Hans Chen

Ph.D. candidate in meteorology and atmospheric science at Penn State

  • Hans W. Chen
  • Doctoral Graduate Student
  • Department of Meteorology and Atmospheric Science
  • The Pennsylvania State University

News: Our paper on the robustness of the mid-latitude atmospheric response to Arctic sea-ice loss has just appeared in Journal of Climate. You can download the paper here (PDF).


I am a Ph.D. student in the Department of Meteorology and Atmospheric Science at Penn State, working with my advisor Fuqing Zhang and my co-advisor Richard Alley. My research interests include climate variability on the intraannual to multidecadal time scale, synoptic-scale atmospheric dynamics, fluxes and transport of atmospheric carbon dioxide, and the application of data assimilation to these topics. At the moment my research focuses on improving our knowledge of regional carbon dioxide sources and sinks in the contiguous United States, which is important both for climate management and to quantify and reduce the uncertainties of future climate projections.


2013–present Ph.D. in Meteorology and Atmospheric Science, Department of Meteorology and Atmospheric Science, The Pennsylvania State University, University Park, Pennsylvania, United States.
2010–2012 M.S. in Atmospheric Sciences, Oceanography and Climate, Department of Meteorology, Stockholm University, Stockholm, Sweden.
2007–2010 B.S. in Meteorology, Department of Meteorology, Stockholm University, Stockholm, Sweden.

More information can be found in my CV (PDF).

Life outside of research

I have a keen interest in programming and the Linux ecosystem, and have made various contributions to free and open-source software, most notably the KDE project. I also enjoy outdoor activities and like to take my mountain bike on off-road adventures. To relax, I like to listen to music and play the classical guitar.

Research interests

I am interested in understanding the dynamical and physical processes that drive climate variability and climate change. What are the mechanisms behind natural climate variability? How does human-induced climate change affect these mechanisms? And what are the ultimate impacts of the shifting weather and climate on ecosystems and human society? These are some of the broad questions I tackle in my research. My focus is on the atmosphere from a regional-scale to global-scale perspective and on intraannual to multidecadal time scales, but my work has so far also involved Arctic sea ice and the carbon cycle. The research topics I am interested in include:

  • Climate variability and climate change
  • Synoptic-scale atmospheric dynamics
  • Transport and fluxes of carbon dioxide
  • Data assimilation

Current research projects

Fluxes and transport of carbon dioxide in the contiguous United States

Atmospheric Carbon and Transport - America

Our knowledge of carbon dioxide (CO2) emissions and removal by sinks is limited, largely due to a lack of observations of CO2 fluxes between the Earth's surface and the atmosphere. This information is crucial to better understand how the climate will change in the future. Furthermore, as the world's nations are moving towards international agreements to limit greenhouse gas emissions, it has become evident that we need to be able to verify the self-reported emission estimates by countries using independent data and methods.

In this project I will develop a data assimilation system based on the Ensemble Kalman Filter to constrain the CO2 fluxes in the contiguous United States by assimilating observed atmospheric CO2 concentration. Using this system, we will be able to assess the impact of different assumptions and sources of errors on the estimated CO2 fluxes, and answer questions such as: How much do uncertainties in the atmospheric transport contribute to the total uncertainties? Are we able to constrain CO2 fluxes on a regional scale using our current observations of CO2 concentration? How much do we benefit from satellite observations in terms of constraining the fluxes?

This work is part of the NASA-funded ACT-America project.

Past research projects

Regional sea-ice changes in the Arctic

The aim of this project was to objectively classify different regions in the Arctic based on the interannual variability of sea ice concentration in each region, and to quantify the regional sea-ice extent variability and trends in each Arctic sub-region. We used Self-Organizing Map clustering to classify the Arctic sea-ice cover into nine regions in autumn and six regions in early winter. The regional sea-ice extents were detrended using an adaptive nonlinear method based on the Ensemble Empirical Mode Decomposition technique, which resulted in a better fit than a linear trend. Finally, we investigated the statistical relationship between the regional sea-ice variations and weather patterns in winter and found that sea-ice reductions in the Barents-Kara Seas and the Beaufort Sea are both related to unusually cold winters over the Eurasian continent, but are associated with distinctly different wintertime circulation patterns.

