New processes to counteract sedimentation of coarse dust particles are required for climate models to agree with aircraft observations

Abstract. Recent observations reveal coarser (d > 2.5 µm) mineral dust particles transported unexpectedly great distances from their Saharan source. Transport models represent this coarse dust transport poorly, which has important feedbacks on the Earth's radiative budget, carbon and hydrological cycles, and human health. In this study, we carry out sensitivity tests on the trans-Atlantic transport and deposition processes governing the long-range evolution of mineral dust size distribution in a climate model (HadGEM3-GA7.1). We test the sensitivity of coarser particle transport to sedimentation, convective and turbulent mixing, impaction scavenging, and dust shortwave absorption by removing or scaling the magnitude of these processes. Reductions in sedimentation of 80 % are shown to have the greatest impact on bringing coarse particle transport in the model into better agreement with in-situ aircraft observations. Other tested processes do not result in the multiple orders of magnitude changes in transported coarser dust mass required for agreement with observations. Since sedimentation is a well-understood physical process, we infer that some additional process/es which are misrepresented or not represented at all in the model must be acting to counteract sedimentation. We find that convective and turbulent mixing in the model have minimal impact on coarse particle long-range transport, but are key in controlling the vertical distribution in the Saharan air layer and marine boundary layer, respectively. This study adds to the growing body of evidence that points to processes involved in coarser mineral dust transport and deposition which are not represented accurately or at all in models, which counteract the sedimentation of coarse particles in the real-world. The work in this paper brings the community one step closer to better understanding and modelling of the full dust size distribution and its impacts on weather and climate.