Climatology of Dry Layers in the Tropical Troposphere Derived from CONTRAST Observations and GFS Analyses

Abstract:

A recent airborne field experiment in the remote tropical western Pacific, CONTRAST (CONvective Transport of Active Species in the Tropics), obtained over 80 vertical profiles of water vapor, ozone and numerous trace species from the surface to above 14 km during January-February 2014. These observations showed the frequent occurrence of extreme dry layers (relative humidity < 20%) co-located with enhanced ozone, occurring primarily over altitudes ~4-10 km. We compare these water vapor measurements with analyses from the NCEP Global Forecast System (GFS), and find good overall agreement. We then use the GFS analyses to quantify the climatological variability of extreme dry layers in the tropical troposphere, finding frequent occurrence in the subtropics of both hemispheres during winter-spring. The space-time structure of the dry layers, in addition to the strong anti-correlation between water vapor and ozone, suggests frequent transport to the tropics from the extratropical upper troposphere – lower stratosphere (UTLS).