Abstract
Abstract. In this study we examine the simulated downward transport and mixing of stratospheric air into the upper tropical troposphere as observed on a research flight during the SCOUT-O3 campaign in connection with a deep convective system. We use the Advanced Research Weather and Research Forecasting (WRF-ARW) model with a horizontal resolution of 333 m to examine this downward transport. The simulation reproduces the deep convective system, its timing and overshooting altitudes reasonably well compared to radar and aircraft observations. Passive tracers initialised at pre-storm times indicate the downward transport of air from the stratosphere to the upper troposphere as well as upward transport from the boundary layer into the cloud anvils and overshooting tops. For example, a passive ozone tracer (i.e. a tracer not undergoing chemical processing) shows an enhancement in the upper troposphere of up to about 30 ppbv locally in the cloud, while the in situ measurements show an increase of 50 ppbv. However, the passive carbon monoxide tracer exhibits an increase, while the observations show a decrease of about 10 ppbv, indicative of an erroneous model representation of the transport processes in the tropical tropopause layer. Furthermore, it could point to insufficient entrainment and detrainment in the model. The simulation shows a general moistening of air in the lower stratosphere, but it also exhibits local dehydration features. Here we use the model to explain the processes causing the transport and also expose areas of inconsistencies between the model and observations.
Highlights
The air in the tropical upper troposphere is subject to mean upward transport: above the level of zero radiative heating, the Brewer–Dobson circulation leads to slow upwelling, which can be penetrated by rapid overshooting convection (Corti et al, 2006; Pommereau, 2010; Randel and Jensen, 2013)
A number of passive tracers have been introduced to study the role of transport, including transport from the stratosphere to the troposphere
This study presents ozone and carbon monoxide in situ measurements that indicate downward transport from the stratosphere
Summary
The air in the tropical upper troposphere is subject to mean upward transport: above the level of zero radiative heating, the Brewer–Dobson circulation leads to slow upwelling, which can be penetrated by rapid overshooting convection (Corti et al, 2006; Pommereau, 2010; Randel and Jensen, 2013). Turrets were performed, which penetrated into the stratosphere up to potential temperatures of 417 K; see Corti et al (2008) and de Reus et al (2009) for details These studies show that the observed cloud particles were a result of convective overshooting and did not form in situ. The kinematic trajectories were calculated with LAGRANTO (Wernli and Davies, 1997) based on ECMWF operational analysis fields with a horizontal grid spacing of 0.125◦ and 60 vertical levels and were initialised on 30 November 2005 at 06:00 UTC, at different altitudes, i.e., 75, 80, 85, 90 hPa, above the Tiwi Islands From this analysis it was concluded that the measured tracers were not significantly influenced by a large-scale stratospheric filament (not shown), but were rather modified by local processes. Overall, the NSSL scheme performed best, and was chosen for the rest of the study
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