| 1. |
- Mallick, Kaniska, et al.
(författare)
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A Surface Temperature Initiated Closure (STIC) for surface energy balance fluxes
- 2014
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Ingår i: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257. ; 141:5, s. 243-261
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Tidskriftsartikel (refereegranskat)abstract
- The use of Penman-Monteith (PM) equation in thermal remote sensing based surface energy balance modeling is not prevalent due to the unavailability of any direct method to integrate thermal data into the PM equation and due to the lack of physical models expressing the surface (or stomatal) and boundary layer conductances (g(S) and g(B)) as a function of surface temperature. Here we demonstrate a new method that physically integrates the radiometric surface temperature (T-S) into the PM equation for estimating the terrestrial surface energy balance fluxes (sensible heat, H and latent heat, lambda E). The method combines satellite T-S data with standard energy balance closure models in order to derive a hybrid closure that does not require the specification of surface to atmosphere conductance terms. We call this the Surface Temperature Initiated Closure (STIC), which is formed by the simultaneous solution of four state equations. Taking advantage of the psychrometric relationship between temperature and vapor pressure, the present method also estimates the near surface moisture availability (M) from T-S, air temperature (T-A) and relative humidity (R-H), thereby being capable of decomposing lambda E into evaporation (lambda E-E) and transpiration (lambda E-T). STIC is driven with T-S, T-A, R-H, net radiation (R-N), and ground heat flux (G). T-S measurements from both MODIS Terra (MOD11A2) and Aqua (MYD11A2) were used in conjunction with FLUXNET R-N, G, T-A, R-H, lambda E and H measurements corresponding to the MODIS equatorial crossing time. The performance of STIC has been evaluated in comparison to the eddy covariance measurements of lambda E and H at 30 sites that cover a broad range of biomes and climates. We found a RMSE of 37.79 (11%) (with MODIS Terra T-S) and 44.27 W m(-2) (15%) (with MODIS Aqua T-S) in lambda E estimates, while the RMSE was 37.74(9%) (with Terra) and 44.72 W m(-2) (8%) (with Aqua) in H. STIC could efficiently capture the lambda E dynamics during the dry down period in the semi-arid landscapes where lambda E is strongly governed by the subsurface soil moisture and where the majority of other lambda E models generally show poor results. Sensitivity analysis revealed a high sensitivity of both the fluxes to the uncertainties in T-S. A realistic response and modest relationship was also found when partitioned lambda E components (lambda E-E and lambda E-T) were compared to the observed soil moisture and rainfall. This is the first study to report the physical integration of T-S into the PM equation and finding analytical solution of the physical (g(B)) and physiological conductances (g(S)). The performance of STIC over diverse biomes and climates points to its potential to benefit future NASA and NOAA missions having thermal sensors, such as HyspIRI, GeoSTAR and GOES-R for mapping multi-scale lambda E and drought. (C) 2013 Elsevier Inc. All rights reserved.
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| 2. |
- Niu, Shuli, et al.
(författare)
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Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms.
- 2012
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Ingår i: New Phytologist. - : Wiley. - 1469-8137 .- 0028-646X. ; 194:3, s. 775-783
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Tidskriftsartikel (refereegranskat)abstract
- • It is well established that individual organisms can acclimate and adapt to temperature to optimize their functioning. However, thermal optimization of ecosystems, as an assemblage of organisms, has not been examined at broad spatial and temporal scales. • Here, we compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms. • We found that the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites. Shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration. • Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably reflecting associated evolutionary adaptation of organisms within ecosystems, and has the potential to significantly regulate ecosystem-climate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in changing environments and benchmarking global models.
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| 3. |
- Yi, Chuixiang, et al.
(författare)
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Climate control of terrestrial carbon exchange across biomes and continents
- 2010
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 5:3
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO2 exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid-and high-latitudes, (2) a strong function of dryness at mid-and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45 degrees N). The sensitivity of NEE to mean annual temperature breaks down at similar to 16 degrees C (a threshold value of mean annual temperature), above which no further increase of CO2 uptake with temperature was observed and dryness influence overrules temperature influence.
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