the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Jul 2025
Dynamical Linkages Between Planetary Boundary Layer Schemes and Wildfire Spread Processes
Abstract. Wildfires can significantly enhance surface sensible heat and modify the state of the near-surface atmosphere, becoming key factors in triggering turbulence and restructuring the boundary layer. This study uses high-resolution simulations with WRF-Fire, combined with hourly observational data from six meteorological stations (five national and one emergency stations) during the Jinyun Mountain wildfire in Chongqing, China, to systematically evaluate the performance of five planetary boundary layer (PBL) schemes (MYJ, MYNN2, MYNN3, BouLac, UW) in simulating temperature, wind speed, and turbulence intensity. Results show that all schemes can reproduce the diurnal trends of temperature and wind speed but exhibit significant differences in amplitude response and simulation errors. The MYNN3 scheme captures the spatiotemporal variations of turbulence intensity and wind speed at different heights more accurately, thereby better representing the 2-meter temperature and 10-meter wind speed response and reduces the model’s cold bias in high-temperature simulations. The BouLac and MYNN2 schemes also show some response to thermal disturbances at certain sites but perform poorly under strong perturbations and exhibit large fluctuations. The MYJ and UW schemes show overall weaker turbulence and fail to capture local circulation variations effectively. Turbulent energy budget analysis of MYNN3 indicates a buoyancy-dominated turbulence generation mechanism, with vertical transport promoting upper-level disturbances. This reveals MYNN3’s greater sensitivity to wildfire thermal perturbations and more complete feedback processes, providing a scientific basis for selecting PBL schemes in mountainous wildfire simulations.
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Status: open (until 20 Dec 2025)
- CEC1: 'Comment on egusphere-2025-3072', Astrid Kerkweg, 30 Jul 2025 reply
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RC1: 'Comment on egusphere-2025-3072', Anonymous Referee #1, 16 Nov 2025
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This manuscript evaluates five PBL schemes within the WRF-Fire coupled atmosphere-fire model using simulations of a wildfire case in a mountainous region in China. The study evaluates the simulations using near-surface variables (temperature at 2 m and wind at 10 m), and vertical turbulence structures (TKE and PBL height). The manuscript is well written, the figures are high quality, and there's scientific value in the analysis. However, the manuscript doesn't seem to include a model development component, so it may be best to transfer to another journal.
Issues to be addressed:
It is not entirely clear what is the role of including the fire component in these simulations, given that fire spread was not compared or discussed. It seems the manuscript's primary goal is to evaluate the PBL schemes under “stressed” conditions, which happens to be fire heat. The manuscript does not compare the effect of the different PBL in the resulting fire spread, and does not quantify the magnitude of the fire perturbation to the PBL (e.g. TKE in coupled simulation minus TKE in uncoupled simulation). A revised analysis including results of the simulated fire would be a substantial contribution to the existing coupled fire modeling literature.
Comparing the PBL schemes and their representation of the processes during a fire provides helpful guidance on the characteristics of these schemes under non-typical conditions. However, fire-induced processes occur at the microscale, where turbulent eddies propagate in the 3-D space, which are not represented by a PBL scheme. Most, if not all, of these schemes are 1-D. There are no fluxes outside of the vertical column over each grid point. Hence, it's expected that all PBL schemes will show a poor representation of the atmospheric state in the vicinity of the fire, especially at a spatial resolution that can be resolved by LES. That's also the case in regions of complex topography.
The grid spacing of 300 is arguably too fine for using a PBL scheme, especially with fire-induced perturbations. It would be more reasonable to simulate the innermost domain using LES, with the PBL scheme in d02 providing the boundary conditions to d03, which is the typical configuration used with WRF-Fire. It's also important to consider the vertical resolution: with WRF's default configuration used in this study (i.e. CONUS), the 1st model level represents a layer starting at the surface to approx. 50 m AGL. This is an important aspect of the model configuration considering the fire heat is at the 1st few meters above the surface. A vertical layer of 50 m cannot properly represent the actual intensity of the heat and the magnitude of the perturbation it produces.
The current manuscript does not discuss these caveats, which are important aspects to be considered when simulating the atmospheric processes induced by the fire and driving the fire spread. From the perspective of someone interested in simulations that represent the fire-atmosphere coupled processes and their impact on the chaotic nature of fire spread, the model configuration used in the manuscript would be hard to justify. However, LES simulations (including LES nests within WRF) are computationally expensive, and perhaps there are other use cases where microscale processes may not be so important. Since this manuscript is not necessarily interested in addressing the typical challenges associated with WRF-Fire, I suggest the authors strengthen the motivation and potential application of this study, along with caveats in the model configuration.
The authors' motivation for using a PBL parameterization in a 300 m domain, along with a 50-m vertical layer as the 1st model level must be clearly stated. I strongly recommend they include a simulation using one of WRF's LES options in this analysis, which can then be configured with the 1st model level closer to the ground. This will provide a better understanding of the effects of using a PBL scheme instead of LES, and whether this is a critical aspect to be accounted for in these simulations. It would be desirable and insightful (but not critical) to also include one of the 3-D PBL options, which can be configured following the other PBL schemes.
Their comparison with observations is based on 2-m temperature and 10-m winds. However, the simulations' temperature and winds represent a 0-50 m deep layer. Although WRF's atmosphere and land surface are coupled, the fire heat is only coupled to the atmosphere. This means the soil temperature can only increase from the increased air temperature, not from the fire directly. This leads to more inertia in the heat transfer and underrepresented heat gradients. Considering that T2 is a diagnostic variable derived from T, and T represents a 50-m deep layer, I'd expect the fire feedback to be substantially diluted. The same applies to 10-m winds. These variables are interpolated to 2 and 10 m, they are not resolved by the model at these scales. For this reason, I suggest including a control simulation with fire feedback turned off (or just no fire at all). This will show the magnitude of the fire perturbations affecting the PBL schemes. They may be quite minor. In addition, this would allow the authors to compare how each scheme transports the fire heat, i.e. by looking at the temperature or energy difference between the control and the corresponding fire simulation for each PBL scheme.
Lastly, the CONUS namelist does not include the fire configuration. Please include the fire configuration, including ignition location and any other entry under “&fire” that is different from the model default.
*It's worth noting that the official release of the WRF model only allows a meso-LES configuration with the YSU scheme. Yet, it is possible to make it work with other schemes with minimal code modification: the variables in the registry associated with the package pbl=0 must include the same variables in the associated pbl scheme.
Citation: https://doi.org/10.5194/egusphere-2025-3072-RC1
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Dear authors,
in my role as Executive editor of GMD, I would like to bring to your attention our Editorial version 1.2: https://www.geosci-model-dev.net/12/2215/2019/
This highlights some requirements of papers published in GMD, which is also available on the GMD website in the ‘Manuscript Types’ section: http://www.geoscientific-model-development.net/submission/manuscript_types.html
In particular, please note that for your paper, the following requirements have not been met in the Discussions paper:
As you use WRF-Fire for your analysis, please add something like “a case study using WRF-FIRE version x.y” to the title of your manuscript upon submission of the revised version to GMD.
Best regards, Astrid Kerkweg