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

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值，是基于针对南非本土条件适配改造的Bristow与Campbell（1984）公式（Bristow and Campbell (1984) equation）生成的。该每日辐射数据可作为其他公式的输入参数，例如用于计算作物潜在蒸发量的Penman-Monteith公式（Penman-Monteith equation）。针对南非地区的每日太阳辐射载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，且与伴随高昼夜温差（diurnal temperature range）的最高气温正相关；而多云/降雨天气则对应低昼夜温差。基于该前提，研究团队针对南非本土条件对Bristow与Campbell（1984）公式进行了两项适配改造：一是针对大气水汽含量的晴空消光项进行区域优化表达，二是针对多云/降雨天气下的太阳辐射消光项构建逐月份的区域化表达。除优化得到的区域/月度系数，以及通过三角几何函数计算得到的地外辐射量之外，本次估算仅使用每日最高气温与最低气温作为输入变量。该两类气温数据是基于Schulze与Maharaj（2004）提出的方法，针对1950年至1999年这50年时段，在南非全境429700个1弧分（arc minute，对应实地尺度1.7×1.7千米）的栅格点（raster points）上计算得到的。