Observational Evidence of Horizontal Transport-Driven Dehydration in the TTL

A recent airborne study obtained extensive measurements in the tropical tropopause layer (TTL) over the western Pacific and provided the first opportunity to examine the relationship between water vapor and temperature in the coldest region and season of the TTL using high-resolution in situ data. Analysis of this data set verifies key hypotheses in Lagrangian simulations of TTL transport and freeze drying. Furthermore, the observations provide a number of new insights into the transport process: In the layer below the lapse rate tropopause, vertical transport from upward motion dominates the relative humidity structure; final dehydration, dominated by large-scale horizontal advection, occurs in the layer transacting the cold point tropopause that is often above the lapse rate tropopause, resulting in water vapor mixing ratios with corresponding frost points consistent with the coldest temperatures of the region, lower than the temperatures of the local cold points.