Sensitivities of modelled water vapour in the lower stratosphere: temperature uncertainty, effects of horizontal transport and small-scale mixing

Poshyvailo, Liubov; Müller, Rolf; Konopka, Paul; Günther, Gebhard; Riese, Martin; Podglajen, Aurélien; Ploeger, Felix

Water vapour (inline-formulaH2O) in the upper troposphere and lower stratosphere (UTLS) has a significant role for global radiation. A realistic representation of inline-formulaH2O is therefore critical for accurate climate model predictions of future climate change. In this paper we investigate the effects of current uncertainties in tropopause temperature, horizontal transport and small-scale mixing on simulated inline-formulaH2O in the lower stratosphere (LS).

To assess the sensitivities of simulated inline-formulaH2O, we use the Chemical Lagrangian Model of the Stratosphere (CLaMS). First, we examine CLaMS, which is driven by two reanalyses, from the European Centre of Medium-Range Weather Forecasts (ECMWF) ERA-Interim and the Japanese 55-year Reanalysis (JRA-55), to investigate the robustness with respect to the meteorological dataset. Second, we carry out CLaMS simulations with transport barriers along latitude circles (at the Equator, 15 and 35inline-formula N/S) to assess the effects of horizontal transport. Third, we vary the strength of parametrized small-scale mixing in CLaMS.

Our results show significant differences (about 0.5 ppmv) in simulated stratospheric inline-formulaH2O due to uncertainties in the tropical tropopause temperatures between the two reanalysis datasets, JRA-55 and ERA-Interim. The JRA-55 based simulation is significantly moister when compared to ERA-Interim, due to a warmer tropical tropopause (approximately 2 K). The transport barrier experiments demonstrate that the Northern Hemisphere (NH) subtropics have a strong moistening effect on global stratospheric inline-formulaH2O. The comparison of tropical entry inline-formulaH2O from the sensitivity 15inline-formula N/S barrier simulation and the reference case shows differences of up to around 1 ppmv. Interhemispheric exchange shows only a very weak effect on stratospheric inline-formulaH2O. Small-scale mixing mainly increases troposphere–stratosphere exchange, causing an enhancement of stratospheric inline-formulaH2O, particularly along the subtropical jets in the summer hemisphere and in the NH monsoon regions. In particular, the Asian and American monsoon systems during a boreal summer appear to be regions especially sensitive to changes in small-scale mixing, which appears crucial for controlling the moisture anomalies in the monsoon UTLS. For the sensitivity simulation with varied mixing strength, differences in tropical entry inline-formulaH2O between the weak and strong mixing cases amount to about 1 ppmv, with small-scale mixing enhancing inline-formulaH2O in the LS.

The sensitivity studies presented here provide new insights into the leading processes that control stratospheric inline-formulaH2O, which are important for assessing and improving climate model projections.

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Poshyvailo, Liubov / Müller, Rolf / Konopka, Paul / et al: Sensitivities of modelled water vapour in the lower stratosphere: temperature uncertainty, effects of horizontal transport and small-scale mixing. 2018. Copernicus Publications.

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