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Development of Daily Weather and Fire Danger Scenarios Using Two General Circulation Models

Daily data for North America were obtained from two general circulation models (GCMs), namely the Canadian Climate Centre (CCC) and Hadley Centre models. CCC data consisted of modeled surface temperatures, relative humidities (RH), wind speeds, and precipitation variables from the Canadian Global Circulation Model (CGCM1) for the years 1975–1995, 2020–2040, and 2080–2100. Similar weather variables for the periods 1975–1990 and 2080–2099 were obtained for the Hadley 3 (Had3GGa1) model. The period 1975–1995 roughly corresponds to the 1 x CO₂ scenario and the period 2080–2100 to the 3 x CO₂ scenario.

The grid cell size for the CGCM1 is 3.75° x 3.75°, whereas the grid for the Hadley models has slightly better resolution, at 3.75° x 2.5°. Within the Hadley model, the wind is modeled on a different grid than the other variables. All variables from the Hadley model were converted to a common grid structure for the analysis.

CCC grid structure	Hadley grid structure

Model Calibrations

The modeled variables were compared with actual, observed data during the fire season for the 1975–1995 time period, and adjustments were made to RH and precipitation. These adjustments were based on rainfall and humidity frequency distributions. Several different correction methods for RH and rainfall were tested on both models. The rainfall correction was a subtraction of 2.0 mm daily for the CCC data and 1.5 mm daily for the Hadley data. For the CGCM1, the RH correction involved first correcting the modeled specific humidity (SQ) values on the basis of observed values. Then RH was calculated from the adjusted SQ and the saturation mixing ratio. Had3GGa1 RH was calculated from the noon vapor pressure and the saturation vapor pressure. The final correction factors were then applied to the future scenarios.

The following maps illustrate this process for RH with CCC model data, with carbon dioxide (CO₂) levels unchanged from the present.

Mean daily RH (%)	Actual mean RH
0–20 21–40 41–50 51–60 61–70 71–80 81–90 91–100
CCC mean RH, 1 x CO2 (unadjusted)	CCC mean RH, 1 x CO2 (adjusted)

CCC temperature difference (3x - 1x)	Had3GGa1 temperature difference (3x - 1x)

CCC mean seasonal rainfall, 1 x CO2	CCC mean seasonal rainfall, 3 x CO2
Had3GGa1 mean seasonal rainfall, 1 x CO2	Had3GGa1 mean seasonal rainfall, 3 x CO2

FWI Calculations

The daily weather output from the GCMs was used to create noon fire weather values for use in the Canadian Forest Fire Weather Index (FWI) System. Output from the FWI System gives a measure of possible fire danger across the country. The results for each model and time period were compared to identify changes in future fire danger and also differences between the models.

The Seasonal Severity Rating (SSR) is a seasonal (May to August) average of the Daily Severity Rating (DSR), which is a weighted version of FWI that gives a measure of the daily fire control difficulty. SSR ratios of 3 x CO₂ to 1 x CO₂ were calculated for the fire season for both models. This is a good overall measure of fire danger over the next century. In most regions of North America the SSR appears to be increasing greatly.This increase may correspond to an increase in fire activity, which will affect forest biodiversity, productivity, and carbon dynamics. Fire management agencies may need to adjust their strategies to deal with a changing fire regime.

CCC SSR ratio (3x/1x)	Had3GGa1 SSR ratio (3x/1x)

Acknowledgments

We thank Walter Skinner (Climatologist, Meteorological Service of Canada) for his contributions of weather station data, David Viner (Climate Change LINKS Project) for providing the daily Hadley data sets, the Canadian Climate Centre Modeling Group for allowing access to their daily GCM data sets, and the Climate Change Action Fund (CCAF) for providing funding in support of this research.

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