1 An Improved Fire Model in LPJ-SPITFIRE By Leilei Dong University of Bristol 14/10/2008
2 What is LPJ-SPITFIRE Modelling? LPJ-SPITFIRE simulates the whole process of the uncontrolled wildland surface fire, from its occurrence and behavior to the trees killing and fire emissions.
3 What can LPJ-SPITFIRE do? 1.It predicts daily, monthly and annual number of fire caused by both lightning strike and human ignition. 2.It predicts the combustion intensity of each fire. 3.It predicts daily, monthly and annual global area burnt of each fire. 4.It predicts daily, monthly and annual global CO2 emission from each fire. 5.It predicts daily, monthly and annual global trace gas emissions from each fire. 6.So It can predicts (1) the fire regime, including fire frequency, fire size, fire severity, fire seasonality and fire effects in the global ecosystem; (2) the relationship between climate and fire regime because the characteristics of a fire regime is primarily determined by climate, which determines the weather and duration of the fire season. These climatic characteristics drive a host of critical fire-related variables, such as fuel moisture content and the frequency of lightning-caused ignitions. On the other hand, the emissions of fire also provides a pathway by which the biosphere affects climate.
7 Assumptions Made in Fire Model – Fire Occurrence 1.Averaged monthly lightning climatology averaged from 1995 to 2000 (vs. the fact that lightning activities vary from year to year). 2.Every lightning flash has the same intensity and current. 3.Each lightning stroke strikes different locations in the gridcell outside the area burnt. 4.Smouldering combustion for more than one day is not considered (vs. the fact that fire can start on the day when there is no ignition source).
8 Assumptions made in Fire Model – an Idealized Fuel Model 1.Fuel is distributed uniformly and continuously in the fuel bed. This implies that (1) each of them has the constant physical and chemical properties; (2) fire can spread uninterrupted; (3) under the same conditions, fire spreads in a steady state with neither fire accelerating nor fire decelerating, i.e., there is no change in fire spread rate. 2.The fuel bed remains flat with no slope.
10 Fuel Type 1.Fuel Type: light fuel and heavy fuel characterized by 5 size class from 9 PFT. 2.The timelag is the time required for the fuel to reach 63% of the difference between the initial moisture content and the equilibrium moisture content. The categories are named for the “midpoint” of the response time of each fuel category: 1-hour fuels respond in less than 2 hours, 10-hour fuels respond in 2 to 20 hours, 100-hour fuels respond in 20 to 200 hours, and 1000 hour fuels respond in greater than 200 hours. 3.1-hour time lag fuels are those with the diameter less than 0.625cm (0.25 inch). These include fallen needle and leaf litter, grassy fuels, lichens, and small twigs. They are the most important for carrying surface fires and their moisture content governs fire behavior. 4.10-hour timelag fuels are those with the diameters between 0.25 – 1 inches. These include small branches and woody stems. 5.100-hour timelag fuels are those with the diameters between 1 –3 inches. These include larger downed woody debris. 6.1000-hour timelag fuels are those with the diameters larger than 3 inches. These include large downed branches, logs, and tree stumps, etc.
11 Fire Ignition 1.For ignition to occur, all the following things must exist simulaneously: (1) sufficient air supply; (2) Enough fuel vapor in the air; (3) sufficient heat to provide the required energy to reach the ignition temperature (approximately 380 degrees centigrade) for the chemical reaction to start). If there is insufficient energy, the molecules do not react with each other. The energy to cause ignition must be in the form of heat and makes the molecules of fuel and of oxygen nearby move faster. This energy is called kinetic energy. If they move fast enough, they will react when they collide. If they do not move fast enough, they will bounce off each other when they collide. 2. Ignition normally takes place in the dead component of the fine fuels. Living fuel (herbaceous vegetation) in the fine fuel complex reduces the probablity of ignition.
12 Fire Ignition Sources Natural Sources: lightning, volcano eruption, spontaneous combustion, etc. Human Sources: intentional and negligence
13 Lightning - Caused Fires – Lightning Lightning is a release of charge buildup that occurs within a cloud. This exchange of charge can occur within a cloud, between clouds, from a cloud to clear, or between a cloud and ground.
