Publications

2022

1. Thesis

   Liu, Q. (2022). Understanding climate feedbacks with idealized models. University of Exeter. Retrieved from http://hdl.handle.net/10871/128290

   Abs Bib PDF Code

   The global mean surface air temperature change in response to global warming, namely climate sensitivity, plays a central role in climate change studies, and the estimates of climate sensitivity depend critically on the climate feedbacks, the processes that can either amplify or dampen the responses of the climate system to external perturbations. The goal of this thesis is to understand climate feedbacks through idealized climate models. The first part explores the roles of climate feedbacks in polar amplification of surface temperature change. By running idealized aquaplanet simulations with a hierarchy of radiation schemes (without sea ice and clouds), and by decomposing the total surface temperature responses into different components through the radiative kernel method, we find the poleward heat transport, the lapse rate and Planck feedbacks contribute to amplified surface temperature changes in the polar region, while the forcing and water vapor feedback dominates the tropical temperature change. The second part investigates the underlying causes of cloud feedback uncertainty with a simple cloud scheme. The scheme diagnoses the cloud fraction from relative humidity and other variables such as inversion strength, and its optical properties such as effective radius and cloud water content are prescribed as simple functions of temperature. The simulations show this scheme can capture the basic feature of cloud climatology. Through a series of perturbed parameter ensemble global warming simulations, part of the inter-model spread of cloud feedbacks among general circulation models can be reproduced. In addition, the low cloud amount feedback, especially over the low-latitude subsidence regions, is the largest contributor to the net cloud feedback uncertainty. The cloud controlling factor analysis suggests that the sea surface temperature (SST) and estimated inversion strength (EIS) have opposite impacts on marine low cloud amounts, but their responses to SST rather than EIS seem to bring larger uncertainty. Finally, the equilibrium climate sensitivity and cloud feedback over tropical subsidence regimes show a robust linear relationship, implying a possible constraint for climate sensitivity.

   @thesis{Liu2022understanding,
     abbr = {Thesis},
     bibtex_show = {true},
     title = {{Understanding climate feedbacks with idealized models}},
     author = {Liu, Qun},
     year = {2022},
     url = {http://hdl.handle.net/10871/128290},
     pdf = {https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.846052},
     code = {https://github.com/lqxyz/PhD_Thesis},
     publisher = {University of Exeter}
   }

2021

1. GMD

   SimCloud version 1.0: A simple diagnostic cloud scheme for idealized climate models

   Qun Liu, Matthew Collins, Penelope Maher, Stephen I Thomson, and Geoffrey K Vallis

   Geoscientific Model Development. 2021, 14 (5): 2801–2826.

   Abs Bib DOI PDF Code

   A simple diagnostic cloud scheme (SimCloud) for general circulation models (GCMs), which has a modest level of complexity and is transparent in describing its dependence on tunable parameters, is proposed in this study. The large-scale clouds, which form the core of the scheme, are diagnosed from relative humidity. In addition, the marine low stratus clouds, typically found off the west coast of continents over subtropical oceans, are determined largely as a function of inversion strength. A “freeze-dry” adjustment based on a simple function of specific humidity is also available to reduce an excessive cloud bias in polar regions. Other cloud properties, such as the effective radius of cloud droplet and cloud liquid water content, are specified as simple functions of temperature. All of these features are user-configurable. The cloud scheme is implemented in Isca, a modeling framework designed to enable the construction of GCMs at varying levels of complexity, but could readily be adapted to other GCMs. Simulations using the scheme with realistic continents generally capture the observed structure of cloud fraction and cloud radiative effect (CRE), as well as its seasonal variation. Specifically, the explicit low-cloud scheme improves the simulation of shortwave CREs over the eastern subtropical oceans by increasing the cloud fraction and cloud water path. The freeze-dry adjustment alleviates the longwave CRE biases in polar regions, especially in winter. However, the longwave CRE in tropical regions and shortwave CRE over the extratropics are both still too strong compared to observations. Nevertheless, this simple cloud scheme provides a suitable basis for examining the impacts of clouds on climate in idealized modeling frameworks.

   @article{Liu2021simcloud,
     abbr = {GMD},
     bibtex_show = {true},
     title = {{SimCloud version 1.0: A simple diagnostic cloud scheme for idealized climate models}},
     author = {Liu, Qun and Collins, Matthew and Maher, Penelope and Thomson, Stephen I and Vallis, Geoffrey K},
     journal = {Geoscientific Model Development},
     volume = {14},
     number = {5},
     pages = {2801--2826},
     year = {2021},
     publisher = {Copernicus GmbH},
     doi = {10.5194/gmd-14-2801-2021},
     url = {https://gmd.copernicus.org/articles/14/2801/2021/},
     pdf = {https://gmd.copernicus.org/articles/14/2801/2021/gmd-14-2801-2021.pdf},
     code = {https://github.com/lqxyz/cloud_scheme_manuscript_figs},
     selected = {true}
   }

2018

1. ASL

   Uncertainties in simulated El Niño–Southern Oscillation arising from internal climate variability

   Chao Sun, Li Liu, Li-Juan Li, Bin Wang, Cheng Zhang, Qun Liu, and Rui-Zhe Li

   Atmospheric Science Letters. 2018, 19 (3): e805.

