Dry layers in the tropical troposphere observed during CONTRAST and global behavior from GFS analyses

The Convective Transport of Active Species in the Tropics (CONTRAST) experiment was an aircraft-based field campaign conducted from Guam (14°N, 145°E) during January–February 2014. Aircraft measurements included over 80 vertical profiles from the boundary layer to the upper troposphere (~15 km). 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 National Centers for Environmental Prediction Global Forecast System (GFS) analyses, we find good agreement for dry layers, including profile-by-profile comparisons and statistical behavior. We then utilize GFS data to evaluate the frequency of occurrence and 3-D structure of dry layers for the CONTRAST period to provide perspective to the campaign measurements and evaluate the global climatological behavior based on a longer record. 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. 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 (21°S, 56°E). The climatologically frequent occurrence of dry, ozone-rich layers, plus their vertical and spatial structures linked to the subtropical jets, all suggest that dry layers are linked to quasi-isentropic transport from the extratropical UTLS and suggest a ubiquitous UTLS influence on the subtropical middle troposphere.