14 Lightning - Caused Fires – Lightning Characteristics 1.The current of each stroke within a lightning flash is thousands of amps in strength. 2.Lightning flashes aren’t all the same shape or size, and they don’t all carry the same amount of electrical current. 3.Most lightning occurs with positive charge near the top and negative charge from middle to cloud base. There are about 10% to 20% of all cloud-to-ground flashes discharge positive charge to the Earth, and carries as much as ten times the current of negative CGs, often last longer, and are often separated from rain-bearing parts of a storm.
15 Lightning Climatology – Input Data File OTD-LIS data set from 1995 to 2005. LIS – Lightning Imaging Sensor on the Tropical Rainfall Measuring Mission Satellite operating from 1995 to 2000. OTD – Optical Transient Detector flying aboard the Microlab-1 Spacecraft operating from 1998 to 2005. Each Satellite saw only a part of the Earth. Lightning flash rate density is calculated by a counting experiment, i.e., Lightning flash rate density at a given grid location = number of lightning flash detected / sensor viewing time
19 Physical Model – Fire Spreading through a Given Fuel Bed
20 Physical Model – Fire Spreading through a Given Fuel Bed
21 Physical Model – Fire Spreading through a Given Fuel Bed Heat from the combusting fuel, I.e., the flame, is transferred to the neighrouring unignited fuel via three heat transfer mechanisms: radiation, convection and conduction. Radiation is the primary heat transfer mechanism when there is no wind. Radiation is being emitted from two sources: (1) the glowing surface of the fuel bed; and (2) the flame attached to this surface. When wind is present, wind will bring the hot combusted product closer to the unignited fuel and replace them to the air that surrounds the unignited fuel. Conduction occurs between different layers in the fuel bed. Because wood is a porous material and it is a poor heat conductor, conduction is usually not the primary heat transfer mechanism in a wildland fire. The heat being transferred to (i.e., received by) the unignited fuel raises its temperature from ambient to ignition point if the heat received is high enough to vaporize al the water contained in the fuel and volatize enough high amount of fuel gas from the fuel ready for burn. Once the adjacent fuels are ignited by the upstream flame, this newly started flame acts as the ignition source, to ignite the unburned fuel repeating the process 1 and 2.
22 Fire Spread Model – Rothermel’s Empirical Solution 1.Fire spread rate is determined by the first thermodynamic law (energy conservation) under the steady state: Under the steady state, for the unit mass of the fuel, we have Where is the heat received by unignited fuel ahead of the fire, m is the heat transfer mechanism, and m=3, representing radiation, convection and conduction; is the heat required to ignite the fuel, and n is the number of fuel type in the fuel bed; is the fuel moisture content; V is the latent heat of vaporization.
23 Environmental Factors Driving Fire Behavior Three Principle Environmental Factors driving the Fire Behavior: Fuel type and characteristics, weather conditions and terrain. Weather conditions include wind (speed and direction), temperature, humidity of air, precipitation and fuel moisture, etc.
24 Effects of Wind 1.Wind results in a forward lean of the flame front, which reduces the distance between the flame and the uignited fuel and thus enhances radiative heat transfer. 2.Wind also increases the rate of convective heat transfer between the hot combustion product and heated air and the unignited fuel. 3.Wind passing through the interior of the fuel bed speeds the exchange of moisture between the ambient air and the fuel, reducing or increasing the energy required for ignition. Effects of Temperature: Temperature affects how much heat is needed to ignite the fuel.
25 Effects of Humidity: High humidity of the air could dampens the fuel, slowing down the spread of flames. Because humidity is greater at night, fires will often burn less intensely at this time under normal circumstances. Effects of Terrain Shape of the Landscape; Slope Steepness; Slope Aspects
26 Effects of Fuel Moisture 1.Fuel moisture content is a weather-related variable. 2.Fuel moisture of dead fuel varies daily according to the ambient temperature and humidity, while fuel moisture of live fuel varies on a seasonal basis as the plant goes through the phonological cycle through growth to drying. 3.Increased values of fuel moisture contribute to reduced rates of fire spread because additional energy is required to vaporize the water before bring the respective fuel particles to ignition.