. Honomichl, 2016: Dry layers in the tropical troposphere observed during CONTRAST and global behavior from GFS analyses. J. Geophys. Res., in press.