   Abs Bib DOI

   Significant uncertainties exist in El Niño–Southern Oscillation (ENSO) simulations. To investigate the source of these uncertainties, previous studies have primarily focused on the model itself; however, internal climate variability (ICV) as a source of uncertainty has not been sufficiently explored to date. Using the Community Earth System Model–Last Millennium Ensemble (CESM–LME) modeling project and the Coupled Model Intercomparison Project (CMIP), an investigation into uncertainties in simulated ENSO arising from ICV is performed. Results show that external forcing can significantly increase the uncertainties arising from ICV when the simulation length is greater than  40 years. In addition, the spread in ENSO amplitude arising from ICV accounts for 50% of the total spread within the CMIP5 historical simulations. Finally, the impact of ICV on ENSO varies considerably with simulation length and stabilizes at the threshold of 300-400 years.

   @article{Sun2018uncertainties,
     abbr = {ASL},
     bibtex_show = {true},
     title = {{Uncertainties in simulated El Ni{\~n}o--Southern Oscillation arising from internal climate variability}},
     author = {Sun, Chao and Liu, Li and Li, Li-Juan and Wang, Bin and Zhang, Cheng and Liu, Qun and Li, Rui-Zhe},
     journal = {Atmospheric Science Letters},
     volume = {19},
     number = {3},
     pages = {e805},
     year = {2018},
     doi = {10.1002/asl.805},
     url = {https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/asl.805},
     publisher = {Wiley Online Library}
   }

2017

1. GRL

   Reduction of initial shock in decadal predictions using a new initialization strategy

   Yujun He, Bin Wang, Mimi Liu, Li Liu, Yongqiang Yu, Juanjuan Liu, Ruizhe Li, Cheng Zhang, Shiming Xu, Wenyu Huang, Qun Liu, Yong Wang, and Feifei Li

   Geophysical Research Letters. 2017, 44 (16): 8538–8547.

   Abs Bib DOI

   A novel full-field initialization strategy based on the dimension-reduced projection four-dimensional variational data assimilation (DRP-4DVar) is proposed to alleviate the well-known initial shock occurring in the early years of decadal predictions. It generates consistent initial conditions, which best fit the monthly mean oceanic analysis data along the coupled model trajectory in 1 month windows. Three indices to measure the initial shock intensity are also proposed. Results indicate that this method does reduce the initial shock in decadal predictions by Flexible Global Ocean-Atmosphere-Land System model, Grid-point version 2 (FGOALS-g2) compared with the three-dimensional variational data assimilation-based nudging full-field initialization for the same model and is comparable to or even better than the different initialization strategies for other fifth phase of the Coupled Model Intercomparison Project (CMIP5) models. Better hindcasts of global mean surface air temperature anomalies can be obtained than in other FGOALS-g2 experiments. Due to the good model response to external forcing and the reduction of initial shock, higher decadal prediction skill is achieved than in other CMIP5 models.

   @article{He2017reduction,
     abbr = {GRL},
     bibtex_show = {true},
     title = {Reduction of initial shock in decadal predictions using a new initialization strategy},
     author = {He, Yujun and Wang, Bin and Liu, Mimi and Liu, Li and Yu, Yongqiang and Liu, Juanjuan and Li, Ruizhe and Zhang, Cheng and Xu, Shiming and Huang, Wenyu and Liu, Qun and Wang, Yong and Li, Feifei},
     journal = {Geophysical Research Letters},
     volume = {44},
     number = {16},
     pages = {8538--8547},
     year = {2017},
     doi = {10.1002/2017GL074028},
     url = {https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL074028},
     publisher = {Wiley Online Library}
   }

2015

1. GMDD

   Importance of bitwise identical reproducibility in earth system modeling and status report

   Li Liu, Shuai Peng, Cheng Zhang, Ruizhe Li, Bin Wang, Chao Sun, Qun Liu, Li Dong, Lijuan Li, Yanyan Shi, Yujun He, Wenjie Zhao, and Guangwen Yang

   Geoscientific Model Development Discussions. 2015, 8 (6): 4375–4400.

   Abs Bib DOI

   Reproducibility is a fundamental principle of scientific research. Bitwise identical reproducibility, i.e., bitwise computational results can be reproduced, guarantees the reproduction of exactly the same results. Here we show the importance of bitwise identical reproducibility to Earth system modeling but the importance has not yet been widely recognized. Modeled mean climate states, variability and trends at different scales may be significantly changed or even lead to opposing results due to a slight change in the original simulation setting during a reproduction. Out of the large body of Earth system modeling publications, few thoroughly describe the whole original simulation setting. As a result, the reproduction of a particular simulation experiment by fellow scientists heavily depends on the interaction with the original authors, which is often inconvenient or even impossible. We anticipate bitwise identical reproducibility to be promoted as a worldwide standard, to guarantee the independent reproduction of simulation results and to further improve model development and scientific research.

   @article{Liu2015bitwise,
     abbr = {GMDD},
     bibtex_show = {true},
     title = {Importance of bitwise identical reproducibility in earth system modeling and status report},
     author = {Liu, Li and Peng, Shuai and Zhang, Cheng and Li, Ruizhe and Wang, Bin and Sun, Chao and Liu, Qun and Dong, Li and Li, Lijuan and Shi, Yanyan and He, Yujun and Zhao, Wenjie and Yang, Guangwen},
     journal = {Geoscientific Model Development Discussions},
     volume = {8},
     number = {6},
     pages = {4375--4400},
     year = {2015},
     doi = {10.5194/gmdd-8-4375-2015},
     url = {https://gmd.copernicus.org/preprints/gmd-2015-83/},
     publisher = {Copernicus GmbH}
   }