| The terrestrial ecosystem is the basis for human survival and sustainable development, and the global climate change is gradually changing its natural process, which greatly affects the migration and transmission of the energy, momentum and matter in the surface-atmosphere system of the Earth. The plants in terrestrial ecosystem absorb the solar radiation for photosynthesis and convert it into chemical energy for promoting the material and energy circulation between the biosphere and the atmosphere. At the same time, the aerosol direct and indirect radiative effects alter the radiation balance of earth-atmosphere system and regulate the radiation components and photosynthesis, which cause great uncertainties on the distribution pattern, process and function of surface ecological system. This will also directly or indirectly change the spatial-temporal distribution of vegetation productivity. Recently, there is a widespread concern about the effects of increasing aerosols on climate change and terrestrial vegetation ecosystem among the international scientific community, and China has became one of the key zones with great uncertainties in the aerosol radiative forcing due to the rapid economic development and the increasing anthropogenic pollution emissions. This will change the regional terrestrial ecosystem carbon cycle process, and further affect the global terrestrial ecosystem carbon balance. In order to scientifically understand the impacts, adaptation and mitigation of climate change, a comprehensive study of atmospheric processes-ecological system should be carried out for investigating the interactions and feedback mechanism between vegetation productivity and atmosphere process.In this paper, the solar radiation components, including ultraviolet radiation (UV) and photosynthetically active radiation (PAR) at Wuhan has been observed and simulated for studying their long-term characteristics and the interactions between aerosol, radiation and vegetation productivity. The aerosol optical characteristics and its direct radiative forcing at Wuhan have been quantitatively analyzed. Five different gross primary production (GPP) modes have been tested and calibrated at eight sites in China, the relationships between eddy CO2 flux and model parameters are also analyzed for further investigating the model application at various land cover types. The spatial and temporal distribution of the vegetation net Primary production (NPP) and its relationships with climatic factors in Wuhan during 2001-2011 have been further investigated. The main research contents are summarized as following:The diurnal and seasonal variations of PAR, UV and global solar radiation (G) observed at Wuhan have been analyzed, the results show that the annual mean PAR and UV are 22.39±11.11mol m-2 d-1 and 0.49 MJ m-2, respectively, the maximum values for PAR (UV) appear at July, and the minimum in January. The combined effects of aerosols, clouds, water vapor and radiation path have great effects on the ratios of PAR and UV on G, for example, the UV/G is higher in July (4.46±0.25%), and lower in January (3.23±0.1%). The variation characteristics of PAR and UV under different sky conditions (overcast, cloudy and clear sky) and different cloud amounts have been further analyzed, and it is discovered that aerosols, clouds and water vapor selectively absorb and scatter the solar radiation components.Different PAR and UV estimation models have been calibrated and developed at Wuhan using atmospheric parameters (clearness index, optical air mass and cloud modification factors), and the model accuracies have been analyzed and compared in detail. The results show that the relative difference (RE), mean bias error (MBE), mean absolute error (MAE) and root mean square error (RMSE) are 4.98%,-0.71%,3.51% and 4.57%, respectively for the best PAR model; RE, MBE, MAE and RMSE are 5.98%,-0.68%,7.1% and 9.87%, respectively for the best UV model. The daily PAR and UV values are then reconstructed based on the measured G at Wuhan during 1961-2011, the results show that the average PAR is 23.12 mol m-2d-1, and PAR decreases at the rate of -11.2 mol m -2per decade from 1961 to 2011; the average daily UV is 0.488 MJ m -2 d-1, and decreases at the rate of -0.018 MJm-2 per decade during 1961-2011, but UV increases at the rate of 0.003 MJ m-2 per year since 1990s at Wuhan, China. Above PAR and UV models are further calibrated at 115 stations in China, and the daily PAR and UV values are finally obtained using remote sensing data processing technology. The results show that the average PAR in China is 27.61 mol m-2d-1, and PAR decreases at the rate of -0.15 mol m -2 per decade from 1961 to 2012, but PAR increases at the rate of 0.01 mol m-1 d-1 per year; he average daily UV is 0.61 MJ m-2d-1, and decreases at the rate of -2.72 KJ m-2d-1 per decade during 1961-2011, but UV increases at the rate of 0.7 KJ m-2 d-1 per year since 1990s in China. The seasonal variations of PAR and UV during 1961-2012 in the whole country are further analyzed in this study.Aerosol optical properties including aerosol optical depth (AOD), Angstrom exponent (AE), single scattering albedo (SSA), aerosol size distribution (ASD) and refractive index (RI) at Wuhan are investigated based on the measurements from a CIMEL sun-photometer during 2007-2013. The results show that the annual mean AOD and AE are 1.05±0.66 and 1.23±0.27, respectively, the average of SSA at 440 nm for total, fine and coarse-mode particles are 0.9±0.05,0.92±0.05,0.73±0.09, respectively; The ASD (bi-modal pattern) show distinct differences in particle radius for different seasons, the radius for fine-mode particles generally increase from spring to summer month, for example, the highest peak is around radius 0.19?0.3?m (1.8?3.0 ?m for coarse-mode particles) in Summer, while the peak radius is around 0.15-0.25?m in Spring. The annual mean values for real parts of RI at 440,670,870 and 1020 nm are 1.45±0.07,1.46±0.07,1.47±0.07 and 1.48±0.07, respectively, the imaginary parts are 0.0149±0.0133,0.0302±0.0362,0.0359±0.0402 and 0.039±0.0412, respectively. The seasonal variations of above aerosol optical properties and their interrelations have also been analyzed, the results indicate that the anthropogenic fine-mode particles are the dominated aerosol types at Wuhan, SSA is generally higher in summer and lower in winter, and the scattering capacity is stronger for the fine-mode particles; there are also greater emissions of absorbing coarse-mode particles in spring months in this area. The aerosol direct radiative forcing (ADRF) and direct radiative forcing efficiency (ADRFE) under clear sky condition at Top of Atmosphere (TOA) and surface are calculated using SBDART radiative transfer model, the average of ADRF at TOA and surface are-133.25±64.05 and-20.52±27.57 W m-2, respectively, the ADRFE is 179.66±78.76 and-25.48±34.30 W m-2, respectively, the seasonal variations of ADRF and ADRFE at Wuhan have been further analyzed and compared with values at other sites in China.Five vegetation productivity models, Vegetation Photosynthesis Model (VPM), Temperature and Greenness model (TG), Alpine Vegetation Model (AVM), Greenness and Radiation model (GR) and MODIS algorithm are tested and calibrated at eight sites in China. Results indicate that the first four models provide more reliable GPP estimations than MODIS GPP products/algorithm, and the VPM model is the most suitable model for GPP estimation. The relationships between eddy CO2 flux and model parameters (EVI, PAR, LST, air temperature and LSWI) are analyzed for further investigating the model application at various land cover types, which indicates that different parameters play different roles for each GPP model. The regional NPP (net primary production) model has been also improved by modifying the model parameters (PAR and light use efficiency for different vegetation types), and the spatial and temporal distribution of NPP in Wuhan from 2001 to 2011 are then investigated. The results indicate that the average NPP for the whole study area is about 500 gCm-2a-1, and there are big differences in NPP values for different vegetation types. The lagged cross-correlation analysis is further used to study the delayed and continuous effects of NPP dynamic to climatic factors, the results show that rainfall and G are the major factors determining the seasonal variation of NPP in Wuhan, for example, the effects of rainfall on NPP will delay 32-d and last for 64-d. |
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