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

摘要

A new global real-time Lagrangian diagnostic system forstratosphere-troposphere exchange (STE) developed for Environment Canada (EC)has been delivering daily archived data since July 2010. The STE calculationsare performed following the Lagrangian approach proposed inBourqui (2006) using medium-range, high-resolution operational globalweather forecasts. Following every weather forecast, trajectories are startedfrom a dense three-dimensional grid covering the globe, and are calculatedforward in time for six days of the forecast. All trajectories crossingeither the dynamical tropopause (±2 PVU) or the 380 K isentrope andhaving a residence time greater than 12 h are archived, and also used tocalculate several diagnostics. This system provides daily global STEforecasts that can be used to guide field campaigns, among otherapplications. The archived data set offers unique high-resolution informationon transport across the tropopause for both extra-tropical hemispheres andthe tropics. This will be useful for improving our understanding of STEglobally, and as a reference for the evaluation of lower-resolution models.This new data set is evaluated here against measurements taken during aballoon sonde campaign with daily launches from three stations in easternCanada (Montreal, Egbert, and Walsingham) for the period 12 July to 4 August2010. The campaign found an unexpectedly high number of observedstratospheric intrusions: 79% (38%) of the profiles appear to show thepresence of stratospheric air below than 500 hPa (700 hPa). An objectiveidentification algorithm developed for this study is used to identify layersin the balloon-sonde profiles affected by stratospheric air and to evaluatethe Lagrangian STE forecasts. We find that the predictive skill for theoverall intrusion depth is very good for intrusions penetrating down to 300and 500 hPa, while it becomes negligible for intrusions penetrating below700 hPa. Nevertheless, the statistical representation of these deepintrusions is reasonable, with an average bias of 24%. Evaluation of theskill at representing the detailed structures of the stratospheric intrusionsshows good predictive skill down to 500 hPa, reduced predictive skillbetween 500 and 700 hPa, and none below. A significant low statistical biasof 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 themcoincide with candidate clusters of trajectories that pass through Montreal,but that are too dispersed to be detected in the close neighbourhood of thestation. Within the limits of this study, this allows us to expect anegligible bias throughout the troposphere in the spatially averaged STEfrequency derived from this data set, for example in climatological maps ofSTE mass fluxes. This first evaluation is limited to eastern Canada in onesummer month with a high frequency of stratospheric intrusions, and furtherwork is needed to evaluate this STE data set in other months and locations.