On the emissions and transport of bromoform: sensitivity to model resolution and emission location

Abstract

Abstract. Bromoform (CHBr 3 ) is a short-lived species with an important but poorly quantified ocean source. It can be transported to the Tropical Tropopause Layer (TTL), in part by rapid, deep convective lifting, from where it can influence the global stratospheric ozone budget. In a modelling study, we investigate the importance of the regional distribution of the emissions and of model resolution for the transport of bromoform to the TTL. We use two idealized CHBr 3 emission fields (one coastal, one uniformly distributed across the oceans) implemented in high-and coarse-resolution (HR and CR) versions of the same global model and focus on February as the period of peak convection in the West Pacific. Using outgoing long-wave radiation and precipitation as metrics, the HR version of the model is found to represent convection better. In the more realistic HR model version, the coastal emission scenario leads to 15-20 % more CHBr 3 in the global TTL, and up to three times more CHBr 3 in the TTL over the Maritime Continent, than when uniform emissions of the same tropical magnitude are employed. Using the uniform emission scenario in both model versions, the distribution of CHBr 3 at 15.7 km (approximately the level of zero net radiative heating) is qualitatively consistent with the differing geographic distributions of convection. However, averaged over the whole tropics, the amount of CHBr 3 in the TTL in the two model versions is similar. Using the coastal scenario, in which emissions are particularly high in the Maritime Continent because of its long coastlines, the mixing ratio of CHBr 3 in the TTL is enhanced over the Maritime Continent in both model versions. The enhancement is larger, and the peak in CHBr 3 mixing ratio occurs at a higher altitude, in the HR model version. Our regional-scale results indicate that using aircraft measurements and coarse global models to infer CHBr 3 emissions will be very difficult, particularly if (as is possible) emissions are distributed heterogeneously and in regions of strong convective activity. In contrast, the global-scale agreement between our CR and HR calculations suggests model resolution is less vital for studies focused on the transport of bromine into the global stratosphere.

A multi-model intercomparison of halogenated very short-lived substances (TransCom-VSLS): linking oceanic emissions and tropospheric transport for a reconciled estimate of the stratospheric source gas injection of bromine

“…Such parameterisations are often complex, relying on assumptions regarding detrainment levels, they trigger thresholds for shallow, mid-level and/or deep convection and they vary in their approach to computing updraft (and downdraft) mass fluxes. Furthermore, the vertical transport of model tracers is also sensitive to interactions of the convective parameterisation with the boundary layer mixing scheme (also parameterised) (Rybka and Tost, 2014). On the above basis and considering that the TransCom-VSLS models implement these processes in different ways (Table 2), it was not possible to disentangle transport effects within the scope of this project.…”

Surface fluxes of bromoform and dibromomethane over the tropical western Pacific inferred from airborne in situ measurements

“…Previous studies have highlighted similar issues (e.g. Russo et al, 2015). Figure 3 shows, however, that CAST and CONTRAST data can only reduce flux uncertainties by about 10 %-60 % over the study regions at this coarse model resolution, limited by the density and coverage of the available data.…”

“…This fitting can be achieved using heuristic techniques or more established Bayesian optimization methods (e.g. Liang et al, 2010;Ordóñez et al, 2012;Ashfold et al, 2014;Russo et al, 2015). The short atmospheric lifetime of CHBr 3 poses particular difficulties for the top-down approach because atmospheric mole fractions are highly variable (Ashfold et al, 2014).…”

Transport of short-lived halocarbons to the stratosphere over the Pacific Ocean

“…The transport of VSL halocarbons into the lower stratosphere is by ascent through the tropical tropopause layer (TTL) (Fueglistaler et al, 2009). An important factor influencing the loading of the VSL bromocarbons in the TTL is the strength of the convective transport from the boundary layer where the bromocarbons are emitted (Hosking et al, 2012;Yang et al, 2014;Russo et al, 2015;Hepach et al, 2015;Fuhlbrügge et al, 2016;Krzysztofiak et al, 2018). This is poorly quantified and, when taken together with the large variations in boundary layer concentrations and the uncertainties associated with the model representation of convection, limits our ability to model the bromine budget in the current and future atmosphere (Liang et al, 2010(Liang et al, , 2014Russo et al, 2011Russo et al, , 2015Schofield et al, 2011;Aschmann and Sinnhuber, 2013;Fernandez et al, 2014;Hossaini et al, 2016;Krzysztofiak et al, 2018).…”