تغییرپذیری زمانی – مکانی بارش‌های حدی در ایران

Extended abstract
1- Introduction
Precipitation is usually recognized as the most important and variable factor of climate which is considered from different aspects in environmental sciences especially meteorology and climatology. Because of precipitation temporal-spatial variability importance, and its role in communication and economical disasters, it has attracted much attention by many researchers (Rodriguez et al. 1998 Ochola and Kerkides 2003 Baigorria et al. 2007 Jianting et al. 2009 Nastos and Zerefos 2009).
Spatially, the amount and characteristics of precipitation will change greatly, especially in the arid countries and middle-latitudes in distances less than one kilometer. In order to investigate the precipitation variability, various time scales are taken into account. Scales to study the precipitation element varies from different geological eras consisting of several thousands and millions year periods to month, hour, and minute (Livada and Asimakopoulos 2005).

2. Methodology
The main data under study are 24-hour maxima precipitation frequency for each month of a year in the whole statistical period. We used 40 weather stations which spread out over the whole country along with a full statistical records and a statistical period of 40-years (1966-2005).
Harmonic analysis is usually used to study the periodical changes, and includes oscillations or changes display in a time series obtained from sinus and cosines function set (Kirkyla and Hameed 1989 Wilks 2006).
The first harmonic shows an annual cycle with the greatest amplitude as compared to other harmonics. The second harmonic indicates a semi-annual change, while the third one illustrates 4-months seasonal changes in details besides it is an index of inter-annual patterns of frequency. Eventually, the values of demonstrate the time motion of maxima frequency along the time axis.

3. Discussion
3.1 Harmonic Analysis
In monthly data, 6 harmonics are required to explain the data variance. In the studies, also 3 harmonics are used to explain data variance. Of course, since the first two harmonics explain nearly 90% of the total variance, in some research only the first two harmonics are used to study the annual changes (Livada et al. 2008 Nastos and Zerefos 2010). According to the statements discussed above, first harmonic consists of more PVR1 and plays a significant role in variance distribution in central and southern stations. The amount of first harmonics role decreases in north-west stations.
3.1.1 The First Harmonic
The 1st harmonic indicates annual changes and explains variance largely in most stations. The first harmonic which signifies an annual alternation in regions with regular precipitation has longer amplitude in comparison with other harmonics. Spatial distribution of the first harmonic demonstrates the following:
1. In northern coasts of Persian Gulf, this value consists of a high value of about 60-70%.
2. In the west, PVR1 values are approximately 80%.
3. The lowest value of PVR1 is seen in southeastern and northwestern of Iran.

3.1.2 The Second Harmonic
The 2nd harmonic indicating oscillations with shorter time periods (six months) as compared to 1st harmonic in sections with more PVR1 consists of lower values. PVR2 values in the northwest of the country in Azerbaijan region, the values are remarkable (60%). Spatial distribution of T2 in this section has more complexity.

4. Conclusions
Because PVR component indicates the role of each harmonic in explaining and interpretation of variance, in each section which consists of a high value, the higher the portions of changes, the higher the share of changes will be.
1. In many regions in Iran, the 1st harmonic plays a significant role in variance distribution.
2. The highest values of PVR1 for extreme precipitations were observed in Kashan.
3. After the 1st harmonic, the 2nd harmonic has a very important role in variance distribution.
4. Extreme precipitation has a more harmonious pattern in the central parts of Iran around Kashan, southern coasts of Caspian Sea and Persian Gulf's shores as compared to other parts.
5. In northwest and southeast sections, the PVR value of the 1st harmonic is less. On the contrary, PVR value of 2nd harmonic increases especially in the northwest.