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8.4 the Influence of Atmospheric Instability on Fire Behaviour in the Northwest Territories, Canada

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8.4 the Influence of Atmospheric Instability on Fire Behaviour in the Northwest Territories, Canada 8.4 THE INFLUENCE OF ATMOSPHERIC INSTABILITY ON FIRE BEHAVIOUR IN THE NORTHWEST TERRITORIES, CANADA B. Kochtubajda1 , M.D. Flannigan2, J. R. Gyakum3, and R.E. Stewart4 1Meteorological Service of Canada, Edmonton, Alberta, CANADA 2Canadian Forest Service, Edmonton, Alberta, CANADA 3McGill University, Montreal, Quebec, CANADA 4Meteorological Service of Canada, Downsview, Ontario, CANADA 1. INTRODUCTION The objectives of this study are to examine and compare the relationship of atmospheric severity Wildfires pose the greatest danger within the indices and large-scale circulations with lightning boreal ecosystem for fire managers in the activity and fire behaviour in the Northwest Northwest Territories (NWT). Although they are Territories. Two fire seasons were selected. The beneficial to the control of diseases and insects summer of 1995 was an extreme fire year, during as well as maintaining biological diversity, fires which fires burned 2.8 Mha In 1998, another also threaten human life, property and valuable above average fire year, fires burned about 1.4 commercial resources. The fire season typically Mha. starts in late May and usually ends by early September. 2. STUDY AREA Weather is critical to forest fires. Not only does it The Mackenzie River Basin covers about 1.8 affect the ignition of fires through lightning, but it million km2, and encompasses five fire also influences fire behaviour. Strong winds, management jurisdictions (see inset, Figure 1). high temperatures and low humidity enhance the The NWT extends poleward of the 60th latitude rate of fire growth. Atmospheric instability, and occupies about 47.1% of the basin area. another important factor in fire growth, can The NWT is divided into five fire regions (Fig. 1). influence the spread and intensity of wildfires. For example, in an unstable atmosphere, smoke The major topographic features include the column circulations can be strengthened which in Mackenzie Mountains west of the Mackenzie turn may entrain additional oxygen from the River and the Great Bear and Great Slave Lakes surrounding environment to intensify the fire. east of the River. The rocky Canadian Shield characterizes the eastern region of the NWT, Various thermodynamic and severity indices while arctic tundra typifies the area in the north. have been developed to help fire managers The forested region of the NWT covers about assess severe weather potential and fire risks. 615,000 km2. Black spruce, lodgepole, jack pines Indices such as George“s K, the Lifted Index, and and trembling aspen are the common tree the Total-Totals are used to predict thunderstorm species in the NWT (Rowe 1972). Numerous development. In 1988, Haines introduced the small lakes and marsh areas dot the landscape. lower atmospheric stability index (LASI) for wildland fires (Haines, 1988). LASI is based The climate is influenced by several factors upon the stability and moisture content of the including: latitude, solar radiation, topography, lower atmosphere. Low, middle and high level and weather systems (Phillips, 1990). Although indices were developed to reflect regional incoming solar energy which arrives at low elevations. An appropriate level is selected that angles limits the amount of surface warming, is high enough above the surface to avoid increased day-lengths in the summer offset this significant diurnal variation. Research studies limitation. At Yellowknife, sunlight varies from carried out in the United States have shown a about 20 h in June, while at Inuvik the sun does relationship between the vertical structure of the not set in the midsummer. Summer average atmosphere and fire activity (Werth and Ochoa, monthly maximum temperatures are about 20°C, 1993). however, daily temperatures can reach well above 30°C. Corresponding author address: Annual precipitation totals over the NWT range from 200 to 500 mm. Convective systems are a B. Kochtubajda, Meteorological Service of common feature during the summer months over th Canada, Room 200, 4999 98 Ave. Edmonton, most areas and they account for the majority of AB. CANADA, T6B 2X3; e-mail: the annual precipitation (Stewart et al. 1998). [email protected] However, there is considerable variation from year to year. The convective storm season and resultant years, an annual average of about 310 fires have lightning activity is characterized as short but consumed approximately 650,000 ha in the NWT intense with a strong peak in cloud-to-ground (Kasischke and Stocks, 2000). The number of lightning during June and July. The diurnal fires and the area burned are highly variable from distribution of strikes indicates that most of the year to year (Fig. 2). The lowest and highest lightning is linked with daytime-heating initiated number of fires in any year occurred in 1997 and thunderstorms (Kochtubajda et al. 2001) . 