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Turbulence data (2008–2012) from a 325 m meteorological tower in Beijing, which consisted of three layers (47, 140, and 280 m), was used to analyze the vertical distribution characteristics of turbulent transfer over Beijing city according to similarity theory. The conclusions were as follows. (1) Normalized standard deviations of wind speeds σ i / u * were plotted as a function only of a local stability parameter. The values under near-neutral conditions were 2.15, 1.61, and 1.19 at 47 m, 2.39, 1.75, and 1.21 at 140 m, and 2.51, 1.77, and 1.30 at 280 m, showing a clear increase with height. The normalized standard deviation of wind components fitted the 1/3 law under unstable stratification conditions and decreased with height under both stable and unstable conditions. (2) The normalized standard deviation of temperature fitted the ‒1/3 law in the free convection limit, but was quite scattered with different characteristics under near-neutral conditions. The normalized standard deviations of humidity and the CO2 concentration fitted the ‒1/3 law under unstable conditions, and remained constant under near-neutral and stable stratification. The normalized standard deviation of scalars, i.e., temperature, humidity, and CO2 concentration, all increased with height. (3) Compared with momentum, and the water vapor and CO2 concentrations, the turbulence correlation coefficient for heat was smaller under near-neutral conditions, but larger under both stable and unstable conditions. A dissimilarity between heat, and the water vapor and CO2 concentrations was observed in urban areas. The relative correlation coefficients between heat and each of momentum, humidity, and CO2 concentration (| r wT / r uw |, | r wT / r wc | and | r wT / r uq |) in the lower layers were always larger than in higher layers, except for the relative correlation coefficient between heat and humidity in an unstable stratification. Therefore, the ratio between heat and each of momentum, humidity, and CO2 concentration decreased with height.

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.

The default fractional vegetation cover and terrain height were replaced by the estimated fractional vegetation cover, which was calculated by the Normalized Difference Vegetation Index (NDVI) of Earth Observing System Moderate-Resolution Imaging Spectroradiometer (EOS-MODIS) and the Digital Elevation Model of the Shuttle Radar Topography Mission (SRTM) system. The near-surface meteorological elements over northeastern China were assimilated into the three-dimensional variational data assimilation system (3DVar) module in the Weather Research and Forecasting (WRF) model. The structure and daily variations of air temperature, humidity, wind and energy fields over northeastern China were simulated using the WRF model. Four groups of numerical experiments were performed, and the simulation results were analyzed of latent heat flux, sensible heat flux, and their relationships with changes in the surface energy flux due to soil moisture and precipitation over different surfaces. The simulations were compared with observations of the stations Tongyu, Naiman, Jinzhou, and Miyun from June to August, 2009. The results showed that the WRF model achieves high-quality simulations of the diurnal characteristics of the surface layer temperature, wind direction, net radiation, sensible heat flux, and latent heat flux over semiarid northeastern China in the summer. The simulated near-surface temperature, relative humidity, and wind speed were improved in the data assimilation case (Case 2) compared with control case (Case 1). The simulated sensible heat fluxes and surface heat fluxes were improved by the land surface parameterization case (Case 3) and the combined case (Case 4). The simulated temporal variations in soil moisture over the northeastern arid areas agree well with observations in Case 4, but the simulated precipitation should be improved in the WRF model. This study could improve the land surface parameters by utilizing remote sensing data and could further improve atmospheric elements with a data assimilation system. This work provides an effective attempt at combining multi-source data with different spatial and temporal scales into numerical simulations. The assimilation datasets generated by this work can be applied to research on climate change and environmental monitoring of arid lands, as well as research on the formation and stability of climate over semiarid areas.

On the basis of a simple snow-atmosphere-soil transfer (SAST) model previously developed, this paper presents an improved snow-atmosphere-soil transfer (ISAST) model that has a new numerical scheme and an improved method of layering the snowpack. The new model takes the snow cover temperature and ice content in the snow cover as prognostic variables. This approach, which effectively solves the snow cover temperature distribution when the snow cover is melting or freezing, lessens the iteration time and computation time, which is important for GCM simulation. In this model, the snow cover is divided into three layers (ISAST3) or seven layers (ISAST7). The simulation results obtained using the ISAST7 model agree well with observations in terms of snow depth, snow equivalent water and snow cover lifetime at five Russian sites. The new ISAST model has better simulation capacity for snow cover than the previous SAST model. When the snow cover is deep, the simulation of the ISAST7 model is better than that of the ISAST3 model. Testing shows that our ISAST model is approximately 20% faster than the SAST model.

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% below 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 season (May to September) at UG79 site, with maximum values approaching 0.08 mg C m−2 s−1 in August of 2008. The diurnal average NEE of 2007 was larger than 2008 at SACOL site, with maximum values of 0.07 mg C m−2 s−1 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 occurred 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−2 at SACOL site. Alternation between carbon 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.

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 C d and C h 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 C d and C h 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.