晴空条件下光合有效辐射中散射辐射的测量模型

为了解决散射辐射地面测量中难以摆脱遮光装置的问题,本研究通过调整传统的直接测量方法,拟建立晴空条件下的散射辐射估算模型。具体方法是,选择晴朗无云的天气条件,在美国华盛顿州普罗瑟市郊无绿色植物覆盖的空旷场地上测试07:30—19:30(间隔15 min)的北向局部散射辐射数据(PAR_f-N)和总体散射辐射数据(PAR_f)。测试同时拍摄天空图像,并计算其灰度特征值(H)和颜色特征值(B)。结果分析表明:1)PAR_f-N与PAR_f之间呈指数函数关系,决定系数r²为0.768;2)天空图像的H值与PAR_f之间呈三次函数关系,r²为0.688;3)结合PAR_f-N、H和B值,及常见气象因素,构建多元线性回归模型,最优模型引入了7个变量,其校正和预测相关系数及校正和预测均方根误差分别为0.951、0.958和27.897、25.526。研究表明该估算模型可在较大程度上提高预测性能及其稳定性。

Model of diffuse radiation of photosynthetically active radiation under clear sky

To eliminate the blocking effect caused by the shadow-ring or shadow-ball,a new model is presented for measuring diffuse radiation of photosynthetically active radiation (PAR) under clear sky.A total of 11 days tests were conducted in an open test field belonging to Washington State University in Prosser,WA,US from June to August in 2012.The measurement interval was set at 15 min.One quantum sensor was placed vertically and was used to measure the standard value of diffuse radiation (PAR_f).Another quantum sensor was specially placed horizontally and faced toward North.The partial diffuse radiation (PAR_f-N) was recorded without any blocking device in most of daytime.The photos of sky were also shot at four directions (east,south,west and north) to attempt to represent the atmospheric condition.The image characteristic value was defined as the minimum value of histogram (H) and B-mode (B) calculated among four photos.Results showed that the correlation between the PAR_f-N and the PAR_f is 0.768.The correlation between the image characteristic values and the diffuse radiation is 0.688.A new multiple linear regression (MLR) model was developed using the SPSS software.The variables included the PAR_f-N,the image characteristic value and some other meteorological factor like temperature,wind speed and relative humidity.The optimal model was obtained when seven different arguments were input,with the correlation coefficients and the root mean square error of calibration and prediction (r_c,r_p,RMSEC and RMSEP) of 0.951,0.958,27.987 and 25.526,separately.The results indicate that the MLR model with PAR_f-N and the image characteristic value offers a more effective way to estimate the diffuse radiation under clear sky.Further studies with different weather condition should be done to improve the applicability,accuracy and robustness of models.