Eddy Covariance Tilt Corrections over a Coastal Mountain Area in South-east China:Significance for Near-Surface Turbulence Characteristics

Turbulence characteristics of an atmospheric surface layer over a coastal mountain area were investigated under different coordinate frames. Performances of three methods of coordinate rotation： double rotation （DR）, triple rotation （TR）, and classic planar-fit rotation （PF） were examined in terms of correction of eddy covariance flux. Using the commonly used DR and TR methods, unreasonable rotation angles are encountered at low wind speeds and cause significant run-to-run errors of some turbulence characteristics. The PF method rotates the coordinate system to an ensemble-averaged plane, and shows large tilt error due to an inaccurate fit plane over variable terrain slopes. In this paper, we propose another coordinate rotation scheme. The observational data were separated into two groups according to wind direction. The PF method was adapted to find an ensemble-averaged streamline plane for each group of hourly runs with wind speed exceeding 1.0 m s-1. Then, the coordinate systems were rotated to their respective best- fit planes for all available hourly observations. We call this the PF10 method. The implications of tilt corrections for the turbulence characteristics are discussed with a focus on integral turbulence characteristics, the spectra of wind-velocity components, and sensible heat and momentum fluxes under various atmospheric stabilities. Our results show that the adapted application of PF provides greatly improved estimates of integral turbulence characteristics in complex terrain and maintains data quality. The comparisons of the sensible heat fluxes for four coordinate rotation methods to fluxes before correction indicate that the PF10 scheme is the best to preserve consistency between fluxes.

Assessment of CH_(4) flux and its influencing drivers in the rice-wheat agroecosystem of the Huai River Basin,China

To understand the CH_(4) flux variations and their climatic drivers in the rice-wheat agroecosystem in the Huai River Basin of China,the CH_(4) flux was observed by using open-path eddy covariance at a typical rice-wheat rotation system in Anhui Province,China from November 2019 to October 2021.The variations and their drivers were then analyzed with the Akaike information criterion method.CH_(4) flux showed distinct diurnal variations with single peaks during 9:00-13:00 local time.The highest peak was 2.15μg m^(-2)s^(-1)which occurred at 11:00 in the vegetative growth stage in the rice growing season(RGS).CH_(4) flux also showed significant seasonal variations.The average CH_(4)flux in the vegetative growth stage in the RGS(193.8±74.2 mg m^(-2)d^(-1))was the highest among all growth stages.The annual total CH_(4) flux in the non-rice growing season(3.2 g m^(-2))was relatively small compared to that in the RGS(23.9 g m^(-2)).CH_(4) flux increased significantly with increase in air temperature,soil temperature,and soil water content in both the RGS and the non-RGS,while it decreased significantly with increase in vapor pressure deficit in the RGS.This study provided a comprehensive understanding of the CH_(4) flux and its drivers in the rice-wheat rotation agroecosystem in the Huai River Basin of China.In addition,our findings will be helpful for the validation and adjustment of the CH_(4) models in this region.

Carbon fluxes and their response to environmental variables in a Dahurian larch forest ecosystem in northeast China

The Dahurian larch forest in northeast China is important due to its vastness and location within a transitional zone from boreal to temperate and at the southern distribution edge of the vast Siberian larch forest. The continuous carbon fluxes were measured from May 2004 to April 2005 in the Dahurian larch forest in Northeast China using an eddy covariance method. The results showed that the ecosystem released carbon in the dormant season from mid-October 2004 to April 2005, while it assimilated CO2 from the atmosphere in the growing season from May to September 2004. The net carbon sequestration reached its peak of 112 g.m^-2.month ^-1 in June 2004 （simplified expression of g （carbon）.m^-2.month^-1） and then gradually decreased. Annually, the larch forest was a carbon sink that sequestered carbon of 146 g-m^-2.a^-1 （simplified expression of g （carbon）.m^-2.a^-1） during the measurements. The photosynthetic process of the larch forest ecosystem was largely affected by the vapor pressure deficit （VPD） and temperature. Under humid conditions （VPD 〈 1.0 kPa）, the gross ecosystem production （GEP） increased with increasing temperature. But the net ecosystem production （NEP） showed almost no change with increasing temperature because the increment of GEP was counterbalanced by that of the ecosystem respiration. Under a dry environment （VPD 〉 1.0 kPa）, the GEP decreased with the increasing VPD at a rate of 3.0 μmol.m^-2.s^-1kPa -1 and the ecosystem respiration was also enhanced simultaneously due to the increase of air temperature, which was linearly correlated with the VPD. As a result, the net ecosystem carbon sequestration rapidly decreased with the increasing VPD at a rate of 5.2 μmol.m^-2.s-1.kPa^-1. Under humid conditions （VPD 〈 1.0 kPa）, both the GEP and NEP were obviously restricted by the low air temperature but were insensitive to the high temperature because the observed high temperature value comes within the category of the optimum range.

