Water Vapor, Clouds, and Saturation in the Tropical Tropopause Layer

The goal of this investigation is to understand the mechanism behind the observed high relative humidity with respect to ice (RHi) in the tropical region between ~14 km (150 hPa) and the tropopause, often referred to as the tropical tropopause layer (TTL). As shown by satellite, aircraft, and balloon observations, high (>80%) RHi regions are widespread within the TTL. Regions with the highest RHi are colocated with extensive cirrus. During boreal winter, the TTL RHi is highest over the Tropical Western Pacific (TWP) with a weaker maximum over South America and Africa. In the winter, TTL temperatures are coldest and upward motion is the greatest in the TWP. It is this upward motion, driving humid air into the colder upper troposphere that produces the persistent high RHi and cirrus formation. Back trajectory calculations show that comparable adiabatic and diabatic processes contribute to this upward motion. We construct a bulk model of TWP TTL water vapor transport that includes cloud nucleation and ice microphysics that quantifies how upward motion drives the persistent high RHi in the TTL region. We find that atmospheric waves triggering cloud formation regulate the RHi and that convection dehydrates the TTL. Our forward domain‐filling trajectory model is used to more precisely simulate the TTL spatial and vertical distribution of RHi. The observed RHi distribution is reproduced by the model, and we show that convection increases RHi below the base of the TTL with little impact on the RHi in the TTL region. Plain Language Summary Satellite, aircraft, and balloon observations show that the upper tropical tropospheric humidity is close to saturation. This high humidity is the result of the near‐continuous upward movement of water vapor from the midtroposphere into the colder upper troposphere that results in extensive cirrus formation. Bulk and trajectory model simulations show how this process works and that convective injection of water into the tropical upper troposphere is relatively unimportant.