Standard Deviation Of Daily Solar Radiation (Mj/M2/Day) February

These daily estimates of solar radiation were derived using modifications for South African conditions of the Bristow and Campbell (1984) equation. The daily values can be used as input in other equations such as the Penman-Monteith equation for potential crop evaporation. The method of estimation of daily solar radiation loadings over South Africa and its verification are as follows: based on the premise that clear sky solar radiation is determined essentially by extraterrestrial solar radiation modulated by an atmospheric extinction function, and dependent on high maximum temperatures associated with a high diurnal range in temperature, while cloudy/rainy conditions are associated with a low diurnal temperature range, the Bristow and Campbell (1984) equation was modified for South African conditions by regionally optimised expressions of clear sky depletion for atmospheric water vapour content, and by month-by-month regionalised expressions for solar radiation extinction by cloudy/rainy conditions. Other than the optimised regional/monthly coefficients and the computation of extraterrestrial radiation, which is undertaken with trigonometrical geometry functions, the only other variables used were daily maximum and minimum temperatures. The latter were determined for the 50 year period 1950 - 1999 at each of 429 700 one arc minute (1.7 x 1.7 km) raster points over South Africa by techniques outlined in Schulze and Maharaj (2004).

本数据集的每日太阳辐射估算结果，是通过针对南非本土工况修正后的布里斯托-坎贝尔（Bristow and Campbell, 1984）方程计算得到的。该每日辐射数据可作为其他方程的输入项，例如用于潜在作物蒸发量计算的彭曼-蒙泰斯（Penman-Monteith）方程。针对南非区域的每日太阳辐射负荷估算方法及其验证过程如下：其核心前提为，晴空太阳辐射本质上由受大气消光函数（atmospheric extinction function）调制的地外太阳辐射（extraterrestrial solar radiation）决定，且与伴随大昼夜温度变幅的高温强相关；而阴雨多云天气则对应小昼夜温度变幅。基于该前提，研究团队针对南非本土工况对布里斯托-坎贝尔（Bristow and Campbell, 1984）方程完成了两项改进：一是针对大气水汽含量优化得到区域化晴空辐射衰减表达式，二是针对阴雨多云天气下的太阳辐射消光效应建立逐月份的区域化表达。除了优化得到的区域/月度系数以及采用三角几何函数（trigonometrical geometry functions）计算的地外太阳辐射量之外，该模型仅使用每日最高、最低气温作为输入变量。上述气温数据由Schulze与Maharaj（2004）提出的方法生成，覆盖南非全境429700个1弧分（对应1.7×1.7 km）栅格点（raster points），时间跨度为1950年至1999年的50年序列。