1994, when 105, and 627 fires were started, respectively. The lowest area burned was 37,000 Associated lightning activity can also generate ha in 1974, while a record of more than 3 Mha of major forest fires. Lightning typically starts forest was consumed in 1994. approximately 80% of the forest fires in the NWT (Epp and Lanoville, 1996). Over the past 30 Figure 1: A map of the study area. The upper air stations used in the analysis are superimposed. Fire History in the NWT (1970-1999) 700 3.5 600 3 ) a 500 2.5 h M s ( e r d i e F 400 2 n f r No. of Fires o u r b Area burned ( Mha) e a b 300 1.5 e r m a u l N a 200 1 t o T 100 0.5 0 0 0 5 0 5 0 5 7 7 8 8 9 9 9 9 9 9 9 9 1 1 1 1 1 1 YEAR Figure 2: Annual variation of the number of fires and forest area burned in the Northwest Territories, from 1970 to 1999. 3. DATA AND METHODS values represent a greater likelihood of extreme fire behaviour. The study used a variety of data sources. These Fires larger than 200 ha comprise the Large Fire include the archived lightning strike data from the Database. Although small in number, large fires Northwest Territories government; fire data from account for approximately 97% of the total area the Canadian Forest Service“s national Large burned (Stocks et al. 2001). Fires larger than Fire Database (Stocks et al. 2001), and the 200 ha, 1000 ha, and 10,000 ha were used to territorial fire database; sounding data from the determine whether a relationship between Environment Canada upper air digital archive; stability indices and fire size existed. and the historical gridded data from the National Centers for Environmental Prediction (NCEP). To investigate whether the various thermodynamic and severity indices were related Daily lightning strike statistics were determined to fire behavior we calculated the head fire from the lightning detection network archive intensity (HFI kW/m) for each large fire in the operating in the Northwest Territories. The NWT during the 1995 and 1998 fire season using characteristics of the network are described in the Canadian Fire Behaviour Prediction (FBP) Kochtubajda et al. (2001). System (Forestry Canada 1992). We assumed a standard fuel type, black spruce, in this case. Thermodynamic and severity indices were Meteorological variables required to calculate the calculated from the 00 UTC radiosonde releases HFI include temperature, relative humidity, wind at the upper air stations near Fort Smith and speed and 24-h precipitation. These values were Norman Wells (Fig. 1). The thermodynamic interpolated from surrounding meteorological indices are defined in Table 1. The 00 UTC stations in a procedure outlined in detail in Amiro soundings were selected instead of the 12 UTC et al. (2001). Using SAS version 8 we calculated soundings because few ”nocturnal" and early- the variance explained in the HFI using each morning lightning strikes are detected, and index separately. We also calculated the because the atmospheric conditions at this time variance explained in the daily lightning strike are more representative of the conditions when data using all the indices in SAS. forest fires are most active. 4. RESULTS and DISCUSSION Data from the Norman Wells station were used to determine lapse rates and moisture values for Stability, moisture and LASI index distributions all fires in the Inuvik, Sahtu, and Deh Cho for the large fires in both seasons are shown in regions, and the Fort Smith station was used for Figure 3. The indices were also calculated for all fires in the North and South Slave regions. the entire 1995 and 1998 fires seasons (May 1 - Haines mid-level index was considered most Sep30) to provide a climatology on the relative applicable to the NWT (Haines, 1998). frequencies of the indices. The analysis of the mid-level stability shows that Factor A is able to The mid-level stability and moisture index was discriminate large fires. We found that less than calculated by determining the temperature 1% of the fires started when the mid-level difference between the 850 and 700 mb levels temperature difference is less than 6°C and that and the atmospheric moisture content at the 850 about 60% of the fires began when the mid-level mb level. The stability factor (A) is assigned a temperature difference is greater than 11 °C. value between 1 and 3 based on the temperature lapse rate between the levels. If the temperature The mid-level moisture, Factor B, on the other lapse rate between the 850 and 700 mb level is hand was not useful in discriminating large fires. greater than or equal to 11°C, the stability factor About 20% of the fires were initiated when the is given a value of 3. If the lapse rate is less dew point depression was greater than 13°C, than 6°C, a value of 1 is given to the stability and about 40% of the fires began when the dew factor.
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