MOSAIC Bug Fix: Error In Modal_aero_amicphys.F90

Introduction

This article addresses a bug encountered in the MOSAIC (Model for Simulating Aerosol Interactions and Chemistry) module, specifically within the modal_aero_amicphys.F90 file. This issue was identified during simulations involving NH3 related chemicals using CAM-Chem (Community Atmosphere Model with Chemistry). The error occurred during the case building process, preventing successful execution. This article outlines the problem, the steps to reproduce it, the solution, and other relevant details.

The Bug: Logical Expression Error in modal_aero_amicphys.F90

The primary issue lies within the modal_aero_amicphys.F90 file, where the variable mwaer_from_advmass is incorrectly declared as an integer instead of a logical. This leads to an error during compilation because the IF clause in the code requires a scalar logical expression. Specifically, the error message encountered was:

_5635 |                if ( mwaer_from_advmass ) then
         |                   1
Error: IF clause at (1) requires a scalar LOGICAL expression_

In simpler terms, the code expects a true or false value for mwaer_from_advmass within the IF statement. However, because it was defined as an integer, the compiler flags it as an error. This error prevents the model from building correctly, hindering any subsequent simulations or analyses. Let’s delve deeper into why this seemingly small error can have significant consequences for atmospheric modeling.

Atmospheric models, like CAM-Chem with MOSAIC, are complex systems with numerous interacting components. The correct declaration and usage of variables are crucial for the model to accurately represent real-world processes. In this case, mwaer_from_advmass likely controls a specific pathway or calculation related to aerosol mass transfer. If this variable is not correctly interpreted, the model's representation of aerosol dynamics and chemical reactions can be significantly skewed. This, in turn, can impact the accuracy of simulations concerning air quality, climate change, and other environmental phenomena. For instance, incorrect aerosol representation can lead to inaccurate predictions of cloud formation, radiative forcing, and the transport and deposition of pollutants. Therefore, identifying and rectifying such bugs is paramount for ensuring the reliability of scientific findings derived from these models.

Furthermore, this type of error highlights the importance of rigorous code review and testing procedures in scientific software development. Even a single incorrect variable declaration can have cascading effects, underscoring the need for thorough quality assurance practices. In collaborative projects, where numerous researchers contribute to the codebase, maintaining code integrity is a continuous challenge. Robust version control systems, automated testing suites, and standardized coding practices are essential tools for mitigating the risk of such errors. The open-source nature of models like CAM-Chem allows for community-driven bug identification and resolution, which ultimately strengthens the reliability and usability of the model for the broader scientific community. The swift identification and correction of this bug in MOSAIC exemplifies the power of collaborative software development in scientific research.

Steps to Reproduce the Bug

To reproduce this bug, follow these steps:

1. Clone the MOSAIC source code from the GitHub repository using the following command:

   git clone https://github.com/ESCOMP/CAM -b zz_cam6_3_018_mosaic0.3 MOSAIC_CODE
   

2. Within your CESM2.2.2 directory, rename the original components/cam directory to components/cam_origin. This preserves the original CAM code.

   mv CESM2.2.2/components/cam CESM2.2.2/components/cam_origin
   

3. Copy the downloaded MOSAIC code into the components directory and rename it to cam:

   cp -r MOSAIC_CODE CESM2.2.2/components/cam
   

4. Set up a FCHIST case. This compset (component set) is used for testing and historical simulations.

5. Build the case. The error will occur during the build process when the compiler encounters the type mismatch for mwaer_from_advmass.

It's important to note that while the bug is reproducible in the FCHIST compset, MOSAIC is not intended to function fully within this configuration. The user in this instance used FCHIST for testing purposes due to the data download requirements of FCnudged, which is another compset. The primary goal here is to demonstrate the error encountered during the build process, regardless of the specific compset used. Replicating the error in a controlled environment is crucial for confirming the bug and verifying the effectiveness of the fix. By providing detailed steps, other users can independently verify the issue and contribute to the development of robust solutions.

