Abstract
Abstract. Stratospheric water vapour (SWV) is a climatically important atmospheric constituent due to its impacts on the radiation budget and atmospheric chemical composition. Despite the important role of SWV in the climate system, the processes controlling the distribution and variation in water vapour in the upper troposphere and lower stratosphere (UTLS) are not well understood. In order to better understand the mechanism of transport of water vapour through the tropopause, this study uses the high-resolution Global Environmental Multiscale model of the Environment and Climate Change Canada to simulate a lower stratosphere moistening event over North America. Satellite remote sensing and aircraft in situ observations are used to evaluate the quality of model simulation. The main focus of this study is to evaluate the processes that influence the lower stratosphere water vapour budget, particularly the direct water vapour transport and the moistening due to the ice sublimation. In the high-resolution simulations with horizontal grid spacing of less than 2.5 km, it is found that the main contribution to lower stratospheric moistening is the upward transport caused by the breaking of gravity waves. In contrast, for the lower-resolution simulation with horizontal grid spacing of 10 km, the lower stratospheric moistening is dominated by the sublimation of ice. In comparison with the aircraft in situ observations, the high-resolution simulations predict the water vapour content in the UTLS well, while the lower-resolution simulation overestimates the water vapour content. This overestimation is associated with the overly abundant ice in the UTLS along with a sublimation rate that is too high in the lower stratosphere. The results of this study affirm the strong influence of overshooting convection on the lower stratospheric water vapour and highlight the importance of both dynamics and microphysics in simulating the water vapour distribution in the UTLS region.
Highlights
Stratospheric water vapour (SWV) strongly influences the Earth radiation budget (IPCC, 2013) and stratospheric chemistry (e.g., Anderson et al, 2012)
This study shows that the ratio of water vapour content in the Global climate models (GCMs) to that from satellite observations can be as large as two to five in the mid-latitude upper troposphere and lower stratosphere (UTLS) region
Global reanalyses suffer from SWV biases, including the ModernEra Retrospective Analysis for Research and Applications (MERRA), its newer release MERRA2 and the Interim Reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) (Jiang et al, 2015)
Summary
Stratospheric water vapour (SWV) strongly influences the Earth radiation budget (IPCC, 2013) and stratospheric chemistry (e.g., Anderson et al, 2012). Water can be transported to the lowermost extratropical stratosphere by poleward transport from the TTL, by isentropic transport due to planetary wave activity from the tropical troposphere and by deep convection in the extratropics Among these mechanisms, the vertical transport by mid-latitude convection, demonstrated to be impactful by studies using in situ and remote sensing measurements, remains poorly understood (Poulida et al, 1996; Hegglin et al, 2004; Dessler and Sherwood, 2004; Ray et al, 2004; Hanisco et al, 2007; Weinstock et al, 2007; Homeyer et al, 2014, 2017; Sun and Huang, 2015; Smith et al, 2017). We conclude with a summary of the findings and perspectives for further studies
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