A new global real-time Lagrangian diagnostic system for stratosphere-troposphere exchange: evaluation during a balloon sonde campaign in eastern Canada

Abstract

Abstract.A new global real-time Lagrangian diagnostic system for stratosphere-troposphere exchange (STE) developed for Environment Canada (EC) has been delivering daily archived data since July 2010. The STE calculations are performed following the Lagrangian approach proposed in Bourqui (2006) using medium-range, highresolution operational global weather forecasts. Following every weather forecast, trajectories are started from a dense three-dimensional grid covering the globe, and are calculated forward in time for six days of the forecast. All trajectories crossing either the dynamical tropopause (±2 PVU) or the 380 K isentrope and having a residence time greater than 12 h are archived, and also used to calculate several diagnostics. This system provides daily global STE forecasts that can be used to guide field campaigns, among other applications. The archived data set offers unique high-resolution information on transport across the tropopause for both extra-tropical hemispheres and the tropics. This will be useful for improving our understanding of STE globally, and as a reference for the evaluation of lower-resolution models. This new data set is evaluated here against measurements taken during a balloon sonde campaign with daily launches from three stations in eastern Canada (Montreal, Egbert, and Walsingham) for the period 12 July to 4 August 2010. The campaign found an unexpectedly high number of observed stratospheric intrusions: 79 % (38 %) of the profiles appear to show the presence of stratospheric air below than 500 hPa (700 hPa). An objective identification algorithm developed for this study is used to identify layers in the balloon-sonde profiles affected by stratospheric air and to evaluate the Lagrangian STE forecasts. We find that the predictive skill for the overall intrusion depth is very good for intrusions penetrating down to 300 and 500 hPa, while it becomes negligible for intrusions penetrating below 700 hPa. Nevertheless, the statistical representation of these deep intrusions is reasonable, with an average bias of 24 %. Evaluation of the skill at representing the detailed structures of the stratospheric intrusions shows good predictive skill down to 500 hPa, reduced predictive skill between 500 and 700 hPa, and none below. A significant low statistical bias of about 30 % is found in the layer between 500 to 700 hPa. However, analysis of missed events at one site, Montreal, shows that 70 % of them coincide with candidate clusters of trajectories that pass through Montreal, but that are too dispersed to be detected in the close neighbourhood of the station. Within the limits of this study, this allows us to expect a negligible bias throughout the troposphere in the spatially averaged STE frequency derived from this data set, for example in climatological maps of STE mass fluxes. This first evaluation is limited to eastern Canada in one summer month with a high frequency of stratospheric intrusions, and further work is needed to evaluate this STE data set in other months and locations.

Input Data Requirements for Lagrangian Trajectory Models

“…Many different types of trajectory models have been developed over the years (Stohl 1998), and a variety of different models are currently in use around the world for research and operational applications. Model trajectories have been verified at different scales using balloon tracking (Reisinger and Mueller 1983;Knudsen et al 1996;Baumann and Stohl 1997), tracer releases (Haagenson et al 1987;Draxler 1991;Stohl et al 1998), satellite observations of volcanic SO^ clouds in the stratosphere (Schoeberl et al, 1993), satellite ozone data (Bowman 1993), in situ observations of natural tracers (Bourqui et al 2012), and comparisons with conserved meteorological parameters (Stohl and Seibert 1998), Model trajectories have been verified at different scales using balloon tracking (Reisinger and Mueller 1983;Knudsen et al 1996;Baumann and Stohl 1997), tracer releases (Haagenson et al 1987;Draxler 1991;Stohl et al 1998), satellite observations of volcanic SO^ clouds in the stratosphere (Schoeberl et al, 1993), satellite ozone data (Bowman 1993), in situ observations of natural tracers (Bourqui et al 2012), and comparisons with conserved meteorological parameters (Stohl and Seibert 1998),…”

“…The accuracy of numerical solutions to the trajectory equations, and the potential sources of error, have been investigated by many authors for a wide range of meteorological situations (Walmsley and Mailhot 1983;Kuo et al 1985;Kahl and Samson 1986;Merrill et al 1986;Rolph and Draxler 1990;Schoeberl et al 1992;Bowman 1993;Doty and Perkey 1993;Seibert 1993;Stohl et al 1995;Stohl and Seibert 1998;Bourqui 2006;Davis and Dacre 2009). Errors due to spatial and temporal interpolation of wind fields can be quantified by carrying out model simulations at high resolution and using the meteorological data at the original and at degraded resolutions for trajectory calculations.…”

Observation of a tropopause fold by MARA VHF wind-profiler radar and ozonesonde at Wasa, Antarctica: comparison with ECMWF analysis and a WRF model simulation

“…Assimilation of satellite limb measurements has been proposed as one way to improve detectability of such stratospheric intrusions using global meteorological databases (Semane et al, 2007). A new global real-time Lagrangian diagnostic system for STE has been developed by Bourqui et al (2012) and first tests, using comparison with a campaign of daily ozonesonde launches in Eastern Canada, showed an unexpectedly high number of observed stratospheric intrusions with stratospheric air below 500 hPa (700 hPa) found in 79 % (38 %) of profiles.…”

Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS) experiment: design, execution and science overview

“…Table 3 shows the key launch sites during BORTAS-B; for interest, Table 3 also shows the corresponding launches for 2010 which were supporting another measurement intensive to understand stratosphere-troposphere exchange (Bourqui et al, 2012) were used to help evaluate the GEOS-Chem atmospheric chemistry transport model (Parrington et al, 2012) as part of modelling activities supporting BORTAS-B.…”