基于Copula函数的陕西苹果晚霜冻特征分析

Analysis of the characteristics of apple later frost risks in Shaanxi Province based on Copula functions

为探讨利用Copula函数对苹果晚霜冻进行特征分析的适用性,该研究首先基于陕西省苹果主产区7个气象站点1971-2018年的逐日最低气温(T min)数据集,提取出晚霜冻事件的历时和强度两个特征变量。然后,基于6种不同的Copula函数构建晚霜冻特征变量的联合分布,并进行拟合优度评价。最后,利用优选的Copula函数分析晚霜冻发生的概率及重现期。结果表明:陕西苹果产区各站点1971-2018年受晚霜冻的影响在空间分布上由东南向西北方向加重,各站点晚霜冻的历时和强度之间均具有显著的正相关关系。当晚霜冻强度和历时增大时,其联合累积概率也相应增大,且增大趋势变缓。各站点联合重现期代表的“或”事件比同现重现期所代表的“且”事件更容易发生。当单变量重现期取值较小时,可将联合重现期和同现重现期视为单变量重现期的两种极端情况,对其实际范围进行估计。总体而言,陕西苹果产区各站点发生长历时且高强度晚霜冻事件的概率较小,但是位于延安果区的站点相较于其他果区站点更容易发生高强度或长历时的晚霜冻事件,以及高强度长历时同时发生的晚霜冻事件,需要重点加以关注。该研究可为陕西苹果产区应对晚霜冻灾害提供理论依据。

Late frost is one of the most destructive meteorological disasters in the Loess Plateau of China.A great threat can be posed to the sustainable production of apples,leading to great economic losses in the apple industry.Thus,it is a high demand to explore the occurrence of late frost events for the prevention of apple late frost disasters in the decision-making of the local apple industry.In this study,the late frost return periods were investigated on the duration and severity of late frost events using the Coupla functions.The reliability of the model was then verified to analyze the characteristics of apple late frost.The study area was taken as the apple-producing areas of Shaanxi Province in western China.The meteorological datasets were collected from the seven weather stations from 1971-2018.The daily minimum temperature(Tmin)of 0℃was taken as the critical temperature for the occurrence of apple late frost events,in order to extract the two characteristic variables of duration and severity of late frost events.These characteristic variables of late frost events were first fitted by seven common distribution functions,respectively.Kolmogorov Smirnov(K-S)test was then carried out to verify the model.The joint distributions of late frost characteristic variables were constructed to evaluate the goodness-of-fit using six Copula functions.The occurrence probability and return period of late frost events were analyzed with the optimized Copula functions.The results showed that the severity of late frost risks generally increased from the southeast to northwest in the study area from 1971 to 2018.The optimal marginal distribution of late frost duration was in the log-normal distribution,while there was a great difference in the optimal marginal distributions of late frost severity.A significant positive correlation was found between the duration variables and the severity of late frost at each station.The joint cumulative probability increased significantly,as the severity and duration of late frost increased,but the increasing trend was much slower than before.A much more significant increase was observed in the co-occurrence return period under the same increase of univariate value,compared with the joint.The univariate return period was always between the joint and co-occurrence return period.Once the univariate return period was small enough,the optimal range of the actual univariate return period can be estimated,according to the joint and the co-occurrence return period.In general,a low probability was found in the late frost events with long duration and high severity at the weather stations in apple-producing areas in Shaanxi Province of China.However,the stations in the Yan’an area were more susceptible to the late frost events with high severity or long duration,as well as the late frost events with both high severity and long duration.Thus,more attention can be paid to late frost risks in the Yan’an area.This finding can provide a theoretical base to deal with the late frost disaster in apple production.