| Based on the longer term data of NOAA/AVHRR GIMMS NDVI from 1982 to 2000, such as Hunan and Hubei, the temporal and spatial characteristics of vegetation cover and climate factor variations and their relationships in the-mid reaches of YANGTZE RIVER zone are analyzed, by the methods of trend analysis, correlation analysis, SVD and numerical simulation. Meantime the numerical simulations on the effects of the vegetation cover area dicreasing on regional climate in the research area are carried out by using the WRF model. The main conclusions are as following:(1) The vegetation index of NDVI had the trend of increasing in the-mid reaches of YANGTZE RIVER from 1982 to 2000. The year border fluctuation change was obvious and the yearly average rate of change reached 3.74%. In the entire district vegetation year changes in the whole presented the very strong seasonal characteristic, and in the annual 12 months NDVI was lowest in January and was highest in August. Looking from the season, spring and the autumn presented an enhancing tendency, summer and winter for reduced tendency from 1982 to 2000. In the research area, the vegetation NDVI had an indistinct increase trend in the-mid reaches of YANGTZE RIVER from 1982 to 2000, but there was reduction trend in parts of area. Vegetation reduction area existed in Mountain area in the Northwest and Northeast of Hunan.(2)The annual mean temperature had the trend of increasing obviously within nearly 20 years with a linear regression trend of 0.0479, 1998 was the warmest year. The average temperature of the four seasons also all presented increase trend, except for some difference in range between them, and the increase velocity in winter increased quickest. The whole study area presented an enhancing tendency with temperature, and from south to north the tendency is increasing gradually. The annual mean precipitation also exhibited increase trend. The precipitation in spring and summer exhibited increase trend, with the one in autumn and winter presented negative change trend.(3)The dynamic change of the annual vegetation NDVI last year related remarkable with the yearly average precipitation in the same year, explained that last year the NDVI change had certain influence to same year's precipitation, but NDVI and temperature related was not remarkable, must be weak compared with the precipitation to vegetation's lag response. There was an obvious relationship between the summer temperature, precipitation and the Normalized Difference Vegetation Index of preceding winter. It showed that preceding vegetation growth condition had certain influence to the later summer temperature and precipitation, but different season had different influence.(4)Numerical simulation has been carried out in the-mid reaches of YANGTZE RIVER by using WRF. The result indicated that the simulation result further confirmed the conclusion of the statistical analysis, and the research area vegetation cover change is influential to this region's climatic element. It finds out that precipitation decreases in vegetation reduction area, but increases in part of the downwind area. Lake in the vegetation reduction area plays an important role, and it can strengthen the rainfall in part of nearby. In the vegetation reduction area, the surface temperature increases obviously, which of the most range can reach 2—3℃. Vegetation reduction can affect the ground wind field and the low-level circulation, and weaken or bar the wind through the area.
|
PDF Full Text Request