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 (14degN, 145degE) 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 (21degS, 56degE) 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.