Cold Spells

In South Africa cold spells are defined as 3 or more consecutive days with minimum temperatures below 2.5 degree Celsius, severe cold spells are 3 or more consecutive days with minima below freezing point, i.e. 0 degree Celsius, while extreme cold spells are defined as 3 or more consecutive days with temperatures below -2.5 degree Celsius. In South Africa the main causes of cold spells are outbreaks of Antarctic air which follow cold fronts, and in the southern hemisphere with its vast tracts of oceans, these cold fronts are frequently well developed. Cold spells affect many sectors, including health, especially exposure to low temperatures, energy, with marked increases in electricity consumption for heating purposes. In agriculture some consequences of cold spells include frost and freeze impacts, especially if the cold spells occur early or livestock losses, where the livestock has not been protected from the cold. The temperature database created for South Africa by Schulze and Maharaj (2004) was used to compute frequencies of cold spells. In this temperature database daily maximum and minimum temperatures were generated for the 50 year period 1950 - 1999 at 429 - 700 one arc minute grid points covering the area, by using observed daily temperatures of varying durations from 973 temperature stations with quality controlled data which were infilled and extended to the common 50 year period by techniques outlined in Schulze and Maharaj (2004) and interpolated/extrapolated using regional and monthly temperature lapse rates. What was not accounted for in mapping cold spell occurrences at each of the grid points was whether a point was located in a valley, and therefore subject to cross-sectional nocturnal cold air drainage from crest to valley bottom in the winter months, or on a crest. Schulze and Maharaj (2004) found some differences between minimum temperatures from stations located in valleys vs those on crests, but were not able to map such temperature differences because of the complex dynamics of determining longitudinal cold air drainage within a valley, i.e. from higher altitudes of the upper valley to the lower altitudes of the lower valley.南非对寒潮（cold spells）的定义为：连续3天及以上最低气温低于2.5摄氏度；强寒潮（severe cold spells）指连续3天及以上最低气温低于冰点（即0摄氏度）；极端寒潮（extreme cold spells）则定义为连续3天及以上气温低于-2.5摄氏度。 南非寒潮的主要成因是伴随冷锋（cold fronts）活动的南极气团暴发；在广袤海洋遍布的南半球，这类冷锋通常发育得较为成熟。 寒潮会对多个领域造成影响：包括健康领域，尤其是低温暴露风险；能源领域，供暖相关电力消耗会显著攀升；农业领域，寒潮（尤其是发生在早季时）会引发霜冻与冻害，若未对牲畜采取防寒措施，还会造成牲畜死亡。 本研究采用Schulze与Maharaj（2004）为南非构建的气温数据库（temperature database）来计算寒潮发生频率。该数据库通过973个经过质量控制（quality controlled）的气象站逐日观测气温数据（观测时长各不相同），针对覆盖研究区域的429~700个1弧分（arc minute）网格点，生成了1950-1999年共50年的日最高、最低气温序列；具体方法为先通过Schulze和Maharaj（2004）中所述的技术手段将观测数据插补延长至统一的50年时段，再结合区域及逐月气温直减率（temperature lapse rates）完成插值与外推。 但在各网格点的寒潮发生情况制图过程中，未考虑网格点是否位于山谷——这类点位在冬季会受到夜间沿山脊向山谷底部横向流动的冷空气下泄影响，或是位于山脊之上。Schulze与Maharaj（2004）曾发现山谷站点与山脊站点的最低气温存在一定差异，但由于山谷内纵向冷空气下泄（即从河谷上游高海拔区域至河谷下游低海拔区域）的动力学机制极为复杂，无法绘制这类气温差异分布。