عنوان مقاله [English]
Precipitation is a principal element of the hydrological cycle, so that understanding its behavior may be of profound social and economic significance. The detection of oscillations in precipitation time series yields important information for the understanding of climate. These oscillations can be seen as a response of the climate system either to external forcing or feedback processes.
In addition, precipitation is a very variable climatic element. It is highly variable, both spatially and temporally at different scales (interannual and intra-annual). The search for cyclist in the climatic record can resolve some of the complexities of the atmospheric system .Many authors analyzed the precipitation patterns in several parts of Iran (e.g. Omidvar K., 2007, Babayi fini A. and Farajzadeh M., 2002, Kavousi A., Meshkani M.R., 2007 Masoudian A., 2005 ,Masoudian A., 2009). Iran is located in worldâs dry belt, so studying the precipitation characteristics and its periodicity of different harmonics in order to forecasting its behavior has significant role on water resources management of the country
The main objective of this study is spatial and temporal analysis of inter-annual variability of frequency of monthly precipitation over Iran using harmonic analysis.
One of the most important features in analyzing the climatology of any region is to study the precipitation as the most variable climatic element. In addition, the abnormal variability and uneven spatial-temporal distribution of precipitation are often the direct reasons for the extreme flood and draught events. Harmonic analysis represents the variations of a time series in the form of sine functions.
The purpose of the applying of harmonic analysis to the frequency precipitation amounts, for each month, is to determine the characteristic parameters of the variances of these frequencies as well as the determining the occurrence date of maximum values of precipitation (Tk) over Iran. To do so, long monthly precipitation time series data from 33 well-scattered synoptic stations are studied with basic harmonic analysis in 25 years statistical period (1980-2005). Herein, the amplitude of frequencies, phase angle and basic statistical parameters are calculated in order to depict spatial characteristics of precipitation, revealing the various climatologically patterns of Iran in the form of contour maps.
First, relative frequency of precipitation for each month was calculated and characteristics of monthly precipitation over the whole country were analyzed in form of six harmonics. It is important to note that, the first harmonic represents a single annual cycle with the largest amplitude in comparison to the other harmonics. The second harmonic represents a probable semi-annual variation, while the third harmonic describes the seasonal 4-month variations, in more detail, and is also an indicator of the inter-annual patterns of the examined frequencies. Finally, the Tk values indicate the displacement of the maximum frequency along the time axis.
3-Results and Discussion
The harmonic analysis showed that the first three harmonics are the most important ones for explanation of inter-annual variability of precipitation over the country. As mentioned before, the first harmonic represents a single annual cycle with the largest amplitude in comparison to the other harmonics, so that this harmonic revealed more than 70 percent of monthly precipitation variability in most of regions except Northwest and Southeast of the country. The highest values of first harmonic PVR are related to Khoramabad (92.52%), Babolsar (92.08%) and Mashhad (90.20%) stations, respectively. Also, the results showed that the highest values of second harmonic variances are related to Ardebil (53.74%), Tabriz (48.66%) and Orumieh (33.97%) stations, while the effect of third harmonic in western, central and southeast of the country, such as Iranshahr (8.92%) and Tehran (6.92%) stations is considerable. In addition, in Kermanshah, Hamedan, Sanandaj, Esfahan and Bojnord stations the third harmonic has significant role. It is concluded that the occurrence date of maximum values of precipitation (Tk) for the first harmonic varies from mid-December to March over whole country except in the southern coasts of Caspian Sea. Tk for second harmonic varies from the October to March, while for the third harmonic it varies from October to February. The amplitude of the first harmonic which shows the spatial distribution of the size of particular types of seasonal variations is more important in western, eastern and central parts of the country.
This paper has focused on the spatial and temporal characteristics of precipitation over Iran using harmonic analysis. It is observed that only the first and the second harmonics are sufficient to explain more than 90% of the climatologically variations in Iran.
It is important to note that the fourth, fifth and sixth harmonics did not represent significant impact on precipitation variability over whole country. Also, it is concluded that the occurrence date of maximum values of precipitation (Tk) for the first harmonic varies from mid-December to March over whole country except in the southern coasts of Caspian Sea.