The global dust cycle and uncertainty in CMIP5 (Coupled Model Intercomparison Project phase 5) models

Abstract

Abstract. The dust cycle is an important component of the Earth system and has been implemented in climate models and Earth system models (ESMs). An assessment of the dust cycle in these models is vital to address their strengths and weaknesses in simulating dust aerosol and its interactions with the Earth system and enhance the future model developments. This study presents a comprehensive evaluation of the global dust cycle in 15 models participating in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The various models are compared with each other and with an aerosol reanalysis as well as station observations. The results show that the global dust emission in these models varies by a factor of 4–5 for the same size range. The models generally agree with each other and observations in reproducing the “dust belt”, which extends from North Africa, the Middle East, Central and South Asia to East Asia, although they differ greatly in the spatial extent of this dust belt. The models also differ in other dust source regions such as North America and Australia. We suggest that the coupling of dust emission with dynamic vegetation can enlarge the range of simulated dust emission. For the removal process, all the models estimate that wet deposition is smaller than dry deposition and wet deposition accounts for 12 %–39 % of total deposition. The models also estimate that most (77 %–91 %) dust particles are deposited onto continents and 9 %–23 % of dust particles are deposited into oceans. Compared to the observations, most models reproduce the dust deposition and dust concentrations within a factor of 10 at most stations, but larger biases by more than a factor of 10 are also noted at specific regions and for certain models. These results highlight the need for further improvements of the dust cycle especially on dust emission in climate models.

Contribution of the world's main dust source regions to the global cycle of desert dust

“…We constrain the total flux of PM20 dust deposited to oceans to be 0.8 (0.6-1.3) × 10 3 Tg. This is approximately double the median deposition flux to oceans that was obtained in recent model ensemble studies (Table 5 in Checa-Garcia et al, 2020; Tables 8 and 9 in Wu et al, 2020). This larger dust deposition flux to oceanic regions likely occurs because we correct the underestimation (or omission) by current models of dust with geometric diameters between 10 to 20 µm (Adebiyi and Kok, 2020), which we find makes up approximately half (46 (35-66) %) of this deposition flux to oceans.…”

“…We consider the results presented in the previous section to be more accurate constraints on the source region-resolved dust loading, concentration, DAOD, and deposition flux than results obtained directly from regional and global model simulations (e.g., Tanaka and Chiba, 2006;Mahowald, 2007;Huneeus et al, 2011;Wu et al, 2020). Nonetheless, our results are subject to a number of important limitations.…”

“…However, estimates of the contributions of the world's major source regions to the global dust cycle diverge widely. For instance, model estimates of emissions from the main source regions vary by up to an order of magnitude between different global model simulations (Huneeus et al, 2011;Wu et al, 2020).…”

“…Many past studies of the contributions of the main dust source regions to the global dust cycle have been based on global model simulations (Tanaka and Chiba, 2006;Chin et al, 2007;Huneeus et al, 2011;Wu et al, 2020). These simulations exhibit substantial biases when compared to observations of dust abundance and of dust microphysical properties like size distribution and mass extinction efficiency (Kok et al, 2014a;Ansmann et al, 2017;Checa-Garcia et al, 2020).…”

“…Another recent study by Albani et al (2014) regionally tuned model sources to match concentration, deposition, and aerosol optical depth observations using optimal estimation combined with expert opinion to incorporate geochemical tracer information in dust deposition measurements (Albani et al, 2014). Although both these studies are steps towards obtaining more reliable constraints on the contribution of each main source region to the global dust cycle, remaining deficiencies include (1) substantial biases in dust microphysical properties such as size distribution and extinction efficiency , (2) the use of a single model to represent dust transport and emission even though the spread in predictions between models is large (Huneeus et al, 2011;Checa-Garcia et al, 2020;Wu et al, 2020), and (3) the lack of robust uncertainty estimates in the contributions of the different source regions that can be propagated into calculated dust impacts such as dust radiative forcing. As such, although clear progress has been made in understanding the relative contributions of the world's source regions to the global dust cycle, current knowledge is not yet sufficient to constrain regionally varying dust impacts or to reliably inform the provenance of dust in deposition records.…”