起伏地形下日照时间计算模型的修正

通过对日照时间计算模型的原理分析,发现该模型具有系统误差,误差产生的原因在于模型中忽略了气象站的日照百分率是基于天文可照时间计算这一事实。文中利用平原、丘陵、山区的实际资料对模型进行了试验分析,探讨了其系统误差的变化规律。针对原模型误差的产生原因,提出了去除系统误差的日照时间计算修正模型。从公式解释和实际资料两方面对修正模型进行了系统误差和非系统误差的分析,证明修正模型消除了系统误差,使总体误差大大减小,其总的误差仅由天空状况不一致及DEM分辨率导致的误差引起,误差大小与测站到计算点的距离及天气系统有关。在对实际资料计算结果的分析后,指出在目前的气象站点密度配置条件下用修正模型计算日照时间能够满足实际业务及服务的需要。

Corrections for Sunshine Duration Models under Rugged Terrain Conditions

It was found that there exist systematic errors in models for calculating sunshine duration based on theoretical analyses in that these models do not account for the fact that the sunshine percentage is calculated from the astronomical sunshine duration. In this paper, the author investigated the variability in the systematic errors through testing the model outputs with measurements from plain, hilly and mountainous areas. The characteristics of the systematic errors from the model are: 1) the error in winter is generally greater than that in summer; and 2) the error in mountainous areas is greater than that over flatland. A corrected model for calculating sunshine duration was subsequently proposed to eliminate the systematic errors. The corrected model was implemented through correcting the sunshine percentage involved in the original mode, i.e., the ratio of actual sunshine duration to astronomical sunshine duration. By multiplying a coefficient, the sunshine percentage in the original model was converted into the actual sunshine percentage in the corrected model, i.e., the ratio of actual sunshine duration to geographical sunshine duration. The coefficient is equal to the ratio of the astronomical sunshine duration over the geographical sunshine duration. By this way, the sunshine percentage in the corrected model is much more than that in the original model. This method has the capability to remove the systematic errors. To calculate the geographical sunshine duration, a day can be divided into several equal time steps. After judging whether each period is shadowed by surrounding terrains, the geographical sunshine duration is eventually derived by summation of sunshine duration at each time step not shadowed. Therefore, the actual sunshine duration under rugged terrain conditions can be determined by the proposed method. The shorter the time step partitioned, the more accurate the results would be obtained. Given the time cost and efficiency in calculation, the time step can, however, not be very short. In addition, the author illustrated the error in the correction model, indicating that there is no systematic error in the corrected model. The overall error of the corrected model could be attributed to non-uniform sky conditions and DEM resolution. The magnitude of the error is closely related to the distance from the monitoring stations to the calculation sites and weather systems. Based on the comparison between the corrected model-based outputs and the measurements, it was demonstrated that the corrected model can basically meet meteorological operation and practical applications under current weather station networks.