More information about this project will be available when Chen et al. (2016, In review) has been published.

Robustness of the mid-latitude atmospheric response to Arctic sea-ice loss

In this work we asked the question, can we find a robust atmospheric response when we gradually reduce the Arctic sea-ice cover? To find an answer, we ran a large number of experiments in a general circulation model with systematically perturbed Arctic sea-ice cover, and diagnosed the changes in the jet stream, planetary wave amplitude, teleconnection patterns, and extreme cold events over the continents. The results show that reductions in the Arctic sea-ice cover did not produce detectable changes in our metrics of the jet stream position and planetary wave amplitude, even in the simulations with the most severe sea-ice reduction that resembles the record-low Arctic sea-ice cover in September 2012. Some experiments show a statistically significant response in the frequency of large-scale circulation patterns, with a shift towards a circulation pattern that resembles the negative phase of the Arctic Oscillation, and more frequent extreme cold winter days in eastern Asia. However, the changes are not found in all experiments with reduced Arctic sea-ice cover and do not scale linearly with the sea-ice anomalies. This non-robust behavior of the atmospheric response to Arctic sea-ice loss suggests that the link between Arctic sea-ice anomalies and mid-latitude weather patterns is complex and depends on other factors, such as the state of the climate.

See Chen et al. (2016, Journal of Climate) for more information.

A robust mode of climate variability in the Arctic: The Barents Oscillation

Worldmap of Köppen types

This project is based on my Master's thesis and examines the robustness of an atmospheric mode known as the Barents Oscillation (BO). The BO has a meridional structure and could be important for the heat transport into the Arctic, but has also been speculated to appear purely as an artifact from the Empirical Orthogonal Function (EOF) analysis due to a spatial shift of the leading mode, the Arctic Oscillation. In this work we investigated if the BO can be considered a real physical mode of variability, or if it is an artifact of the EOF analysis. We found that the BO is a robust mode that can be found even during time periods when the Arctic Oscillation was relatively stable, which suggests that the BO is not an artifact. On the project page you can find more information about the BO and obtain the BO index.

Go to project page

Using the Köppen classification to quantify climate variation and change

Worldmap of Köppen types

When trying to quantify the impact of climate variation and climate change, it is common to use a single scalar variable such as surface temperature. However, it is not given that a change in e.g. surface temperature will have the same impact everywhere; some ecosystems are relatively stable, while others are more sensitive to changes in the regional climate. This project explores a different approach using the Köppen climate classification to quantify the impact of climate variations and change on ecosystems in different regions and over various time scales. The results show that the Köppen classification can be used as a diagnostic tool to monitor changes in climatic conditions that are relevant for ecosystems.

Go to project page


  • Chen, H. W., R. B. Alley, and F. Zhang (2016): Interannual Arctic sea-ice variability and associated winter weather patterns: A regional perspective for 1979–2014. Journal of Geophysical Research–Atmospheres. In review.


  • Chen, H. W., F. Zhang, and R. B. Alley (2016): The robustness of midlatitude weather pattern changes due to Arctic sea ice loss. Journal of Climate, 29, 7831-7849, doi:10.1175/JCLI-D-16-0167.1.


  • Chen, H. W., Q. Zhang, H. Körnich, and D. Chen, 2013: A robust mode of climate variability in the Arctic: The Barents Oscillation. Geophysical Research Letters, 40, 2856-2861, doi:10.1002/grl.50551.
  • Chen, D. and H. W. Chen, 2013: Using the Köppen classification to quantify climate variation and change: An example for 1901–2010. Environmental Development, 6, 69-79, doi:10.1016/j.envdev.2013.03.007.


I use presentation slides and posters to complement my speech and they are therefore not very useful on their own. Nevertheless, if you want the slides or poster from a presentation I have given, do not hesitate to send an email to hans.chen@psu.edu.

Oral presentations




Here I have listed various free software I have found useful in my research.

Atmospheric science