A HIERARCHICAL VIEW OF CLIMATIC CONTROLS OF WILDFIRE IN THE WESTERN UNITED STATES

BARTLEIN, P.J. (1), HOSTETLER, S.W. (2), SHAFER, S.L. (3), HOLMAN, J.O. (4) AND SOLOMON, A.M. (5)

(1) Univ. Oregon, Dept. Geography, Eugene, OR 97403-1251; (2) U.S. Geological Survey, Oregon State Univ. and Dept. Geosciences, Corvallis, OR 97333; (3) U.S. Geological Survey, 200 SW 35th St., Corvallis, OR 97333; (4)  Univ. Oregon, Dept. Geography, Eugene, OR 97403-1251; (5) U.S. Environmental Protection Agency, 200 SW 35th St., Corvallis, OR 97333

The incidence of wildfire in the western United States is governed by climatological, meteorological, and ecological controls that operate across a range of spatial and temporal scales, from hemispheric to landscape, and from decadal (and longer) to diurnal. These controls are responsible for fire weather (i.e., the conditions responsible for the ignition, spread, and suppression of individual fires) and fire climate (i.e., the conditions responsible for the severity of a particular fire season). Interannual, seasonal, and monthly variations of atmospheric circulation result in variations of airmass distribution, moisture flux, and large-scale vertical motion on monthly-to-daily time scales, and these in turn determine the meteorological conditions that are directly responsible for the outbreak of fires, such as the surface water- and energy-balances, atmospheric stability, lightning, and wind on daily-to-diurnal time scales. Fire outbreaks over the West often form a coherent pattern associated with the temporal and spatial dynamics of circulation. The severity of a particular fire season is largely determined by the integration across time spans of seasons to months of the water-balance related variables described above, which determine soil-moisture status, as well as by the condition of the vegetation, which determines fuel load and flammability.