Abstract
Recent Arctic warming has led to changes in the hydrological cycle. Circum-Arctic and circumboreal ecosystems are showing evidence of “greening” and “browning” due to temperature warming leading to shrub encroachment, tree mortality and deciduousness. Increases in latent heat flux from increased evapotranspiration rates associated with deciduous-dominated ecosystems may be significant, because deciduous vegetation has extremely high-water use and water storage capacity compared to coniferous and herbaceous plant species. Thus, the impact of vegetation change in boreal ecosystems on regional surface energy balance is a significant knowledge gap that must be addressed to better understand observed trends in water use/availability and tree mortality. To this end, output from a two-source energy balance model (TSEB) with modifications for high latitude boreal ecosystems was evaluated using flux tower measurements and Terra/Aqua MODIS remote sensing data collected over the two largest boreal forest types in Alaska (birch and black spruce). Data under clear and overcast days and from leaf-out to senescence from 2012 to 2016 were used for validation. Using flux tower observations and local model inputs, modifications to the model formulation for soil heat flux, net radiation partitioning, and canopy transpiration were required for the boreal forest. These improvements resulted in a mean absolute percent difference of around 23% for turbulent daytime fluxes when surface temperature from the flux towers was used, similar to errors reported in other studies conducted in warmer climates. Results when surface temperature from Terra/Aqua MODIS estimates were used as model input suggested that these model improvements are pertinent for regional scale applications. Vegetation indices and LAI time-series from the Terra/Aqua MODIS products were confirmed to be appropriate for energy flux estimation in the boreal forest to describe vegetation properties (LAI and green fraction) when field observations are not available. Model improvements for boreal settings identified in this study will be implemented operationally over North America to map surface energy fluxes at regional scales using long time series of remote sensing estimates as part of NOAA’s GOES Evapotranspiration and Drought Information System.
Highlights
In the recent past, a significant increase in air temperature in the Arctic [1], known as the Arctic amplification, is leading to an accelerated increase in air temperatures compared to other parts of the globe inducing broad sea-ice retreat, snow and ice melting and increases in sea level [2,3]
This study focuses on refining and evaluating a diagnostic remote sensing-based energy balance model for estimating seasonal dynamics of surface energy fluxes over the boreal forest, using measurements of land surface temperature retrieved from thermal infrared sensors on satellite platforms as a key boundary condition
To estimate surface energy fluxes, the Two-Source Energy Balance (TSEB) model in its series version [28,31,32] was used with later modifications for Arctic tundra environments and applied in clear sky and cloudy conditions [27]
Summary
A significant increase in air temperature in the Arctic [1], known as the Arctic amplification, is leading to an accelerated increase in air temperatures compared to other parts of the globe inducing broad sea-ice retreat, snow and ice melting and increases in sea level [2,3]. Arctic and sub-Arctic ecosystems in Alaska, dominated by tundra and boreal forest land covers, are witnessing singular changes due to climate warming including widespread permafrost degradation, increases in the area burned and the severity of wildfires, decreased thickness and duration of winter snow cover, acceleration of the hydrological cycle and, rises in river discharge and important vegetation changes in both structure and distribution [4,5,6,7,8,9,10,11]. Due to increased temperatures there has been a rise in deciduous shrubs in the Circum-Arctic, which is responsible for the “greening” of the Arctic tundra [16,18] resulting in nearly a 14% increase in vegetation cover [4]. The transition zone between forest and tundra ecosystems in the northern boundary is expanding both latitudinally and in elevation and is leading to an increase of tree heights and shrub growth leading to denser and taller canopies [16,22]
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