1.7 Dry, ozone-rich layers in the tropical troposphere as a fingerprint of UTLS transport

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.

Sample template image

Figure 1. Upper left: Example of CONTRAST aircraft measurements showing dry, high ozone layers in the tropical Pacific. Lower left: Two-dimensional distribution of ozone vs. relative humidity (RH) for all of the CONTRAST vertical profiles. Right: Global climatology of dry layers derived from GFS meteorological analyses (frequency of air with RH < 20%), during DJF and JJA. These results are for the 330 K isentrope, which is near 5 km in the tropics.

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.