Impact of the vertical velocity scheme on modeling transport in the tropical tropopause layer

Abstract

[1] To assess the impact of the vertical velocity scheme on modeling transport in the tropical tropopause layer (TTL), 3 month backward trajectories are initialized in the TTL for boreal winter and summer 2002. The calculations are done in either a kinematic scenario with pressure tendency as the vertical velocity or in a diabatic scenario with cross-isentropic velocity deduced from various diabatic heating rates due to radiation (clear sky, all sky) and latent, diffusive and turbulent heating. This work provides a guideline for assessing the sensitivity of trajectory and chemical transport model (CTM) results on the choice of the vertical velocity scheme. We find that many transport characteristics, such as time scales, pathways and dispersion, crucially depend on the vertical velocity scheme. The strongest tropical upwelling results from the operational European Centre for Medium-Range Weather Forecasts kinematic scenario with the time scale for ascending from 340 to 400 K of 1 month. For the ERA-Interim kinematic and total diabatic scenarios, this time scale is about 2 months, and for the all-sky scenario it is as long as 2.5 months. In a diabatic scenario, the whole TTL exhibits mean upward motion, whereas in a kinematic scenario, regions of subsidence occur in the upper TTL. However, some transport characteristics robustly emerge from the different scenarios, such as an enhancement of residence times between 350 and 380 K and a strong impact of meridional in-mixing from the extratropics on the composition of the TTL. Moreover, an increase of meridionally transported air from the summer hemisphere into the TTL (maximum for boreal summer) is found as an invariant feature among all the scenarios.

Insight from ozone and water vapour on transport in the tropical tropopause layer (TTL)

“…Figure 5 shows the position of the global (top panels) and in-situ trajectories (bottom panels) when tracing them back in time for 60 days. The main differences observed here, already noticed by Ploeger et al (2010) and Liu et al (2010), are the higher vertical dispersion and the occurrence of descent in the equatorial lower stratosphere for the kinematic trajectories. Consequently, the positions where the kinematic model calculations are initialised are more widespread than those of the diabatic calculations.…”

“…Crossisentropic vertical velocityθ = dθ/dt is taken from the forecast total diabatic heating rate, being the sum of all-sky radiative heating and all other diabatic heating terms, including latent heat release (see e.g., Fueglistaler et al, 2009b;Ploeger et al, 2010). Conversely, kinematic trajectories use the reanalysis vertical wind ω = dp/dt as vertical velocity.…”

Analyzing dynamical circulations in the tropical tropopause layer through empirical predictions of cirrus cloud distributions

“…It is tempting to use this result to weigh in on the discussion regarding the relative merits of the kinematic formulation versus the diabatic and Lagrangian cold point) of cloud probability from four data sources: kinematic trajectories using MERRA temperature and winds (blue bars), diabatic trajectories using MERRA temperature, winds, and total diabatic heating rates (gray), kinematic trajectories using GFS temperature and winds (red), and kinematic trajectories using ERA-interim temperature and winds (black). Shown are explained variances during boreal summer formulation [Danielsen, 1961;Schoeberl et al, 2003;Ploeger et al, 2010Ploeger et al, , 2011Schoeberl and Dessler, 2011]; however, diabatic heating rates from MERRA have known weaknesses-particularly during summer [Bergman et al, 2013;Wright and Fueglistaler, 2013]. Furthermore, there is no significant difference in the cloud predictions by the four data sets during winter (not shown; seven-season averages of r 2 are in the range 0.90-0.91 for all four data sources).…”

Transport of Antarctic stratospheric strongly dehydrated air into the troposphere observed during the HALO-ESMVal campaign 2012

“…The trajectories are driven by the ERA-Interim reanalysis horizontal winds and diabatic heating rates (Dee et al, 2011;Ploeger et al, 2010). To resolve transport processes in the troposphere influenced by the orography and transport processes in the stratosphere where adiabatic horizontal transport dominates, the hybrid σ -θ coordinate ζ is used (Mahowald et al, 2002).…”

“…To resolve transport processes in the troposphere influenced by the orography and transport processes in the stratosphere where adiabatic horizontal transport dominates, the hybrid σ -θ coordinate ζ is used (Mahowald et al, 2002). Therefore, in the stratosphere and in the UT/LS, potential temperature θ is employed as the vertical coordinate of the model and the cross-isentropic velocityθ = Q is deduced from the ERAInterim forecast total diabatic heating rates Q, including the effects of all-sky radiative heating, latent heat release and diffusive heating as described by Ploeger et al (2010). In the tropospheric region defined by the condition σ = 0.3, the vertical model coordinate smoothly transforms into an orography-following σ = p/p s coordinate (p -pressure; p s -surface pressure), with the vertical velocity transforming into the correspondingσ (Pommrich et al, 2014).…”