The simulation of mineral dust in the United Kingdom Earth System Model UKESM1

Abstract

Abstract. Mineral dust plays an important role in Earth system models and is linked to many components, including atmospheric wind speed, precipitation and radiation, surface vegetation cover and soil properties and oceanic biogeochemical systems. In this paper, the dust scheme in the first configuration of the United Kingdom Earth System Model UKESM1 is described, and simulations of dust and its radiative effects are presented and compared with results from the parallel coupled atmosphere–ocean general circulation model (GCM) HadGEM3-GC3.1. Not only changes in the driving model fields but also changes in the dust size distribution are shown to lead to considerable differences to the present-day dust simulations and to projected future changes. UKESM1 simulations produce a present-day, top-of-the-atmosphere (ToA) dust direct radiative effect (DRE – defined as the change in downward net flux directly due to the presence of dust) of 0.086 W m−2 from a dust load of 19.5 Tg. Under climate change pathways these values decrease considerably. In the 2081–2100 mean of the Shared Socioeconomic Pathway SSP5–8.45 ToA DRE reaches 0.048 W m−2 from a load of 15.1 Tg. In contrast, in HadGEM3-GC3.1 the present-day values of −0.296 W m−2 and 15.0 Tg are almost unchanged at −0.289 W m−2 and 14.5 Tg in the 2081–2100 mean. The primary mechanism causing the differences in future dust projections is shown to be the vegetation response, which dominates over the direct effects of warming in our models. Though there are considerable uncertainties associated with any such estimates, the results presented demonstrate both the importance of the size distribution for dust modelling and also the necessity of including Earth system processes such as interactive vegetation in dust simulations for climate change studies.

The projected future degradation in air quality is caused by more abundant natural aerosols in a warmer world

“…NorESM2-LM uses a fixed map of soil erodibility and clay content, but includes interactive vegetation (and soil moisture) effects on dust emissions, such as leaf area index and canopy height 49 . UKESM1-0-LL also includes the effects of interactive vegetation on dust emissions, and a prior analysis showed good agreement between UKESM1-0-LL simulated and observed dust changes 50 . Figure 3c, d shows a multi-model mean increase in surface wind speed throughout most of the region, with enhanced westerly/southwesterly flow, particularly during JAS (Supplementary Fig.…”

Assessing the Impact of Self‐Lofting on Increasing the Altitude of Black Carbon in a Global Climate Model

“…This represents the emission, transport and deposition of BC, organic matter, sulfate and sea salt across a range of soluble and insoluble size modes in which chemical components are internally mixed. Mineral dust is represented separately with a 6‐bin scheme (Woodward et al., 2022). Whilst a detailed description and evaluation of the aerosol performance in UKESM1 is given in Mulcahy et al.…”

“…This represents the emission, transport and deposition of BC, organic matter, sulfate and sea salt across a range of soluble and insoluble size modes in which chemical components are internally mixed. Mineral dust is represented separately with a 6-bin scheme (Woodward et al, 2022). Whilst a detailed description and evaluation of the aerosol performance in UKESM1 is given in Mulcahy et al (2020) and an assessment of the aerosol processes and effective radiative forcings were presented in Mulcahy et al (2018), some aspects of the aerosol simulation are also summarized below.…”

A modern-day Mars climate in the Met Office Unified Model: dry simulations

“…Atmospheric dust is absent upon initialisation and is calculated throughout the model simulation. Dust particles are transported by atmospheric dynamics, turbulence (Lock et al, 2000), saltation (for uplifting larger particles, Woodward, 2001;Woodward et al, 2022) and dry deposition. Absorption and scattering of short-/long- wave radiation is calculated using Mie theory with the assumption that dust particles are spherical.…”

“…Both schemes omit some dust microphysics due to the difficulty in accurately characterising them within the dust scheme, namely surface crusting and surface re-entrainment (Woodward, 2001;Wolff et al, 2009;Madeleine et al, 2011;Woodward et al, 2022), which likely contributes to the disparity between GCM outputs (without forcing) and observations. The differences in spatial distribution could also be caused by the differences in microphysics parameterisation between models, and therefore refining this would undoubtedly improve the representation of dust on Mars.…”