This dialog box allows you to modify fire behavior calculations and other options for the simulation.
This option enables a surface fire to make the transition to some form of crown fire depending on the crown fuel conditions and surface fire behavior. Selecting this option does not force a crown fire, but allows the FARSITE model to determine if transition occurs (see Crown Fire in the Technical Documentation) and adjust fire rate of spread accordingly. Crown Base Height (CBH), Crown Bulk Density (CBD), and Stand Height themes contained in the Landscape (.LCP) File are best for FARSITE to accurately simulate crownfire. However, constants for crown fuels can be entered for the entire landscape when crown fuel variability is low with the Input > Canopy Characteristics command.
Crown bulk density is critical to determining crown fire spread. This option forces the simulation to use the crown cover attributes for each cell to modify the crown bulk density values for that cell. This will be important to crown fire behavior if you are using constant crown bulk density information for the landscape (see Input > Canopy Characteristics) or have specified only the maximum crown bulk densities in the raster theme used in the Landscape (.LCP) File. DO NOT select this option if your crown bulk density theme in your Landscape (.LCP) File contains site specific crown bulk densities that are already representing actual variability of crown fuels.
If the crown fire calculations are enabled, embers may be lofted by torching trees. Embers of a given size class are lofted and their contact points with the landscape are computed by iterating their descent through a modeled wind field. (see Spotting in the Technical Documentation)
This option will enable a variable percentage of embers that land on receptive fuel to ignite new fires. The simulation will grow each of these as new point ignitions. The slider changes the percentage of live embers that cause ignitions. This setting is NOT equivalent to a calculated Probability of Ignition. Simulations takes too long for most purposes with the many spot fires started when the ignition frequency is near 100%. In most situations the ignition frequency should be set at less than 10%.
The Ignition Delay time is simply the time between a ember landing and when it begins a spot fire. Longer times with this option will slow down the spread effect of spot fires.
This option forces the use of the No-Wind No-Slope (NWNS) rate of spread for the spread rate of backing fires (Rothermel 1983). This has been proposed as a solution to problems of fire area with increasing wind speed if the origin of fires assumed to coincide with the rear focus of an ellipse (Bilgili and Methven 1990); if not selected, the backing spread rate is computed as a product of elliptical dimensions (see Limitations and Assumptions, and Elliptical Dimensions in the Technical References).
This option attempts to eliminate illogical expansions of the fire perimeter at each time step. These illogical expansions are caused by small local concavities involving a series of three perimeter points; the orientation off these points forces them to cross over previously burned areas. Enabling this option will not affect the fire shape at the scale of the whole fire, but will affect some localized concave portions of the perimeter. These operations also increase the processing time for the simulation.
Distance checking compares the advance of the simulated fire perimeter to the values of the Distance and Perimeter Resolutions set in the "Parameters" dialog box. If the simulated fire reaches either the Distance or Perimeter Resolution within a time step, FARSITE recalculates the fire behavior characteristics. This prevents fast simulated fires from skipping over changes in the landscape.
The two choices in the "Fire Behavior Options" dialog box are to compare either the maximum spread of an individual fire (the Fire-Level Distance Checking radio button) or the maximum spread of all simulation fires (Simulation-Level Distance Checking radio button) with the parameter resolutions in the distance check. Simulation-Level Distance Checking is required if the simulation includes post-frontal combustion. FARSITE won't let you select the Fire-Level Distance Checking radio button if the Simulate Post-Frontal Combustion checkbox is selected in the "Post-Frontal Calculations" dialog box.
More information on distance checking can be found in the Huygen's Principle section of the Technical References.
Future versions of FARSITE will likely permit some options for methods to determine how the simulation controls the spatial and temporal precision of the calculations. There is currently only one option available in this release of FARSITE.