Change in surface energy balance in Alaska due to fire and spring warming, based on upscaling eddy covariance measurements

Abstract

Warming in northern high latitudes has changed the energy balance between terrestrial ecosystems and the atmosphere. This study evaluated changes in regional surface energy exchange in Alaska from 2000 to 2011 when substantial declines in spring snow cover due to spring warming and large-scale fire events were observed. Energy fluxes from a network of 20 eddy covariance sites were upscaled using a support vector regression (SVR) model, by combining satellite remote sensing data and global climate data. Based on site-scale analysis, SVR reproduced observed net radiation, sensible heat flux, latent heat flux, and ground heat flux; 8 day root-mean-square errors for these variables were 15, 10, 9, and 3 W m À2 , respectively. Based on upscaled fluxes, decreases in spring snow cover induced an increase in surface net radiation, a net heating effect, of 0.56 W m À2 decade À1. This heating effect was comparable to the net cooling effect due to increased fire extent during the study period (up to 0.59 W m À2 decade À1). These land cover effects were larger than the change in the energy forcing associated with CO 2 balance for the Alaska region. Spring warming and postfire land cover change increased the regional latent heat flux. The regional sensible heat flux decreased with the postfire land cover change. Our results highlight the importance of positive spring snow albedo feedback to climate and a postfire negative feedback under the expected warming climate in the Arctic.

Focus on changing fire regimes: interactions with climate, ecosystems, and society

“…This was primarily a function of decreased snow cover in the spring, which was only partially offset by increases in land surface albedo from more fires and thereby more snow exposure beneath tree canopies. This result is in contrast to work showing the magnitude of atmospheric heating from snow cover loss to be roughly equal to that from post-fire albedo cooling in Alaska between 2000 and 2011 (Ueyama et al 2014), but consistent with projections of a reduced cooling impact from post-fire albedo due to earlier snowmelt (Potter et al 2019). Overall, these studies suggest the biophysical cooling effect of boreal fires will decrease in magnitude, and be more than offset by atmospheric heating from declining snow cover in spring.…”

New data‐driven estimation of terrestrial CO2 fluxes in Asia using a standardized database of eddy covariance measurements, remote sensing data, and support vector regression

“…Besides the MODIS products, we used incoming surface shortwave radiation products based on the JAXA Satellite Monitoring for Environmental Studies product (http://kuroshio.eorc.jaxa.jp/JASMES/index.html) for the 2000–2015 period. The products are based on Terra MODIS data with a simple radiative transfer model [ Frouin and Murakami , ] and were evaluated for three EC sites in Asia [ Saigusa et al ., ] and 20 EC sites in Alaska [ Ueyama et al ., ]. Spatial and temporal averaging was conducted by converting the original 5 km grid to 0.25° grids and daily to 8 day temporal resolution.…”

Charred forests accelerate snow albedo decay: parameterizing the post‐fire radiative forcing on snow for three years following fire

“…In response to warmer temperatures and declining snowpacks, large wildfires are increasing in frequency, intensity, and extent across the western USA (Westerling et al, 2006(Westerling et al, , 2011Trouet et al, 2010;Semmens and Ramage, 2012). In turn, forest fire disturbance affects patterns of snow accumulation and ablation by reducing canopy interception, increasing turbulent fluxes, and modifying the net radiation balance (Liu et al, 2005;Winkler et al, 2005;Boon, 2009;Burles and Boon, 2011;Gleason et al, 2013;Ueyama et al, 2014).…”

“…Forest fire disturbance significantly alters the net radiation balance of the underlying snowpack by increasing the transmission of incoming solar radiation through the canopy, decreasing the emission of longwave radiation by the canopy, and decreasing snow surface shortwave albedo (integrated across the spectral range of 0.3-3.0 μm) by deposition of organic debris from the canopy (Pomeroy and Dion, 1996;Winkler et al, 2010;Burles and Boon, 2011;Gleason et al, 2013). Charred forests increase the rate of snowmelt leading to earlier snow disappearance (Winkler et al, 2010;Gleason et al, 2013;Harpold et al, 2014;Micheletty et al, 2014), which amplifies land surface-atmosphere radiative feedback and alters terrestrial-atmosphere hydrological interactions (Liu et al, 2005;Dery and Brown, 2007;O'Halloran et al, 2012;Ueyama et al, 2014).…”