Influence of wildfire induced land-cover changes on clouds and precipitation in Interior Alaska — A case study

As especially observed during the 2004 Alaska fire season, huge wildfires drastically alter land cover leading to a change in the dynamic (roughness length), radiative (albedo, emissivity), vegetative (vegetation type and fraction, stomatal resistance), thermal (soil heating, thawing of permafrost), and hydrological (water loss due to the fire) surface characteristics. A case study was performed with the Mesoscale Model generation 5 (MM5) assuming the landscape prior to and after the 2004 wildfires in Interior Alaska to quantitatively examine the impact of fire-caused land-cover changes on summer cloud and precipitation formation.The results of our model study show that sensible heat fluxes into the atmosphere and air temperatures increase by up to 225 W m− 2 and 3 K over burnt areas. Burnt areas > 600 km2 and > 800 km2 significantly (at the 90% or higher confidence level according to statistical tests) affect sensible and latent heat fluxes, respectively. While burnt areas must exceed 1600 km2 for the increased buoyancy to significantly increase the upward transport of air, burnt areas > 600 km2 already experience enough enhanced lifting to produce areas of increased cloud-water, rainwater, and graupel mixing ratios followed by an area of decreased mixing ratios downwind of them. The spatial and temporal distribution of precipitation changes appreciably and resulted in significant increases of 84 h-accumulated precipitation (by up to 15.1 mm) in the lee of burnt areas > 600 km2, but a decrease by 0.1 mm on the domain average.