[Paper Review] Numerical coupling of aerosol emissions, dry removal, and turbulent mixing in the E3SM Atmosphere Model version 1 (EAMv1), part I: dust budget analyses and the impacts of a revised coupling scheme

Hui Wan, Kai Zhang+6 more|arXiv (Cornell University)|Jun 8, 2023

Atmospheric aerosols and clouds4 citations

TL;DR

This paper proposes a revised numerical coupling scheme in the E3SM Atmosphere Model v1 (EAMv1) that reorders the sequence of aerosol processes—moving surface emissions after dry removal and before turbulent mixing—to better represent the real-world dust lifecycle. The change reduces artificial strengthening of dry removal in source regions, increasing global dust lifetime from 1.9 to 2.6 days and raising the dust burden by 39% without tuning parameters, with significant impacts on dust and sea salt but smaller effects on submicron aerosols.

ABSTRACT

An earlier study evaluating the dust life cycle in the Energy Exascale Earth System Model (E3SM) Atmosphere Model version 1 (EAMv1) has revealed that the simulated global mean dust lifetime is substantially shorter when higher vertical resolution is used, primarily due to significant strengthening of dust dry removal in source regions. This paper demonstrates that the sequential splitting of aerosol emissions, dry removal, and turbulent mixing in the model's time integration loop, especially the calculation of dry removal after surface emissions and before turbulent mixing, is the primary reason for the vertical resolution sensitivity reported in that earlier study. Based on this reasoning, we propose a simple revision to the numerical process coupling scheme, which moves the application of the surface emissions to after dry removal and before turbulent mixing. The revised scheme allows newly emitted particles to be transported aloft by turbulence before being removed from the atmosphere, and hence better resembles the dust life cycle in the real world. Sensitivity experiments are conducted and analyzed to evaluate the impact of the revised coupling on the simulated aerosol climatology in EAMv1.

Motivation & Objective

• To identify the cause of artificial sensitivity of dust lifetime to vertical resolution in EAMv1.
• To address the numerical artifact caused by the sequential ordering of aerosol processes in the time integration loop.
• To improve the physical realism of dust lifecycle simulations by reordering the sequence of emissions, dry removal, and turbulent mixing.
• To evaluate the impacts of the revised coupling on global dust burden, lifetime, and transport across multiple aerosol species.

Proposed method

• Reordering the numerical process coupling in EAMv1 by moving surface aerosol emissions after dry removal and before turbulent mixing.
• Implementing the revised scheme for all aerosol species with surface emissions, including dust, sea salt, black carbon, and organic aerosols.
• Conducting wind-nudged simulations with 1° horizontal resolution and 72 vertical layers to isolate process coupling effects.
• Comparing simulations using the original and revised coupling schemes to quantify changes in dust mass budget, removal rates, and vertical mixing.
• Analyzing instantaneous and annual mean output from EAMv1 simulations to assess changes in dust mixing ratios and removal tendencies.
• Evaluating impacts across multiple aerosol types, distinguishing between surface-emitted and secondary aerosols.

Experimental results

Research questions

• RQ1 Why does dust lifetime in EAMv1 show strong sensitivity to vertical resolution, particularly in high-resolution simulations?
• RQ2 How does the current sequence of process coupling—emissions before dry removal and before turbulent mixing—introduce numerical artifacts in dust removal?
• RQ3 What is the impact of reordering the process coupling sequence to place emissions after dry removal and before turbulent mixing on simulated dust burden and lifetime?
• RQ4 How do the changes in process rates propagate across different aerosol species with varying emission and removal characteristics?
• RQ5 To what extent does the revised coupling reduce artificial dry removal in dust source regions and improve the realism of dust transport?

Key findings

• The revised process coupling increases the global annual mean dust burden by 39% compared to the original EAMv1 when tuning parameters are unchanged.
• The global mean dust lifetime increases from 1.9 days to 2.6 days due to reduced dry removal in source regions.
• The revised scheme weakens dry removal in dust source regions by 30–50% and strengthens vertical mixing, better representing the physical sequence of dust emission and transport.
• The changes in process rates are most pronounced for dust and sea salt, with smaller but still significant impacts on black carbon and primary organic aerosols.
• Secondary aerosols like sulfate and secondary organic aerosols show negligible changes, as they lack significant surface emissions.
• The revised coupling reduces the sensitivity of dust lifetime and removal rates to vertical resolution, indicating a reduction in numerical splitting errors.