Stratosphere-troposphere exchange from the Lagrangian perspective: a case study and method sensitivities

Abstract

Abstract. An important part of extra-tropical stratosphere-to-troposphere transport occurs in association with baroclinic wave breaking and cut-off decay at the tropopause. In the last decade many studies have attempted to estimate stratosphere-troposphere exchange (STE) in such synoptic events with various methods, and more recently efforts have been made to inter-compare these methods. These inter-comparisons show large variations between estimates from different methods. This large uncertainty points to a need to thoroughly evaluate such methods, assess the realism of the resulting STE estimates and determine the sensitivities to intrinsic parameters of the methods. The present study focuses on a trajectory-based Lagrangian method which has been applied in the past to climatological studies. This method is applied here to the quantification of STE in the context of a typical baroclinic wave breaking event. The analysis sheds light on (i) the complex three-dimensional temporal and spatial structures that are associated with the rapid inflow of stratospheric air into the troposphere, (ii) the variation of STE mass flux with the choice of the dynamical tropopause definition within 1.5 to 5 PVU, (iii) the sensitivity of the results to resolution, and in particular the minimum spatial resolution of 1°×1° required to reasonably capture STE fluxes in this wave breaking event, (iv) the effective removal of spurious exchange events using a threshold residence time larger than 8 h.

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

“…Many previous studies have investigated the mechanisms leading to extratropical STE in the Northern Hemisphere and have shown that STE typically occurs during the breaking of baroclinic waves at synoptic-to meso-scales and involves a variety of processes such as those associated with clouds, radiative relaxation, gravity wave breaking and wind shear turbulence (Shapiro, 1978;Langford et al, 1996;Zierl and Wirth, 1997;Stohl et al, 2003;Gray, 2003;Mullendore et al, 2005;Bourqui, 2006). This complexity is unimportant when considering only the global net flux of mass across the tropopause (Holton et al, 1995;Appenzeller et al, 1996).…”

“…The approach proposed by Bourqui (2006) is applied separately to each EC 10-day 00:00 UTC operational global weather forecast. These global weather forecasts are calculated at EC's Canadian Meteorological Centre using the Global Environmental Multi-scale (GEM) numerical weather prediction model (Côté et al, 1998a,b) and provide hourly meteorological fields with horizontal grid spacing of 0.3 × 0.3 • , and 80 hybrid vertical levels up to 0.1 hPa.…”

“…This ill-defined character is probably responsible for a good part of the large range of estimates of separated STE fluxes found in the literature (e.g., Hoerling et al, 1993;Gettelman and Sobel, 2000;Sprenger and Wernli, 2003). To render the fluxes well-defined, it is necessary to include an explicit scale constraint such as the residence time (Wernli and Bourqui, 2002;Hall and Holzer, 2003;Bourqui, 2006). In such a case, the parcel is required to reside on either side of the tropopause for a time interval larger than a given threshold.…”

“…An accurate estimate of separate S→T and T→S mass fluxes requires to resolve the complex processes leading to STE with their underlying scales. Bourqui (2006) suggests that consideration of hourly meteorological fields with horizontal resolution 0.5 × 0.5 • is necessary to resolve the most important contributions from these processes on STE. Furthermore, the nature of the separate S→T and T→S mass fluxes is such that they become infinite at infinitely small scales (Hall and Holzer, 2003).…”

“…These data have been calculated daily since July 2010 at Environment Canada (EC) following the methodology introduced in Bourqui (2006). They consist of global, five-day STE forecasts calculated daily using the 10-day global weather forecast initiated at 00:00 UTC.…”