Abstract
The ventilation index (VI) is an index that describes the potential for smoke or other pollutants to disperse from a source. In this study, a Lagrangian particle dispersion model was utilized to examine smoke dispersion and the diagnostic value of VI during a September 2018 prescribed fire in southwestern Colorado. Smoke dispersion in the vicinity of the fire was simulated using the FLEXPART-WRF particle dispersion model, driven by meteorological outputs from Advanced Regional Prediction System (ARPS) simulations of the background (non-fire) conditions. Two research questions are posed: (1) Is a horizontal grid spacing of 4 km comparable to the finest grid spacing currently used in operational weather models and sufficient to capture the spatiotemporal variability in wind and planetary boundary layer (PBL) structure during the fire? (2) What is the relationship between VI and smoke dispersion during the prescribed fire event, as measured by particle residence time within a given horizontal or vertical distance from each particle’s release point? The ARPS no-fire simulations are shown to generally reproduce the observed variability in weather variables, with greatest fidelity to observations found with horizontal grid spacing of approximately 1 km or less. It is noted that there are considerable differences in particle residence time (i.e., dispersion) at different elevations, with VI exhibiting greater diagnostic value in the southern half of the domain, farthest from the higher terrain across the north. VI diagnostic value is also found to vary temporally, with diagnostic value greatest during the mid-morning to mid-afternoon period, and lowest during thunderstorm outflow passage in the late afternoon. Results from this study are expected to help guide the application of VI in complex terrain, and possibly inform development of new dispersion potential metrics.
Highlights
In this study, FLEXPART-Weather Research and Forecast (WRF) [1], a Lagrangian particle dispersion model, was employed to examine the diagnostic value of the ventilation index (VI) for smoke dispersion in complex terrain in southwestern Colorado
Is a horizontal grid spacing of 4 km comparable to the finest grid spacing currently used in operational models (e.g., the 4-km North American Mesoscale (NAM) [12,13] and 3-km High Resolution Rapid Refresh (HRRR) [14,15]), sufficient to capture the spatiotemporal variability in wind and planetary boundary layer (PBL) structure during the Saul’s Creek fire, or is finer grid spacing necessary? This question has relevance to prescribed burning in the western United States (US), as models with O(4 km) grid spacing are critical components of National Weather Service (NWS) fire weather forecasts of VI and other metrics used by land managers to assess smoke dispersion potential [16]
Before proceeding to the assessment of FLEXPART-WRF simulated particle dispersion and VI diagnostic value, it is important to establish whether the Advanced Regional Prediction System (ARPS) simulations capture the spatiotemporal variations in meteorological variables necessary for FLEXPART-WRF to simulate plume behavior with fidelity
Summary
FLEXPART-WRF [1], a Lagrangian particle dispersion model, was employed to examine the diagnostic value of the ventilation index (VI) for smoke dispersion in complex terrain in southwestern Colorado. VAR category thresholds can vary considerably from state to state and from one NWS forecast office to another, with identical VI value ranges associated with a “Poor” VAR in one location and a “Good” VAR in another [5]. Since VI is used by fire managers across the US, including in areas of complex terrain [6], a study of the relationship, if any, between VI and smoke dispersion in and around complex terrain is potentially of great value. To the best of the authors’ knowledge, only two studies have examined VI and its diagnostic utility for smoke dispersion, in complex terrain or otherwise: [7,8]
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