CESM1.1 and CESM1.1.1: Notable Improvements
Machines / Scripts
CESM1.1 contains numerous new infrastructure capabilities that permit new flexibility and extensibility in the model system. The most significant new features are listed here. Completely new capability has been added to the system to permit users to specify new model grids. This is reflected via the addition of a new top level mapping/ directory. New support has been added for permitting multiple instances of component models to run within a single executable. This capability is being leveraged to carry out data assimilation experiments using DART. The specification of inter-component field transfer in the coupler has been refactored to permit namelist specification of new fields between model compoennts without the necessity of modifications to the driver/coupler code base. The river component has been elevated to its own component (it previously existed within CLM and DLND) and leads to new desired flexibility to incorporate new river schemes in CESM. Finally, new scripting capability has been added to the model system that address shortcomings encountered by users in CESM1.0. In particular, all aspects associated with user customization requirements has been simplified and unified. This includes user specification of new component sets, new grids, new PIO settings, user customization of component namelists and the ability to run multiple experiments all using one model executable.
The namelist and stream generation for all of the CESM1.1 data models have been completely refactored in order to significantly simplify the user custoization of data model settings.
CAM5.2 replaces the finite-volume dynamical core (CAM-FV) with the new spectral element dynamical core (CAM-SE) as the default for calculations using horizontal resolutions of 100 km (1 degree) or finer. The CAM-SE dycore uses accurate, high-order numerical methods on rectangular elements in a cubed-sphere geometry. Polar filters are not needed. CAM-SE possesses nearly linear scaling to processor counts of up to one cpu per element. In addition, CAM-SE has regional mesh-refinement capability.
As part of CAM-SE implementation a set of generalized algorithms were developed for generating surface topographic data sets on arbitrary horizontal grids. These algorithms were used to generate topographic forcing datasets for the CAM-SE ne30np4 configuration in this release.
This release is intended to give researchers access to the CAM-SE infrastructure for development purposes. However, use for extended coupled climate simulations is not recommended due to small but significant biases in low cloud forcing when CAM-SE is used. These biases are believed to be related to differences in vertical advection schemes used in CAM-FV and in the current implementation of CAM-SE.
The new features available in CAM-Chem in this release include:
- coupling of tropospheric and stratospheric chemistry with the CAM5 modal aerosol model 3 and 7
- expansion of the representation of secondary-organic aerosols
- implementation of MEGAN v2 in CLM, including a flexible framework for assigning MEGAN emissions to species represented in the chemistry.
All these are released to give users access to these new capabilities. Only limited evaluation has been performed.
The main new feature in CLM in this release is the availability of new higher resolution raw input datasets (e.g., Plant Functional Type distribution, lake distribution, glacier distribution, organic matter content, etc). Most input datasets are now available at approximately 3' to 5' resolution globally, up from roughly 0.5deg in prior releases. Additionally, the River Transport Model can now be run at either 0.5 deg or 0.1deg.
The MEGAN model for biogenic volatile organic compound emissions has been updated to permit up to 120 species.
New software features include multiple instance capability and support to run CLM on unstructured grids such as the CAM spectral element grid.
As of CESM1.1, CESM supports a new compset type (TG), which allows running standalone CISM driven by output from a previous, coupled CESM simulation. We provide a variety of out-of-the-box forcing data, or you can generate your own forcing data. In order to support TG compsets, there is now support for longer time steps (e.g., 1-year) in both CISM and the CESM scripts.
A number of CISM's default parameter settings have also been changed for the CESM1.1 release, resulting in a more realistic simulation of the Greenland ice sheet. Among these is a change in the default resolution to 5 km rather than 20 km.