PeKing University CouPLer

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2 December 2014

PKUCPL: A Two-Way Coupler to integrating models

We develop a PKUCPL (PeKing University CouPLer) coupler to integrating multiple models in a manner of two-way coupling, i.e., allowing for feedbacks between models.

The idea of developing PKUCPL originated from the fact that current global simulations tend to overestimate the global tropospheric oxidative capacity.

Global chemical transport models, widely used for studying global air pollution and transport, are limited by coarse horizontal resolutions, not allowing for detailed representation of small-scale nonlinear processes over the pollutant source regions. Traditional one-way nested regional models take global model outputs as lateral boundary conditions without feeding back to the global model, and thus do not affect the simulated global atmospheric environment. Global simulations at coarse resolutions typically overestimate the tropospheric oxidative capacity, i.e., with an overestimate in OH and ozone concentrations, and an underestimate in CO concentration, MCF lifetime and CH4 lifetime.

We develop and use PKUCPL to integrate, in a manner of two-way coupling, the global GEOS-Chem CTM (at ~2 degree resolution) and its multiple nested models (at 0.5 or 0.25 degree resolutions) covering Asia, North America and Europe. As an example, see the coupling regions and the coupling flowchart here for global + three nested models.

Effects of two-way coupling between global and nested models of GEOS-Chem

Under the two-way coupling framework, PKUCPL takes global model results as lateral boundary conditions of nested models, and at the same time takes nested model results, conduct a regridding procedure, and replaces the global model results within the nested model domains. This allows for feedback between global and nested models. Thus, the two-way coupling allows smaller-scale nonlinear processes in the nested domains (which are normally major pollution source regions) to be better represented and be able to affect the large-scale atmospheric chemistry across the globe.

As a result, the two-way coupling improves the simulations of both regional chemistry and global transport. In particular, the two-way coupling significantly alleviates the overestimate of the tropospheric oxidative capacity in the global simulation, with large reductions in OH and ozone concentrations and enhancements in CO concentration, MCF lifetime and CH4 lifetime (Yan et al., 2014; Yan et al., 2016).

In Yan et al. (2014):

1. We use CO as a tracer to diagnose the consequence of the two-way coupling (Difference between a two-way coupled and a 'pure' global simulation: An animation from 2008.07.01 to 2008.08.15). Compared to a pure global model, the two-way coupled simulation increases the global tropospheric mean CO concentration in 2009 by 10.4%, with a greater enhancement at 13.3% in the Northern Hemisphere. Correspondingly, the global tropospheric mean hydroxyl radical (OH) is reduced by 4.2%, resulting in a 4.2% enhancement in the methyl chloroform lifetime. (See the global budget of tropospheric OH for 2009 here.) The resulting CO and OH contents and MCF lifetime are closer to observation-based estimates.

2. Various factors differentiate the two-way coupled model from the global model, including the small-scale variability of NOx, CO and VOC, the resolution- and meteorology-dependent natural emissions, and other nonlinear small-scale processes. See the percentage contributions of individual factors to the difference in January 2009 tropospheric CO between the two-way coupled model and the global model.

3. Comparisons with the tropospheric CO measurements over the Pacific Ocean during the HIPPO campaigns (Flight tracks and times) in various seasons between 2009 and 2011 show significant improvements by the two-way coupled simulation on the magnitude and spatiotemporal variability of CO. In particular, the two-way coupled simulation captures the measured vertical profiles of CO, with a mean bias of 1.1 ppb (1.4%) below 9 km compared to the bias at -7.2 ppb (-9.2%) for the global model alone. See the CO time-height distribution across five HIPPO campaigns and the CO vertical profiles in five individual HIPPO campaigns.

4. The two-way coupling also affects simulations of other species. See our preliminary results for 2009: Global tropospheric hydrophobic BC, Global tropospheric hydrophilic BC, Global tropospheric OH, Global tropospheric O3.

In Yan et al. (2016):

We focus on the year of 2009. The simulated global tropospheric mean OH concentration is reduced by 5% and the MCF and CH4 lifetimes are increased by 5%. The global tropospheric ozone mass is reduced by 10%, and the global CO mass is increased by 10%. All these changes help reduce positive/negative biases in the simulation of these variables.

PKUCPL Code applied to GEOS-Chem

PKUCPL has been adopted in GEOS-Chem stanadard model since version 10.

Users can choose to couple the global model with any numbers of nested models, with a straightforward setup.

Users can choose various resolutions for global (e.g., 2.5 long. x 2 lon., 5 long. x 4 lat.) or regional (e.g., 0.667 long. x 0.5 lat., 0.3125 long. x 0.25 lat.) models.

The computational complexity of two-way coupling is minimized by the PKUCPL coupler. Users can learn how to set up a two-way coupled simulation quickly (i.e., within 1-2 hours), following a simple manual. Individual global/nested models can run at different nodes, as desired.

The computational time of the coupled system is determined by and is comparable to that of the slowest individual model. Our test suggests that only 2% or so additional run time is needed beyond the slowest individual model (the nested model for North America).

For more details, see this wiki page.

PKUCPL for integrating other models

PKUCPL is available for public use. It is flexible and can be modified and applied to other types of models (e.g., between chemistry models and climate/meteorological models or between atmospheric and oceanic models of various complexities). Please contact us for collaborations.


Yan, Y.-Y., Lin, J.-T. *, Kuang, Y., Yang, D.-W., and Zhang, L.: Tropospheric carbon monoxide over the Pacific during HIPPO: Two-way coupled simulation of GEOS-Chem and its multiple nested models, Atmospheric Chemistry and Physics, 14, 12649-12663, doi:10.5194/acp-14-12649-2014, 2014 (PDF)

Yan, Y.-Y., Lin, J.-T. *, Chen, J., and Hu, L.: Improved simulation of tropospheric ozone by a global-multi-regional two-way coupling model system, Atmospheric Chemistry and Physics, 16, 2381-2400, doi:10.5194/acp-16-2381-2016, 2016 (PDF)

Authors (3)

  Kuang, Y.

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