Abstract
Abstract. We use NAME, a trajectory model, to investigate the routes and timescales over which air parcels reach the tropical tropopause layer (TTL). Our aim is to assist the planning of aircraft campaigns focussed on improving knowledge of such transport. We focus on Southeast Asia and the Western Pacific which appears to be a particularly important source of air that enters the TTL. We first study the TTL above Borneo in November 2008, under neutral El Niño/Southern Oscillation (ENSO) conditions. Air parcels (trajectories) arriving in the lower TTL (below ~15 km) are most likely to have travelled from the boundary layer (BL; <1 km) above the West Pacific. Few air parcels found above ~16 km travelled from the BL in the previous 15 days. We then perform similar calculations for moderate El Niño (2006) and La Niña (2007) conditions and find year-to-year variability consistent with the phase of ENSO. Under El Niño conditions fewer air parcels travel from the BL to the TTL above Borneo. During the La Niña year, more air parcels travel from the BL to the mid and upper TTL (above ~15 km) than in the ENSO-neutral year, and again they do so from the BL above the West Pacific. We also find intra-month variability in all years, with day-to-day differences of up to an order of magnitude in the fraction of an idealised short-lived tracer travelling from the BL to the TTL above Borneo. These calculations were performed as a prelude to the SHIVA field campaign, which took place in Borneo during November 2011. So finally, to validate our approach, we consider measurements made in two previous campaigns. The features of vertical profiles of short-lived species observed in the TTL during CR-AVE and TC4 are in broad agreement with calculated vertical profiles of idealised short-lived tracers. It will require large numbers of observations to fully describe the statistical distribution of short-lived species in the TTL. This modelling approach should prove valuable in planning flights for the long-duration aircraft now capable of making such measurements.
Highlights
Catalytic cycles involving bromine are known to destroy ozone in the lower stratosphere (Yung et al, 1980; Salawitch et al, 2005)
The NAME trajectory model has been used to investigate the statistical behaviour of air masses reaching the tropical tropopause layer (TTL)
As our method assumes a uniform very short-lived substances (VSLS) concentration in the boundary layer, our results suggest that the measurements examined here were not affected by highly variable “hot spot” emissions
Summary
Catalytic cycles involving bromine are known to destroy ozone in the lower stratosphere (Yung et al, 1980; Salawitch et al, 2005). In a Lagrangian representation, Pisso et al (2010) added a parameterisation of deep convection to their model and found that the fraction of an idealised VSLS emitted at the surface, that goes on to reach the upper TTL, increased by up to an order of magnitude. It is clear that the influence of deep convection on atmospheric composition and chemistry remains an active area of research, with many remaining uncertainties. We consider altitudes at which aircraft might fly rather than using any particular definition of the TTL in which absolute altitude might vary year-to-year (e.g. the cold point as an upper boundary; see Gettelman and Forster, 2002)
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