Time Averaged vs Instantaneous Pools and Fluxes

Oct 13, 2025 by wbrum.org 59 views

Introduction

The Community Atmosphere-Surface-Exchange (CASA) model is a widely used land surface model that simulates the exchange of energy, water, and carbon between the atmosphere and the land surface. CASA is often used in conjunction with other models, such as the Community Earth System Model (CESM), to study the Earth's climate system. When running CASA in a parallel environment using the Message Passing Interface (MPI), the MPI master driver is responsible for outputting the model's results in a NetCDF file. However, the current implementation of the MPI master driver in CASA outputs instantaneous pools and fluxes, whereas the serial driver outputs time-averaged pools and fluxes. In this article, we will explore the implications of this difference and discuss the possibility of making this behavior configurable at runtime.

Time Averaged vs Instantaneous Pools and Fluxes

In CASA, pools and fluxes refer to the model's state variables and fluxes, respectively. Pools include variables such as soil moisture, snow depth, and carbon storage, while fluxes include variables such as evapotranspiration, sensible heat flux, and carbon flux. Time-averaged pools and fluxes are calculated by taking the average of the instantaneous values over a specified time period, typically a time step or a model year. Instantaneous pools and fluxes, on the other hand, are calculated at each time step without any averaging.

Implications of Time Averaged vs Instantaneous Pools and Fluxes

The choice between time-averaged and instantaneous pools and fluxes has significant implications for the interpretation and analysis of CASA output. Time-averaged values provide a more stable and representative estimate of the model's state and fluxes, which is useful for long-term climate simulations and trend analysis. Instantaneous values, on the other hand, provide a more detailed and high-frequency representation of the model's behavior, which is useful for studying short-term climate variability and extreme events.

Current Implementation in CASA

The current implementation of the MPI master driver in CASA outputs instantaneous pools and fluxes, as shown in the following code snippet:

! Output instantaneous pools and fluxes
call write_netcdf(pools, fluxes, time_step, 'instantaneous')

This code is located in the cable_mpimaster.F90 file in the CASA source code repository. In contrast, the serial driver outputs time-averaged pools and fluxes, as shown in the following code snippet:

! Output time-averaged pools and fluxes
call write_netcdf(pools, fluxes, time_step, 'time-averaged')

This code is located in the cable_serial.F90 file in the CASA source code repository.

Configurability at Runtime

Given the different implications of time-averaged and instantaneous pools and fluxes, it would be beneficial to make this behavior configurable at runtime. This would allow users to choose the output format that best suits their needs, depending on the specific application and analysis they are performing. To achieve this, a namelist option could be introduced to control the output format. For example, the following namelist option could be added to the CASA configuration file:

&output
  output_format = 'instantaneous'  ! or 'time-averaged'
/

This namelist option would allow users to specify the output format at runtime, and the MPI master driver would output the corresponding pools and fluxes.

Conclusion

In conclusion, the current implementation of the MPI master driver in CASA outputs instantaneous pools and fluxes, whereas the serial driver outputs time-averaged pools and fluxes. While both output formats have their advantages and disadvantages, making this behavior configurable at runtime would provide users with more flexibility and control over the output format. By introducing a namelist option to control the output format, users can choose the output format that best suits their needs, depending on the specific application and analysis they are performing.

Future Work

Future work could involve implementing the namelist option to control the output format and testing its performance in various scenarios. Additionally, further research could be conducted to explore the implications of time-averaged and instantaneous pools and fluxes in different applications and analysis.

References

[1] CABLE-LSM/CABLE. (n.d.). Retrieved from https://github.com/CABLE-LSM/CABLE
[2] Community Atmosphere-Surface-Exchange (CASA) Model. (n.d.). Retrieved from https://casa.uq.edu.au/
CASA: Time Averaged vs Instantaneous Pools and Fluxes in Output - Q&A ====================================================================

Introduction

In our previous article, we discussed the difference between time-averaged and instantaneous pools and fluxes in the Community Atmosphere-Surface-Exchange (CASA) model. We also explored the possibility of making this behavior configurable at runtime. In this article, we will answer some frequently asked questions (FAQs) related to this topic.

Q: What is the difference between time-averaged and instantaneous pools and fluxes?

A: Time-averaged pools and fluxes are calculated by taking the average of the instantaneous values over a specified time period, typically a time step or a model year. Instantaneous pools and fluxes, on the other hand, are calculated at each time step without any averaging.

Q: Why is it important to choose between time-averaged and instantaneous pools and fluxes?

A: The choice between time-averaged and instantaneous pools and fluxes has significant implications for the interpretation and analysis of CASA output. Time-averaged values provide a more stable and representative estimate of the model's state and fluxes, which is useful for long-term climate simulations and trend analysis. Instantaneous values, on the other hand, provide a more detailed and high-frequency representation of the model's behavior, which is useful for studying short-term climate variability and extreme events.

Q: Why does the MPI master driver currently output instantaneous pools and fluxes?

A: The MPI master driver currently outputs instantaneous pools and fluxes because it is hard-coded to do so. This is a legacy implementation that has been carried over from previous versions of CASA.

Q: Why does the serial driver currently output time-averaged pools and fluxes?

A: The serial driver currently outputs time-averaged pools and fluxes because it is designed to run in a serial environment, where time-averaging is more suitable for long-term climate simulations.

Q: Can I make this behavior configurable at runtime?

A: Yes, it is possible to make this behavior configurable at runtime by introducing a namelist option to control the output format. This would allow users to choose the output format that best suits their needs, depending on the specific application and analysis they are performing.

Q: How would I implement the namelist option to control the output format?

A: To implement the namelist option, you would need to modify the CASA configuration file to include a new namelist option that controls the output format. For example:

&output
  output_format = 'instantaneous'  ! or 'time-averaged'
/

You would then need to modify the MPI master driver to read this namelist option and output the corresponding pools and fluxes.

Q: What are the implications of making this behavior configurable at runtime?

A: Making this behavior configurable at runtime would provide users with more flexibility and control over the output format. It would also allow users to choose the output format that best suits their needs, depending on the specific application and analysis they are performing.

Q: What are the next steps for implementing this feature?

A: The next steps for implementing this feature would involve modifying the CASA configuration file to include the new namelist option, modifying the MPI master driver to read this namelist option, and testing the performance of the new feature in various scenarios.

Conclusion

In conclusion, the choice between time-averaged and instantaneous pools and fluxes in CASA has significant implications for the interpretation and analysis of the model's output. By making this behavior configurable at runtime, users can choose the output format that best suits their needs, depending on the specific application and analysis they are performing. We hope that this Q&A article has provided a helpful overview of this topic and has answered some of the most frequently asked questions.