基于MODIS数据的青藏高原气温与增温效应估算

MODIS-based estimation of air temperature and heating-up effect of the Tibetan Plateau

利用2001-2007年MODIS地表温度数据、137个气象观测台站数据和ASTERGDEM数据,采用普通线性回归分析方法(OLS)及地理加权回归分析方法(GWR),研究了高原月均地表温度与气温的相关关系,最终选择精度较高的GWR分析方法,建立了高原气温与地表温度、海拔高度的回归模型。各月气温GWR回归模型的决定系数(AdjustedR2)都达到了0.91以上(0.91～0.95),标准误差(RMSE)介于1.16～1.58℃;约70%以上的台站各月残差介于-1.5～1.5℃之间,80%以上的台站的残差介于-2～2℃之间。根据该模型,估算了青藏高原气温,并在此基础上,将高原及周边地区7月份月均气温转换到4500m和5000m海拔高度上,对比分析高原内部相对于外围地区的增温效应。研究结果表明:(1)利用GWR方法,结合地面台站的观测数据和MODISTs、DEM等,对高原气温估算的精度高于以往普通回归分析模型估算的精度(RMSE=2～3℃),精度可以提高到1.58℃;(2)高原夏半年海拔5000m左右的高山区气温能达到0℃以上,尤其是7月份,海拔4000～5500m的高山区的气温仍能达到10℃左右,为山地森林的发育提供了温度条件,使高原成为北半球林线分布最高的地方;(3)高原的增温效应非常突出,初步估算,在相同的海拔高度上高原内部气温要比外围地区高6～10℃。

Time series of MODIS land surface temperature （LST） data, together with meteorological data of 137 stations and ASTER GDEM data for 2001-2007, were used to estimate and map the spatial distribution of monthly mean air temperatures of the Tibetan Plateau and neighboring areas. Time series and regression analyses of monthly mean land surface temperature （Ts） and monthly mean air temperature （Ta） were conducted using both ordinary linear regression （OLS） and geographical weighted regression （GWR） methods. Analysis shows that recorded Ta is rather closely related to Ts, and that the GWR method has a much better result （adjusted R2 〉 0.91, root mean square error （RMSE） = 1.16-1.58 ℃） for estimating Ta than OLS. The GWR model, with MODIS Ts and altitude as independent variables, was thus used to estimate Ta for the Tibetan Plateau. For more than 80% of the stations, the Ta retrieved from Ts had residuals lower than 2 ℃. Analysis of the spatial pattern of retrieved Ta data showed that the mean Ta of the summer half year was higher than 0 ℃ even at high altitudes of 50004-600 m of the plateau, especially in the warmest month （July） the Ta in high mountain areas with altitudes of 4000-5500 m could reach as high as 10 ℃. This may help explain why the highest timber line in the northern hemisphere is located on the Tibetan Plateau. According to our results, Ta in July was probably 6-10 ℃ warmer in the inner plateau than in the outer plateau at any given elevation which resulted from the heating up effect of the Plateau.