The Community Fire Behavior model for coupled fire–atmosphere modeling: implementation in the Unified Forecast System

There is an increasing need for simulating the evolution of wildland fires. The realism of the simulation increases by accounting for feedbacks between the fire and the atmosphere. These coupled models combine a fire behavior model with a regional numerical weather prediction model and have been used for fire research during the last few decades. This is the case, for instance, for the widely adopted Weather Research and Forecasting model with fire extensions (WRF-Fire). Typically, the coupling includes specific code for the particular models being coupled such as interpolation procedures to exchange variables between the atmospheric grid to the fire grid, and vice versa. However, having a fire modeling framework that can be coupled to different atmospheric models is advantageous to foster collaborations and joint developments. With this aim, we have created, for the first time, a fire behavior model that can be connected to other atmospheric models without the need for developing specific low-level procedures for the particular atmospheric model being used. To this end, the fire behavior model, referred to as the Community Fire Behavior model (CFBM), makes use of the Earth System Modeling Framework (ESMF) library to communicate information between the fire and the atmosphere. More specifically, CFBM is available as a national unified operational prediction capability (NUOPC) component which allows for an agnostic coupling to atmospheric models and other Earth system components. CFBM closely follows WRF-Fire version 4.3.3 methods in its version 0.2.0 which allows us to verify the adequacy of our implementation. The CFBM can be also run offline using an existing WRF simulation to propagate the fires in what we refer to as the standalone model. Herein we describe CFBM and its implementation in the Unified Forecast System (UFS). Simulations of the Cameron Peak Fire performed with UFS and WRF-Fire are presented to compare results from both models in order to verify our implementation. Results from both models, as well as with the standalone version, are consistent indicating a proper development of the CFBM and its coupling to the UFS-Atmosphere. This is the starting point to go beyond WRF-Fire methods and improve the realism of the simulations in the future. The possibility of using the fire behavior model with other atmospheric models provides an attractive collaborative framework to further improve the realism of the CFBM model in order to meet the growing demand for accurate wildland fire simulations.