تغییرپذیری زمانی و مکانی بیشین? بارش ماهانه در بخش های جنوبی دریای خزر

Introduction: Precipitation is one of important climatic elements that vary considerably over space and time. One of the important aspects of precipitation study is the extreme precipitation. Because of economical effect, in recent years extreme climatic events have proved to be one of the most popular topics in contemporary climatology. It is well understood that climatic features in precipitation records are hidden in relative variables such as monthly and annual precipitation amounts, ۲۴-hour annual precipitation extremes, rainfall intensities and temporal scale of rainfall variation ranges from minutes in a storm cell to decades and longer. The variability and spatial distribution of precipitation at different scales are the main cause of flood and drought events. For analysis of variability of precipitation, we can use harmonic method. Harmonic analysis is a particularly useful tool in studying precipitation temporal patterns as it reveals the spatial variation of various precipitation characteristics. It delineates the geographic extents of various precipitation regimes and highlights the boundaries between them. Materials and methods: North of Iran as a particular region has very different climatic condition. Harmonic analysis with the aid of forty years data of the well distributed network of ۳۲ stations used to study of precipitation variability at this region. A brief explanation of the principles of harmonic analysis is presented concerning the nature and interpretation of this technique. For the monthly values of the examined frequencies f_t (f_t=۰ At the origin), harmonic analysis can be written as follows: (f_t ) ?=f ?+?_(k=۱)^۶?(A_k cos??۲?/۱۲? kt+B_k sin??۲?/۱۲? kt) (۲) Where A_k, B_k are the coefficients of the kTh harmonic (k=۱, ۲ . . . ۶). These coefficients are given by (Panofsky and Brier (۱۹۵۸) and Wilks (۲۰۰۶)) as A_k=۱/۶ ?_(t=۱)^۶??f_t cos(۲?/۱۲ kt) ? (۳) and B_k=۱/۶ ?_(t=۱)^۶??f_t sin(۲?/۱۲ kt) ? (۴) Where f_t represents the monthly frequency of the annual ۲۴-hour maximum precipitation amounts at the tTh month. The amplitude of a given harmonic is C_k=[?A_k?^۲+?B_k?^۲ ]^(۱/۲) (۵) The variance of each harmonic can be calculated (Livada et al. ۲۰۰۸) as: V_k=(C_k^۲)/۲ (۶) And the percentage of variance (PVR(k)) of each harmonic can be determined by the ratio: PVR(k)=V_k?(?_۱^۶??V_k ?) (۷) The phase angle of the kth harmonic can be obtained (Wilks ۲۰۰۶) by: ?_k={?(tan^(-۱) (B_k/A_k ),? A?_k>۰ @tan^(-۱) (B_k/A_k )±?,or±?۱۸۰?^°,A_k

Keywords:

بارش , بیشینه , تغییرپذیری , ماهانه , هارمونیک , واریانس