Investigation On Spatial Representativeness Of Surface And Scale Effect Of Sensible Heat Flux From Mixed Plantation In The Hilly Zone

Two flux observation instruments are commonly used:The Eddy-covariance (EC) the former bases on the point observation and can observe hundreds of meters, and the Large Aperture Scintillometer (LAS) can observe area-average fluxes in the scale of hundreds of meters to tens of kilometers. The footprint and source area are the effective tools in scale transformation of fluxes datas and can describe the spatial representativeness.We develop Eulerian analytical footprint models described by Schmid and Meijningerfor LAS, and the continuous flux measurement over a 31-year aged mixed plantation in the hilly zone of the North China from May to December in 2009, compare variations of footprint and source areas for LAS and EC in different conditions. The average sensible heat flux (H) of Observation area from LAS was corrected and estimated with the application of footprint model. The H from EC was corrected and estimated based on the analysis of the footprint and source area, compare the H observed and calculated by EC and LAS.The main conclusions are as follows:1. Sensitivity analysis:analyzing the sensitivity of the footprint of LAS and the H observed and Calculated by LAS.The results showed that:the foortprint are sensitive to wind direction, Monin-Obukhov length, aerodynamic roughness length, and measurement height. The H observed and calculated by LAS are sensitive to wind speed, Zero-plane displacement, measurement height, are not sensitive to temperature and pressure. And be sensitive to Bowen ratio in humid areas, not sensitive in dryness, partly arid areas.2. Temporal and spatial variation of source area for flux:(1)When stratification conditions are unstable (L<0), the source area under SE wind direction is the smallest, the range is 1.1547km2; the source area under SW wind direction is the biggest, the range is 1.5237km2. When stratification conditions are stable (L>0), the source area under NW wind direction is the smallest, the range is 1.7271km2; the source area under SW wind direction is the biggest, the range is 3.5289km2.The souce area under NW that is the smallest in stable stratification conditions is bigger 11.8% then the souce area under SW that is the biggest in unstable stratification conditions. The LAS can more effectively in measuring when the wind direction is alomst vertical to the light path(2)The range of sourc earea is 1.1483 km2 at 8:00, when stratification conditions bencame unstable exactly; the range of source area source area is 0.4518 km2 at 11:00, as is the smallest in one day.when exchange of turbulence is more severe; the range of source area becomes 0.7137 km2 from 0.4779 km2, when stratification conditions become stable from unstable.The changing trend of source area is consistent with radiation and temperature.(3)The range of source area is 1.834km2,1.680km2,2.043km2,1.671km2,1.380km2,2.118km2,1.587 km2,1.922 km2 in May,June,July,August,September,October,November December.(4) The range of source area for EC is smaller than LAS in any conditions. 3. The correlation coefficient between the H observed and calculated by EC and LAS was 0.97. It proved the feasibility of using LAS measurements to estimate average sensible heat flux over the mixed plantations in the hilly zone, based on the analysis of the footprint and source area.