Characters And Parameterization Comparisons Of Turbulent Transfer Over Maize Field On North China Plain

For best understanding the turbulent exchange of energy and mass betweenatmosphere and land surface, eddy covariance measurements were conductedover a typical agriculture field on the North China Plain. In this paper,we focus on corn growing period (from 11 June to 20 October, 2004), in whichthe characters of land cover vary rapidly with corn growing. Our study includethe data process and quality control technique, the statistical charactersof turbulent transfer and fluxes, the energy balance of surface, the netexchange of CO₂ between corn surface and atmosphere, and the fluxparameterization schemes near surface during corn growing. Main concludesare as following:1. Averaging, coordinate rotation and fluxes correction are importantissues in data processing procedure. Our study indicates that (1) Integraltime scale does not fit to determine the averaging time period because ofits variation with time series length and measurement variables. Fluxesincrease with increasing of average period from 2 min to 30 min, while ithas not visible increase or decrease from 30 min to 1 hour. When average periodlonger than 1 hour, fluxes appear much unstable daily pattern, and withoutsystematical increase or decrease with prolonging averaged period. So, wesuggest that the reasonable averaging period for our data should be neithershorter than 30 min, nor longer than 1 hour. (2) Planar fit coordinaterotation, which is recommended by the fluxes community recently, is performwell when applied to the divided periods according to corn growth rather thanto the whole raw data. (3) Density correction of latent heat flux and CO₂flux are necessary for our data, because they enhance the energy closure andremove the falsity of CO₂ uptake. We commend Liu method (Liu, 2005) insteadof WPL method (Webb et al.,1980) that was generally used in recent decades,because WPL method magnify the water vapor's variation. (4) Both coordinaterotation and fluxes density correction can increase the energy closure, totalas 2.6%, indicate that data processing might become energy sink if withoutproper process procedure. Data process is also important to enhance theestimation accuracy of net exchange of CO₂ between ecosystem and atmosphere.2. The analyses of statistical characters of turbulent transfer showthat: (1) Averaged turbulent intensity are 0.2 and 0.1 for horizontal andvertical wind fluctuation, respectively. (2) All of the variance andcovariance of wind and scalars obey the Monin-Obukhov similarity theory. Thevariance similarity functions from our data are close to that of plain areafrom literature expect for temperature and water vapor in stable condition.Thecovariance similarity functions appear much difference from literature.They show that momentum transfer is more effectively in nature conditionrather than stable and unstable condition. Sensible heat transfer will bestronger with the increase of absolute value of stability. Latent heat fluxis always from surface to atmosphere, and transfer efficiency decrease withthe increase of stability. (3) Spectral of wind components are similar tothose in Kaimal et al (1972) if we exchange u and v direction. But temperaturespectral appears widedegradation zone in the inertial subrange. Thecospectrum of momentum and sensible heatare quite different from literature.They do not obey the -4/3 law of slope in inertial subrange, and predictedthat larger eddies are relatively more efficient in transporting heat thanmomentum in vertical direction.3.Examing of surface energy balance indicates that: (1) The averagedenergy closure is in the order of 78.3%, 12.6% and 77.9% for day time, nighttime and the whole data sets, respectively. Energy closure is alwaysincreased from morning to afternoon during day. While, it is usuallydecreased with corn growth, at the range of 93% to 67% from mid June to earlyOctober. (2) Latent heat is always the main energy consume form of radiationenergy over the whole experiment. It takes 32-54% of net radiation fromseeding period to harvesting period. Meanwhile, sensible heat takes 14-37%,soil heat takes 11-18% and residual energy takes 4-28%, respectively. It isevident that the heat storage of corn canopy contributes to the energyimbalance at a large degree. (3) Discussing of the other causes to energyimbalance, we agree that footprint difference may be a driver to energyimbalance in our experiment. In addition, we consider the lag of time phaseof energy component to net radiation might be one of the reasons for ill energyclosure. (4) Surface albedo varies at the range of 0.07-0.18 during ourexperiment, with the maximum probability of 0.12. Bowen ratio at noon is aslarge as 3.6 during the seeding and harvesting period, and keep in 0.5 duringmain growth period.4.Investigating of net CO2 exchange between corn surface and atmospheregets to following results:(1) Human's field activities (e.g. tillage andfiring the residual) influence on CO2 concentration rather than on CO2 flux.(2) The concentration of CO2 holds diurnal cycle during the whole period ofmeasurement, but CO2 flux has not visible diurnal cycle until the corn canopydeveloped to some degree. The peak (vale) value of CO2 concentration usuallyappears at sunrise (sunset), related to the altering direction of CO2 flux.(3) Corn surface serve as carbon sink to atmosphere from young canopy stageto mature stage, and carbon source before it. The maximum daily carbon takingof corn canopy occurs at the abloom stage, as -10.1 gC m-2day-1, followedby spin-ripe stage (-9 gC m-2day-1) and jointing stage (-7.1 gC m-2day-1).Over the whole period, the net exchange of CO2 flux is -176 gC m-2 atmeasurement height. We noted that the different methods mentioned in chapter2 might bringas large as 160% bias to the total CO2 sink in our experiment.(4) The most proper net radiation condition for corn canopy assimilate CO2is 470 Wm-2. Both high temperature and high moisture is good for corn plantassimilation of CO2 from atmosphere, especially at noon, and more effectivelyfor mature canopy than young canopy.5. The research of flux parameterization shows that: (1) For estimatingwind speed at the height of 16 m during our experiment, it is feasible thataerodynamic roughness length z0 is fixed to 0.27 m through the whole period,and the zero displacement height d is fixed to 0 (1.8 m) before (after) midAugust. (2) By comparing measurement fluxes of momentum, sensible heat andlatent heat with that from parameterization schemes, we find that the schemessuch as Businger(1971), Dyer(1974), Beljaars (1991), which employ z/L asstability variable, perform not better than those of using bulk Richardsonnumber RiB as stability variable, such as Louis(1982) and Uno(1995). Amongthese schemes, Uno(1995) scheme is optimum for our experiment. Results stresson that differentiating ZT and zq from z0 is very important for the simulationof corn surface fluxes.