Dust aerosols play an important role in Earth's energy balance and atmospheric interactions with biogeochemical cycles. It is critical to quantify dust life cycle and radiative effects in the coupled E3SM. The development and evaluation of dust representation in the V1 model have mainly focused on the global constraint of the dust optical depth (AOD) with observational estimates. However, further analysis of the dust model simulations with V1 suggested that the dust shortwave (SW) absorption is too strong compared with observations and other GCMs. While the direct radiative effect of dust at the top atmosphere is constrained to a large extent by AOD, the heating effect of dust in the atmosphere is overestimated. This could lead to changes of the lower troposphere thermostructure and the subsequent cloud and precipitation changes.
The second issue identified is with the dust size distribution at emission used in the model, which leads to over-predicted fine-mode (<1 um) dust particles in the atmosphere as shown by Kok et al. (2017). As a result, the overall cooling effect by dust at TOA may be over-estimated.
The third issue is related to the increasing of model vertical resolution from 30 vertical layers to 72 layers. It leads to an over-deposition of dust in source regions and less long-range transport. Despite the global AOD is constrained, global dust emissions increase by nearly a factor of 2, dust layer height is low-biased compared to observations, and the lifetime of dust is also short than other global models.
Therefore, the following dust updates proposed for V2 will address the above three issues. First, we will update the dust refractive index in the SW with the AERONET (obs)-derived values from the OPAC data (Hess, 1998) that is used in the default configuration. Second, we will update the fractional dust emission fluxes in accumulation and coarse modes, calculated with the size distribution by Kok (2011). With these updates implemented, dust emissions need to be re-tuned for the globally constrained AOD.
To fundamentally address the over-deposition bias in dust, it requires some surgical change in the code - the aerosol turbulence transport term needs to be moved from CLUBB and add to the aerosol emission/deposition equation. This issue might also need affect other aerosols and water vapor, which probably requires extensive model evaluation. In view of the timeline for the V2, a simple fix is thus proposed. We will implement an alternative option that distribute dust evenly in the PBL, instead of the lowest model layer. As insufficient dust transport is common in large-scale models for various reasons, similar approaches have been adopted in other modeling studies to enhance the dust vertical transport. We propose to implement this simple fix as an option in the namelist so that we will have a short-term solution, while we are working on a more physically-based solution.
Requirements
Requirement: (1) update namelist options in the default compset; (2) update size distribution of dust emission fluxes; (3) update the dust emission distribution in the boundary layer
Date last modified: 2019-06-27 Contributors: Yan Feng
The first two updates have been implemented and tested in V1. The third update is implemented and tests are not completed.
Algorithmic Formulations, and Design Implementation
Evaluation: (1) Dust aerosol optical depth (AOD) and absorption aerosol optical depth (AAOD) are compared with the AERONET data (INV Version3) averaged between 2006-2015 for sites near the dust sources. LResTN and LResTNs show better agreement with the observed AAOD
Dust AAOD Dust AOD
Image RemovedImage Removed
AERONET
LRes (Correlation with AERONET data)
LResTN
LResTNs
AOD
0.311
0.299 (0.91)
0.284 (0.96)
0.282 (0.97)
AAOD
0.017
0.04 (0.69)
0.022(0.72)
0.022(0.74)
(2) Kok et al. (2017) showed that the new size distribution (Kok, 2011) compares better with recent emission flux measurements (Fig 1c in their paper) than other size distributions.
(3) Impact on global dust AOD:
Dust AOD
Total
Accumulation
Coarse
LRes
0.026
0.0093
0.0171
LResTN
0.026
0.0040
0.0214
LResTNs
0.025
0.0040
0.0214
Kok, J. F. (2011). Does the size distribution of mineral dust aerosols depend on the wind speed at emission? Atmospheric Chemistry and Physics, 11(19), 10149-10156. <Go to ISI>://WOS:000296357300009
Kok, J. F., Ridley, D. A., Zhou, Q., Miller, R. L., Zhao, C., Heald, C. L., et al. (2017). Smaller desert dust cooling effect estimated from analysis of dust size and abundance. Nature Geoscience, 10(4), 274-278. <Go to ISI>://WOS:000398162800014
Zender, C. S., Bian, H. S., & Newman, D. (2003). Mineral Dust Entrainment and Deposition (DEAD) model: Description and 1990s dust climatology. Journal of Geophysical Research-Atmospheres, 108(D14). <Go to ISI>://WOS:000184611000001