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

Abstract

Bromoform (CHBr₃) 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 CHBr₃ 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 CHBr₃ in the global TTL, and up to three times more CHBr₃ 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 CHBr₃ 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 CHBr₃ 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 CHBr₃ in the TTL is enhanced over the Maritime Continent in both model versions. The enhancement is larger, and the peak in CHBr₃ 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 CHBr₃ 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.