Impact of savanna fire scars on heat and moisture fluxes to the atmosphere and feedbacks to the local boundary layer

The far north of Australia or “Top End” as it is commonly known, is characterised by a beautiful yet fire prone landscape. Dominated by tropical savannas this relatively unpopulated area is subject to frequent burning from both anthropogenic and natural causes. Huge tracts (~250,000km2) of savanna are burnt annually within Australia, resulting in changes to heat and moisture fluxes to the atmosphere and feedbacks, through boundary layer change to local and regional climate.

This study focused on the physical changes to the savanna landscape caused by fire and the resultant effects of fire scars on boundary layer heating. Albedo values were found to almost halve after fire ranging from 0.12 pre-burn to 0.07 post-burn. In addition there was a fundamental change in energy partitioning, with a reduction in evapotranspiration and an increase in sensible heating. This was evident in recorded Bowen ratio values for a moderate burn of 1.6 pre-burn compared to 7.0 post-burn. The effects of these changes in surface properties and energy partitioning on the boundary layer was measured through vertical profiles of pressure, temperature, relative humidity, wind speed and wind direction, using a tethered balloon and radiosonde. A total of 172 soundings were conducted over burnt and unburnt sites during 24-hour periods. It was found that the burnt site was on average between 0.5-2.0oC warmer and up to 15% dryer than the unburnt site up to a maximum measured height of 500m. Maximum heating occurred between 12:00pm and 3:00pm, with the burnt site also losing more heat to the overlying atmosphere at night. Overall it was found that, when not affected by a local area sea breeze, the burnt site (fire scar) was warmer and drier with a deeper more well mixed boundary layer. These boundary layer changes resulting from fire scars can further impact the atmosphere leading to the possible formation of mesocale circulations. With fire scars throughout the Northern Territory often reaching into the hundreds of square kilometres, the impact on the regional scale and beyond may become significant.