Vertical structure of stratospheric water vapour trends derived from merged satellite data

Abstract

Stratospheric water vapour is a powerful greenhouse gas. The longest available record from balloon observations over Boulder, Colorado, USA shows increases in stratospheric water vapour concentrations that cannot be fully explained by observed changes in the main drivers, tropical tropopause temperatures and methane. Satellite observations could help resolve the issue, but constructing a reliable long-term data record from individual short satellite records is challenging. Here we present an approach to merge satellite data sets with the help of a chemistry-climate model nudged to observed meteorology. We use the models' water vapour as a transfer function between data sets that overcomes issues arising from instrument drift and short overlap periods. In the lower stratosphere, our water vapour record extends back to 1988 and water vapour concentrations largely follow tropical tropopause temperatures. Lower and mid-stratospheric long-term trends are negative, and the trends from Boulder are shown not to be globally representative. In the upper stratosphere, our record extends back to 1986 and shows positive long-term trends. The altitudinal differences in the trends are explained by methane oxidation together with a strengthened lower-stratospheric and a weakened upper-stratospheric circulation inferred by this analysis. Our results call into question previous estimates of surface radiative forcing based on presumed global long-term increases in water vapour concentrations in the lower stratosphere.

The airborne mass spectrometer AIMS – Part 1: AIMS-H<sub>2</sub>O for UTLS water vapor measurements

“…Including H 2 O measurements with a frost point hygrometer (Voigt et al, 2010) from a series of campaigns, we envisage constructing a database of high-quality in situ H 2 O measurements in the UTLS which can be used for comparison with lidar observations (Groß et al, 2014), meteorological and balloon sondes (Hurst et al, 2011), satellite data (Hegglin et al, 2013) and for model validation (Hegglin et al, 2014;Solomon et al, 2010). With the flexible airborne mass spectrometer AIMS, we have developed a multitool to address key issues concerning atmospheric composition of the UTLS and processes related to trace gas transport, cloud formation and climate.…”

The global methane budget 2000–2012

“…The oxidation of methane in the stratosphere produces significant amounts of water vapour, which has a positive radiative forcing, and stimulates the production of OH through its reaction with atomic oxygen (Forster et al, 2007). Stratospheric methane thus contributes significantly to the observed variability and trend in stratospheric water vapour (Hegglin et al, 2014). Uncertainties in the chemical loss of stratospheric methane are large, due to uncertain interannual variability in stratospheric transport as well as through its chemical interactions with stratospheric ozone (Portmann et al, 2012).…”

An updated version of a gap-free monthly mean zonal mean ozone database

“…This is very similar to the approach used in our study. While Hegglin et al (2014) used the CCM output to adjust monthly mean values of the different 25 instruments, here the SLIMCAT output is used to adjust individual measurements with a correction that is based on zonal mean comparisons (see Sect. 3.2).…”

“…In Sofieva et al (2014) the gap-free ozone fields of a highly temporally and spatially resolved CTM run were used to characterize sampling biases for coarse satellite samplers when their measurements were used for the calculation of monthly mean zonal mean ozone values. Hegglin et al (2014) used a CCM that was nudged to ERA-Interim reanalysis to correct for offsets and drifts between stratospheric water vapor measurements from multiple satellite instruments. This is very similar to the approach used in our study.…”