پديد آورندگان :
بابایی فینی، ام السلمه نويسنده استادیار Babaee Fini, Omosalameh , صفرراد، طاهر نويسنده دانشگاه تهران, , , كریمی، مصطفی نويسنده استادیار Karimi, Mostafa
چكيده لاتين :
1. Introduction
Dust storm is an important atmospheric phenomenon. It has important environment, health, economic and social activities due to penetration of suspended particles of dust to the atmosphere. Measurements resulting from earth and satellite stations show the fact that particles of dust in storms, remain in the nature for a long time. They can get far away by wind, hundreds of kilometers from their origin and have destructive effects. This phenomenon is one of the most disastrous calamities in deserts and half dried regions of the world. It causes many environmental problems in these regions.
Iran is on the dried and half dried belt in the world. Therefore, it is exposed to local and synoptic dusts repeatedly.
Wind with high speed in short time is called storm, storm with unstable air. If unstable air is humid, it is called storm and lightning. It is also called dust storm.
On the basis of the world Meteorological Organization agreement, in case in a station, the wind is beyond 15 meters per second and horizontal vision is less than one kilometer, dust storms occur. There are different methods and data for studying dust storm. One of them is synoptic-dynamic studying on this phenomenon.
According to the terms of the dust storms formation, bare lands of world desert regions (Africa, the deserts of the Middle East, China, Australia and the United States of America) are prone to such storms. Dust storm creative power can arise from the Ocean systems, such as Hurricane of North Atlantic Or due to high pressure gradient difference between two agent regions of this phenomenon, rare sand storms originating from the desert to reach the Arctic region.
In recent years, during which we have been faced with a lot of dust storms, the study of this phenomenon, especially for half western storms of the country and those derived from West surrounding borders confirms the importance of this phenomenon in all aspects of the inhabitants ' life of this land. The current study seeks to investigate the pattern of atmospheric circulation and surface conditions that shape them in terms of spatial and temporal view, which has shaped after big dust storms identification in the period of 1985 to 2005.
2. Study area
Taking into consideration the accelerating trend of the emergence of dusts in the west of Iran, the mentioned region was selected to conduct the research. The mentioned region in the south covers the northern shores of the Persian Gulf from the 30 degree of the north latitude to the borders of north –north West of Iran.
3. Material and Methods
For doing such a research, the data of frequency of dust and standardized pressure of the sea level for the earth surface were collected from the state meteorology organization (dust data with code 6) and the ocean-atmospheric organization, respectively.
Also geo-potential-meter and wind orbit and longitude data were obtained from the National Organization of ocean-atmosphere. These data with local separation capability of 2/5 degree in longitude and latitude are in the form of daily average. The above data are NCEP-DOE Reanalysis 2, that is advanced version of data in model NCEP Reanalysis with no error in quantity.
To study the significant difference in the abundance average of dust stream in the west region of the state during 1989-2005, T test with the two independent samples was used. The prerequisite of doing this test is the study of normality of frequency data of dusts, because in the event of lack of normality of the frequency of dusts, it is necessary to use the equality of non-parameter of this test, i.e. Mann Whitney U Test. In order to study the normality of data, the Kolmogorov–Smirnov test was employed.
After testing comparative hypothesis, different time cycles, regarding to frequency of dust were specified. Then huge dust stream s occurred from 2000-2010. These data were obtained from the site of U.S.A National Organization of space aviation.
4. Results and Discussion
In order to study the presence of difference in the frequency of the data in the hot season (April to September) and cold season (October to March), the t test with two independent samples were used.
The test is used to calculate confidence interval or difference hypotheses test of two independent samples. In other words, in this test obtained averages from random samples are judged. This means we choose random samples from two different communities (here warm season and cool season) (recorded stations of dust frequency) and also compare average of two communities with each other.
This method is based on the t normal distribution and it is best used for small samples that the data are near normal or normally distributed. So at the beginning of the notion normality of the data must be calculated. In the current study the Kolmogorov - Smirnov test was used.
Results showed that the dust frequency mean has normal distribution in studied stations during 1989 to 2008. Thus, t test with two independent samples can be used for investigation of dust frequency differences in the two warm and cool seasons.
To interpret the t test results with two independent samples first equality or inequality of dust frequency variance should be investigated in the two warm and cold seasons. The test which is used to this purpose is Levene 's test. In this test, first conversion is performed on dust frequency in both warm and cold seasons and thus new data is calculated for each of these two groups. Then, t-test is done on the new data.
Comparative analysis of 40 dust stream occurrence during the last 11 years and considering different periods (calculated in testing difference hypothesis) led to presenting the patterns of occurrence of dust stream During hot periods (April to September) and cold periods (October to March).
The monthly average of dust in the west of Iran indicates the maximum of frequency for the occurrence of dust in April to September and contrary to that, in other months, the frequency of this phenomenon reduces.
The synoptic analysis of dusts in hot seasons: Studying the maps of pressure of ground surface and geo-potential height, the speed and direction of wind at the level of 850 hPa during the hot season, it was specified and that in most cases, a low pressure has taken shape at ground surface located in the west of Iran , i.e. in agreement with the centers of dust (so called Oman-Persian gulf low pressure which are observed in form of a flare of seasonal low pressure of India) and at the level of 850 hPa, a low height in the north, west and north west of Iran is observed which transfer dust into the western regions of Iran.
The synoptic analysis of dusts in the cold season: Studying the maps of the land level and geo-potential height, the speed and direction of wind at 850 hPa during the cold season, it was specified that an asymmetric model leading to the dust (opposite to the hot season which was fixed for all samples) results from different conditions. In line with this, with the study and cultivation of samples collected, it was specified that the emergence of dust in the cold season is mostly a function of the 850 hPa rather than surface.
5. Conclusion
Research results show that the frequency of dust in hot seasons is more than that of the cold season and this difference is meaningful with confidence level of 95%.
This meaningful difference derives from their creator systems and specifications of the earth during hot and cold systems. Although instability in the cold season is more than the hot season, the humidity on the earth surface, prevents the formation of dust. The comparative patterns show that those factors that create dust in cold seasons are more variable due to the variety of pressure systems that affect Iran.
