1Department of Physics, University of Oxford, UK
2Co-operative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
3Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA
4National Centre for Atmospheric Science, University of Leeds, UK
5School of Earth and Environment, University of Leeds, UK
6Met. Office Hadley Centre, Exeter, UK
7National Centre for Atmospheric Science, University of Cambridge, UK
8Department of Chemistry, University of Cambridge, UK
Abstract. Evaluation of the aerosol schemes in current climate models is dependent upon the available observational data. In-situ observations from flight campaigns can provide valuable data about the vertical distribution of aerosol that is difficult to obtain from satellite or ground-based platforms, although they are localised in space and time. Using single-particle soot-photometer (SP2) measurements from the HIAPER Pole-to-Pole Observations (HIPPO) campaign, which consists of many vertical profiles over a large region of the Pacific, we evaluate the meridional and vertical distribution of black carbon (BC) aerosol simulated by the HadGEM3-UKCA and ECHAM5-HAM2 models. Both models show a similar pattern of overestimating the BC column burden compared to that derived from the observations, in many areas by an order of magnitude. However, by sampling the simulated BC mass mixing ratio along the flight track and comparing to the observations, we show that this discrepancy has a rather different vertical structure in the two models.
Using this methodology, we conduct sensitivity tests on two specific elements of the models: biomass-burning emissions and scavenging by convective precipitation. We show that, by coupling the convective scavenging more tightly with convective transport, both the column burden and vertical distribution of BC in HadGEM3–UKCA are significantly improved with respect to the observations, demonstrating the importance of a realistic representation of this process. In contrast, updating from GFED2 to GFED3.1 biomass-burning emissions makes a more modest improvement in both models, which is not statistically significant.
We also demonstrate the important role that nudged simulations (where the large-scale model dynamics are continuously relaxed towards a reanalysis) can play in this type of evaluation, allowing statistically significant differences between configurations of the aerosol scheme to be seen where the differences between the corresponding free-running simulations would not be significant.