PWG Priorities

April 2014

A. Coupling across components and understanding interactions

• Coupling of isotopic water and carbon tracers across components will allow better diagnostics on how the CESM water and carbon cycles perform. 

Isotopic water tracers can be used to identify and track moisture sources of air masses, examine the stratospheric-tropospheric exchange of water vapor, and provide additional diagnostics of the global hydrological cycle and cloud processes. Carbon isotopes (¹⁴C, ¹³C) are used as diagnostic tracers in ocean and land carbon-cycle modeling. Measurements of naturally-produced radiocarbon (¹⁴C) in deep-sea sediment cores have been used to infer deep-ocean ventilation rates for past and present climates.  Adding radiocarbon into CESM will help validate ocean-model circulation and deep-water mass formation processes in the present-day ocean, and will provide a solid test for ocean ventilation rates in the paleo-ocean. Carbon isotopes, measured in atmospheric CO₂ and oceanic dissolved inorganic carbon, have been used to constrain carbon exchanges between the atmosphere, ocean and terrestrial biospheres.  The oceanic distribution of d¹³C is complex, depending on the effects of biology, temperature, gas exchange, and circulation, and thus provides a severe test for model performance. 

B. New parameterizations/processes

• Adding isotope tracers to components of the CESM.

Specifically, adding isotopic water and carbon tracers to the ocean component (pop2), the atmosphere component (cam5), the land component (clm), and isotopic water tracers to the sea ice component (cice) and coupling them. This will be useful for diagnosing biases in the model, as it opens up a new datasets to compare with. It also allows for more direct comparisons to paleoclimate records, which are often in the form of isotopic measurements, which are then interpreted as a climate change signal (warming, cooling, wetter, drier, etc.). The comparison provides validation for the interpretation and use of the proxy. This activity ties directly to the paleoclimate developments outlined in the CESM science plan (paleotracers).

C. High-resolution and new dynamical cores

• Exploring the effects of varying horizontal resolution in CAM5 on the simulations of the isotopic water cycle.

D. Addressing biases/shortcomings

• Comparing isotopic water fluxes to observations provides additional information that can help elucidate the source of model biases. 

E. Software development

• The promulgation of passive isotopic water and carbon tracers throughout all active and data component models of the CESM requires software development within all model components.

Code development is also taking place in the atmospheric data model component as well in order to drive the ocean, land, and sea ice components with observed data sets before coupling for testing and spin-up purposes. Depending on the model component, software development can be extensive or pervasive. This is especially true in CAM because isotopic water differentiates from bulk water due to fractionation at every phase change, and is intimately coupled to cloud and convective processes. In the POP model, restructuring of the Ecosystem module and the development of an ecosystem driver was necessary to allow the addition of biotically coupled carbon tracers. Additional passive trace element tracer modules (geotracers) are also planned for development over these two years within the ocean component (Pa, Th, Nd). Additionally, isotopic water tracers will be incorporated within the land and runoff models to complete the isotopic water cycle.

• Toolkit for setting up Quaternary and Deep-time coupled simulations with CESM1.2.

Develop an online Paleo User Resources site to provide the community with background on the CESM code and dataset modifications that may be necessary to address scientific questions about past Earth climates with the CESM model, including useful tools that are designed to modify the forcing and input files for paleoclimate applications.