基于地基宽带辐射观测资料的云检测算法改进

Modification of Cloud-Screening Method Using Ground-Based Broadband Shortwave Irradiance Measurements

利用地表太阳总辐射和散射辐射对Long and Ackerman(2000)的云检测算法进行了改进,提高了云判别的准确率。首先采用比值概率密度峰值法,初步选出晴天时刻。然后根据晴天时刻的地表太阳总辐射和太阳天顶角余弦值,拟合得到该日晴天总辐射近似表达式。在此基础上,计算各时刻实际观测值与用该拟合式估计的总辐射的比值,并再次利用比值概率密度峰值法,判断该时刻的天空状况。最后利用全天空成像仪观测资料和站点天气记录结果检验算法,结果表明,在天顶角小于75°条件下,本算法判断准确率平均达90.9%。改进的云检测算法减少了因水汽柱总量、气溶胶浓度和系统测量偏差的日变化及天顶角变化造成的误差。应用该检测算法,得到了香河和太湖两地云日发生频率并分析了云地表辐射强迫季节变化特征。两地云出现频率和云地表短波辐射强迫均夏季最大,春秋次之,冬季最小,太湖站云出现频率的季节变化幅度不及香河。香河云地表短波辐射强迫年平均为-39.5 W.m-2,春夏秋冬季节平均分别为-25.9 W.m-2、-70.9 W.m-2、-51.1 W.m-2、-10.8 W.m-2。太湖云地表短波辐射强迫年平均为-66.2 W.m-2,春夏秋冬季节平均分别为-84.6 W.m-2、-89.1 W.m-2、-50.2 W.m-2、-44.1 W.m-2。

A modified automatic cloud-screening method is developed using 1-min measurement of sur[ace down- welling shortwave global and diffuse irradiance. Clear skies are detected firstly by peak frequency density of ratio （PFDR） method. Then the detected clear-sky irradianee measurements are used to fit the expected clear-sky irradi- ance as the function of the cosine of the solar zenith angle by the least squares fitting method. Afterwards, the PFDR method is applied to the ratio of observed total shortwave irradiance to the predicted values for the cloud-free sky condition with the fitting expression. The clear sky detection method is verified using Total Sky Imager data and observer reports. The result shows that the average accuracy is 90. 9% for the solar zenith angle less than 75°. For the modified cloud-screening method, the uncertainties due to both diurnal changes in such variables as column water vapor, aerosol loading, systematic pyranometer offsets, and solar zenith variables are diminished. Cloud occurring frequency and cloud radiative forcing （CRF） on downward shortwave irradianee at ground are calculated based on the clear sky detection results for Xianghe and Taihtu The cloud occurring frequency and the CRF show a distinct sea- sonal variation with maximum in summer and minimum in winter. The annual average of CRF is --39. 5 W · m-2 in Xianghe and --66.2 W·m-2 in Taihu. The seasonal average of CRF in Xianghe is --25. 9 W·m-2 in spring, --70. 9 W· m-2 in summer, --51.1 W·m-2 in autumn, and --10. 8 W· m-2 in winter. And the seasonal average of CRFinTaihuis --84.6 W·m 2, 89.1W. m-2, --50.2 W·m-2, and--44.1W·m-2, respectively, from spring to winter.