Eddy covariance measurements of water vapor and CO_2 fluxes above the Erhai Lake

Measurement of turbulence fluxes were performed over the Erhai Lake using eddy covariance(EC) method.Basic physical parameters in the lake-air interaction processes,such as surface albedo of the lake,aerodynamic roughness length,bulk transfer coefficients,etc.,were investigated using the EC data in 2012.The characteristics of turbulence fluxes over the lake including momentum flux,sensible heat flux,latent heat flux,and CO2 flux,and their controlling factors were analyzed.The total annual evaporation of the lake was also estimated based on the artificial neural network(ANN) gap-filling technique.Results showed that the total annual evaporation in 2012 was 1165 ± 15 mm,which was larger than the annual precipitation(818 mm).Local circulation between the lake and the surrounding land was found to be significant throughout the year due to the land-lake breeze or the mountain-valley breeze in this area.The prevailing winds of southeasterly and northwesterly were observed throughout the year.The sensible heat flux over this plateau lake usually had a few tens of W m-2,and generally became negative in the afternoon,indicating that heat was transferred from the lake to the atmosphere.The sensible heat flux was governed by the lake-air temperature difference and had its maximum in the early morning.The diurnal variation of the latent heat flux was controlled by vapor pressure deficit with a peak in the afternoon.The latent heat flux was dominant in the partition of available energy in daytime over this lake.The lake acted as a weak CO2 source to the atmosphere except for the midday of summer.Seasonal variations of surface albedo over the lake were related to the solar elevation angle and opacity of the water.Furthermore,compared with the observation data,the surface albedo estimated by CLM4-LISSS model was underestimated in winter and overestimated in summer.

Seasonal and inter-annual variation of surface roughness length and bulk transfer coefficients in a semiarid area

Here we report a multiyear study on the surface roughness length and bulk transfer coefficients over the degraded grassland and cropland surfaces in a semiarid area of China. Eddy covariance measurement and the meteorological profile observation data were used to analyze characteristics of these parameters on the diurnal, seasonal, and annual scales. Significant seasonal and annual variations of the aerodynamic roughness length are observed over the two surfaces. A large variation of kB-1 is measured during the day. Both kB-1 and the bulk transfer coefficients exhibit significant seasonal and annual variations. During the growing season (May to October), average Cd and Ch are 3.1×10-3 and 2.5×10-3 over the degrade grassland surface, and 4.7×10-3 and 3.1×10-3 over the cropland surface respectively. During the non-growing season, average Cd and Ch are 2.3×10-3 and 2.0×10-3 over the degrade grassland surface, and 2.9×10-3 and 2.2×10-3 over the cropland surface respectively.

Carbon dioxide exchange processes over the grassland ecosystems in semiarid areas of China

Based on the carbon fluxes measured over the grassland ecosystems in Inner Mongolia （UG79 site）, Loess Plateau （SACOL site）, and Tongyu, Jilin Province （TY site） in the semiarid areas from 2007 to 2008 with the eddy covariance method, we have investigated the carbon exchange processes over semiarid grassland ecosystem and its main affecting environmental variables. The precipitations at UG79 and TY sites in 2007 were below the historical average, especially for TY site, which was 50% be- low the historical average annual precipitation. The precipitation in SACOL site was close to average in 2007 but below average in 2008. The variation of monthly diurnal average NEE showed that the diurnal mean NEE decreased in the order of TY site, UG79 site, and SACOL site. However, a longer net carbon uptake period was observed at SACOL site. The diurnal course of NEE at UG79 site was similar between 2007 and 2008. The diurnal average NEE remained large during July and August in growing sea- son （May to September） at UG79 site, with maximum values approaching 0.08 mg C m^-2 s^-1 in August of 2008. The diurnal av- erage NEE of 2007 was larger than 2008 at SACOL site, with maximum values of 0.07 mg C m^-2 sq in September of 2007. A shorter carbon uptake period was recorded in 2007 at TY site, lasting from July to August. A larger diurnal average NEE oc- curred in 2008 at TY site, with maximum values of 0.12 mg C m^-2 s^-1. The ecosystem respirations of three sites were controlled by both soil temperature and soil volumetric water content （at a depth of 5 cm below the land surface）. Both UG79 site and SACOL site acted as a carbon sink during the growing periods of 2007 and 2008. Annual NEE in the growing seasons of 2007 and 2008 ranged from -68 to -50 g C m^-2 at UG79 site and from -109 to -55 g C m^-2at SACOL site. Alternation between car- bon source and carbon sink was found at TY site, with respective values of annual NEE in the growing seasons of 0.32 g C m^-2 and -73 g C m^-2 in 2007 and 2008. The magnitude and duration of carbon uptake depended mainly on the amount and timing of precipitation and the timing of the first effective rainfall during the growing season in semiarid grassland ecosystems.

SURFACE IMBALANCE ENERGY CALCULATED AND ANALYZED WITH THE DATA OF EBEX-2000

Mainly.three methods have been developed to calculate turbulence heat flux.They are eddy covariance method,Bowen ratio/energy balance method and aerodynamic method.In this paper, all the three methods have been used to calculate sensible heat flux,latent heat flux and imbalance energy near the surface with the experiment data of EBEX-2000.Then comparisons of the three methods and some possible explanations of the surface imbalance energy are given.