A Comparative Study between FLEXPART-WRF and HYSPLIT in an Operational Setting: Analysis of Fire Emissions across complex geography using WRF

Abstract

Transport and dispersion models are frequently used by the meteorological community to understand and predict the trajectories of anthropogenic, natural and accidental chemical releases of hazardous materials. There are several reputable dispersion models that can handle a wide range of applications under the direction of global, synoptic or mesoscale forecasts. One such application is the forecast of smoke emissions from wildfires which is important to operational air quality and meteorology communities. Fire emissions have direct impacts to property and respiratory health. Operational meteorologists are responsible for providing meteorological support to emergency management agencies within their county warning area in the event of incidents involving harmful chemical releases, radiation and smoke emissions. A comparative study between two dispersion models during recent wildfire events across complex geography is presented to identify the sensitivities of each dispersion model and the operational benefits of utilizing each model for smoke emission forecasts. FLEXPART-WRF is a Lagrangian dispersion model that predicts the transport and dispersion of trace gases forward or backward from a point, line or area source. Similar to FLEXPART-WRF, the HYbrid Single Particle Integrated Trajectory (HYSPLIT) model simulates the dispersive nature of the environment. Model configuration differences include the prerequisite meteorological data, density correction, dispersion algorithms and removal calculations. Mesoscale meteorological models are needed to provide the ambient environment as well as simulate the small scale flux exchanges and boundary layer processes that can affect dispersion simulations on a local and regional scale. Therefore, both dispersion models are using meteorological input from the WRF ARW mesoscale atmospheric model using both a 12 km and 4 km grid-resolution domain. Two fire events, one along the coast of the Mid-Atlantic (Evans Road Case) and the other within the Appalachians (South Mountain Case), are investigated for this analysis. Simulations are analyzed to identify the relative performance of each dispersion model given identical meteorological input. The dispersion models are evaluated for accurate dispersive simulations and also on their ability to support operational forecast needs. Satellite observations provided by the National Environmental Satellite, Data and Information Service along with other remote sensing tools are used for evaluation of dispersion model performance. The spatial analysis, based on both case studies and resolutions, indicates that HYSPLIT disperses particles 10-20 degrees to the right of FLEXPART-WRF for at least a portion of the simulations. FLEXPART-WRF better replicates the observed plume and also yields a higher air concentration throughout most of the simulations, especially downwind. These differences in plume compositions and concentrations are likely linked to the differing diffusion equations. While the air concentration differences are small compared to the amount being released, the spatial differences are statistically significant. To account for the air concentration differences, dry deposition is analyzed. HYSPLIT sporadically deposited significantly more mass to the ground compared to FLEXPART-WRF. These deposition differences impact the diffusion process and account for only part of the concentration variations. This study suggests that FLEXPART-WRF performs better compared to HYSPLIT and may serve as an improved operational tool.