Observations of Water Vapor and Total Water in the Extremely Dry Tropical Tropopause Layer (TTL) from the NASA Global Hawk
A. Rollins1, T. Thornberry1, S. Ciciora2, R. McLaughlin1, L. Watts1, R. Gao2 and D. Fahey2
1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309; 303-497-5181, E-mail: email@example.com
2NOAA Earth System Research Laboratory, Boulder, CO 80305
The Airborne Tropical Tropopause Experiment (ATTREX) is a multi-deployment mission utilizing the NASA Global Hawk (GH) Unmanned Aicraft System (UAS) to examine transport and phase transformation of water in the TTL, leading to a more complete understanding of the hydration and dehydration mechanisms taking place in the tropical upper troposphere and lower stratosphere. We recently developed a new Tunable Diode Laser (TDL) based absorption hygrometer for quantification of Water Vapor (WV) and enhanced Total Water (water vapor + condensed phase, TW) and deployed this instrument during the January-March 2013 installment of ATTREX (ATTREX-2).
The instrument exploits newly available TDLs near 2.7 µm where the H2O absorption cross section is more than one order of magnitude higher than that of the commonly used 1.4 µm band. The high absorption cross-section allows for a simple and compact optical design, operated in a closed path configuration. The instrument contained two detection cells for continuous measurements of WV and TW, and an on-board calibration system used to routinely deliver calibration flows to the instrument during science flights on the GH.
During ATTREX-2 science flights, the GH extensively profiled the TTL between 45 and 60 kft throughout the central and eastern tropical Pacific. Numerous high altitude cirrus clouds were encountered with highly elevated water content relative to the extremely dry air (<2 ppmv) in the region. Here we present the new instrument and these measurements from its highly successful first deployment during ATTREX-2.