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 co-located 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 (FDF) 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.

Transport of ice into the stratosphere and the humidification of the stratosphere over the 21st century.

Climate models predict that tropical lower-stratospheric humidity will increase as the climate warms. We examine this trend in two state-of-the-art chemistry-climate models. Under high greenhouse gas emissions scenarios, the stratospheric entry value of water vapor increases by ~1 part per million by volume (ppmv) over this century in both models. We show with trajectory runs driven by model meteorological fields that the warming tropical tropopause layer (TTL) explains 50-80% of this increase. The remainder is a consequence of trends in evaporation of ice convectively lofted into the TTL and lower stratosphere. Our results further show that, within the models we examined, ice lofting is primarily important on long time scales - on interannual time scales, TTL temperature variations explain most of the variations in lower stratospheric humidity. Assessing the ability of models to realistically represent ice-lofting processes should be a high priority in the modeling community.

Stratospheric water vapor feedback.

We show here that stratospheric water vapor variations play an important role in the evolution of our climate. This comes from analysis of observations showing that stratospheric water vapor increases with tropospheric temperature, implying the existence of a stratospheric water vapor feedback. We estimate the strength of this feedback in a chemistry-climate model to be +0.3 W/(m(2)⋅K), which would be a significant contributor to the overall climate sensitivity. One-third of this feedback comes from increases in water vapor entering the stratosphere through the tropical tropopause layer, with the rest coming from increases in water vapor entering through the extratropical tropopause.

A determination of the cloud feedback from climate variations over the past decade.

Estimates of Earth's climate sensitivity are uncertain, largely because of uncertainty in the long-term cloud feedback. I estimated the magnitude of the cloud feedback in response to short-term climate variations by analyzing the top-of-atmosphere radiation budget from March 2000 to February 2010. Over this period, the short-term cloud feedback had a magnitude of 0.54 ± 0.74 (2σ) watts per square meter per kelvin, meaning that it is likely positive. A small negative feedback is possible, but one large enough to cancel the climate's positive feedbacks is not supported by these observations. Both long- and short-wave components of short-term cloud feedback are also likely positive. Calculations of short-term cloud feedback in climate models yield a similar feedback. I find no correlation in the models between the short- and long-term cloud feedbacks.