Abstract
Abstract. Isentropic mixing across and above the subtropical jet is a significant mechanism for stratosphere–troposphere exchange. In this work, we show new observational evidence on the role of this process in moistening the lowermost stratosphere. The new measurement, obtained from the Spatial Heterodyne Observations of Water (SHOW) instrument during a demonstration flight on the NASA's ER-2 high-altitude research aircraft, captured an event of poleward water vapour transport, including a fine-scale (vertically <∼1 km) moist filament above the local tropopause in a high-spatial-resolution two-dimensional cross section of the water vapour distribution. Analysis of these measurements combined with ERA5 reanalysis data reveals that this poleward mixing of air with enhanced water vapour occurred in the region of a double tropopause following a large Rossby wave-breaking event. These new observations highlight the importance of high-resolution measurements in resolving processes that are important to the lowermost-stratosphere water vapour budget.
Highlights
The distribution of water vapour in the upper troposphere and lower stratosphere (UTLS) plays a critical role in the physical processes that couple the region to Earth’s climate
We present a case study of high-spatialresolution observations of UTLS water vapour that has been enabled by new measurement technology
Using the ECMWF ERA5 reanalysis product, we demonstrate that the transport is driven by a large-scale Rossby wave-breaking event and in association with the occurrence of a double tropopause
Summary
The distribution of water vapour in the upper troposphere and lower stratosphere (UTLS) plays a critical role in the physical processes that couple the region to Earth’s climate. The measurements, using the Spatial Heterodyne Observations of Water (SHOW) instrument (Langille et al, 2019) aboard the NASA ER-2 research aircraft during a demonstration flight, captured an event of water vapour transport into the lowermost stratosphere across the subtropical jet. The flight path (Fig. 1), across several degrees of latitude off the western coast of North America, from roughly 34 to 48◦ N along the 124.5◦ W longitude line, was chosen in an attempt to observe potential mixing near the tropopause break in a region known to have a relatively frequent occurrence of double tropopauses in the summer season (Añel et al, 2008) This mixing process often produces fine-scale filaments that are difficult for the satellite measurements and the large-scale models to resolve.
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