In Situ Observations of Water Vapor and Cirrus IWC in the Pacific TTL During ATTREX

Despite its very low mixing ratios relative to the troposphere, water vapor in the lower stratosphere (LS) plays a significant role in Earth's radiative balance and climate system and is an important constituent in stratospheric chemistry. The low H2O content of air entering the LS is established to first order by dehydration processes controlled by the cold temperatures of the tropical tropopause layer (TTL). Cirrus clouds occur with high frequency and large spatial extent in the TTL, and those occurring near the thermal tropopause facilitate the final dehydration of stratosphere-bound air parcels. Uncertainties in aspects of the nucleation and growth of cirrus cloud particles and the sparseness of in situ water vapor and cirrus cloud observations with sufficient spatial resolution limit our ability to fully describe the final stages of the dehydration process before air enters the LS in the tropics. The NASA Airborne Tropical Tropopause Experiment (ATTREX) measurement campaign has yielded more than 140 hours of sampling from the Global Hawk UAS in the Pacific TTL during deployments in 2013 and 2014, including more than 30 hours sampling TTL cirrus. The high spatial and temporal resolution in situ measurements of water vapor and cirrus cloud properties made during ATTREX provide an outstanding dataset by which to characterize the Pacific TTL environment and evaluate our current understanding of the dynamical and microphysical processes that result in the dehydration of stratosphere-bound air in this region. Here we present a statistical analysis of the ATTREX water vapor, relative humidity and cirrus cloud crystal number and ice water content (IWC) data in order to investigate cirrus cloud formation and resulting potential for dehydration.