The ability to reproduce bugs consistently is a cornerstone of software development and quality assurance. It allows developers to isolate the root cause of the problem and implement targeted solutions. Without a clear set of steps to reproduce an issue, troubleshooting becomes significantly more challenging and time-consuming. In the case of scientific models like CAM-Chem, ensuring reproducibility is particularly vital, as it contributes to the transparency and reliability of research findings. The steps outlined above serve as a valuable resource for other researchers encountering similar issues and for developers working on future improvements to the MOSAIC module.

The Solution: Correcting the Variable Type

The solution to this bug is straightforward: the variable mwaer_from_advmass needs to be declared as a logical type instead of an integer. This can be achieved by modifying line 93 in the modal_aero_amicphys.F90 file. Change the declaration from:

integer, public :: mwaer_from_advmass = .false.

to:

logical, public :: mwaer_from_advmass = .false.

This simple change ensures that the variable can hold boolean values (true or false), which is required for its use in the IF statement. After making this correction, the case should build and run successfully. This highlights the importance of paying close attention to variable types in programming, especially in languages like Fortran where explicit type declarations are crucial. A seemingly small oversight can lead to significant errors and prevent the program from functioning as intended. The fix underscores the necessity of careful code review and testing to catch such issues early in the development process.

While the fix itself is concise, understanding the underlying implications of this type of error is paramount. As mentioned earlier, incorrect variable typing can lead to misinterpretations of data within the model, potentially skewing results and impacting the reliability of simulations. In this specific case, ensuring that mwaer_from_advmass is treated as a logical variable is critical for the correct flow of logic within the modal_aero_amicphys.F90 subroutine. This subroutine likely handles calculations related to aerosol mass transfer and interactions, and the boolean value of mwaer_from_advmass might control the activation or deactivation of certain processes within these calculations.

Therefore, the correction not only resolves the immediate build error but also safeguards the integrity of the model's representation of atmospheric processes. This illustrates the interconnectedness of code elements in complex scientific models and emphasizes the need for a holistic approach to bug fixing. Simply addressing the error message without understanding its broader context might lead to overlooking potential downstream consequences. The thorough investigation and clear explanation provided in the original bug report are commendable, as they facilitate a more complete understanding of the issue and its resolution.

Additional Information

• CAM Tag: CAM in CESM2.2.2 was used when the bug was encountered.
• Compiler: The GNU compiler was used.
• Machine: The user encountered the bug on a university cluster. More information about the cluster environment can be found at https://qingli411.github.io/eesrf-hpc-user-guide/hpc1/environment.html.
• Addressing the Bug: The user who reported the bug addressed it themselves by modifying the variable type in the source code.

This information provides context about the specific environment and tools used when the bug was discovered. Knowing the CAM tag helps identify the version of the model affected, while the compiler and machine details can be relevant for understanding potential environment-specific issues. The fact that the user addressed the bug themselves highlights the importance of community contributions in open-source projects. By actively participating in bug fixing and code improvements, users contribute to the overall quality and reliability of the software.

The reference to the university cluster's user guide is particularly useful, as it provides insight into the computing environment where the bug was encountered. This can be helpful for other users running CAM-Chem on similar systems, as it might reveal potential compatibility issues or configuration requirements. The combination of detailed bug reports and readily available information about the computing environment facilitates collaborative troubleshooting and knowledge sharing within the scientific community. This collaborative spirit is a key factor in the success of open-source modeling efforts, as it allows researchers to leverage the collective expertise and resources of a diverse group of users and developers.

Conclusion

This article detailed a bug found in the MOSAIC module within CAM-Chem, specifically in the modal_aero_amicphys.F90 file. The issue involved an incorrect variable type declaration for mwaer_from_advmass, which caused a compilation error. The solution was to change the variable type from integer to logical. This fix ensures the correct execution of the model and the accurate representation of aerosol processes. This example highlights the importance of careful coding practices and community involvement in maintaining the integrity of scientific software.

For further information on CAM-Chem and MOSAIC, please visit the NCAR (National Center for Atmospheric Research) website.