Observational Evidence of Horizontal Transport-Driven Dehydration in the TTL

The conceptual framework of the tropical tropopause layer (TTL) was developed over a 10-year period starting around 2000, and it is largely motivated by the challenge of understanding water vapor transport into the stratosphere. The TTL concept emphasizes the important role of horizontal transport in dehydrating the air masses eventually entering the stratosphere. This conceptual model lead to the conclusion that the amount of water vapor entering the stratosphere is controlled by the temperature of the “cold trap” region over the tropical western Pacific, not by the average tropical tropopause temperature. The Airborne Tropical Tropopause Experiment (ATTREX) using the Global Hawk UAS platform achieved the first significant set of in situ measurements in the TTL over the tropical Pacific during the 2013 and 2014 deployments. From the ATTREX data, we have found clear evidence of horizontal transport-driven dehydration in the TTL. In this work, we present the data analyses using simultaneous measurements of water vapor, ozone, and temperature from the Global Hawk. Vertical distributions of relative humidity derived from the water vapor and temperature measurements reveal a strong signature of remote dehydration from the cold region upstream of the flights when organized in the coordinates relative to the lapse rate tropopause and the cold point. The most significant influence of horizontal transport-driven dehydration is found in the layer between the lapse rate and cold point tropopauses. Ozone distributions also provide a clear signature of large scale horizontal transport and mixing in this layer.