جستجوی مقالات مرتبط با کلیدواژه « فراوانی وقوع » در نشریات گروه « جغرافیا »

This paper intends to identify and statistically analyze monthly changes in the lethal temperature above 50 degrees Celsius (TU50c) in the Northern Hemisphere. For this purpose, the reanalyzed data related to the maximum temperature up to two meters of the earth's surface was used during the period of 73 years from 1948 to 2020, and after adjustment, analyzed on a monthly basis. Descriptive statistics analysis methods, linear trend analysis and non-parametric Mann-Kendall test are used for evaluating the frequency of such occurrences. During the 73-year study period, 9553 days with minimum temperature of 50 degrees Celsius were identified and their coordinates were extracted. Examining the daily frequency of TU50c in the Northern Hemisphere showed that the months of May and January have had the highest frequency, respectively. Also, the highest increasing trend in the number of fatal temperatures was observed in the months of March, April, July and August. In terms of location, the region that could record the highest frequency and the most intense TU50c was related to Africa especially Sudan, West Asia (between Iraq, Southwest of Iran, Kuwait and Saudi Arabia) and India in the Indian subcontinent. Moreover, these temperatures are not rare in America either; but TU50c has not been reported in Europe and East Asia. The highest and most intense temperatures occurred in the fifth decade, between 1988 and 1998.

Atmospheric mineral dust particles play a key role in the radiation budget of the atmosphere and the hydrological cycle, and have an important effect on public health by disrupting climate systems and air pollution. Due to Iran’s location in the arid and semi-arid belt of the world, Iran is constantly exposed to local and regional dust systems. Considering the importance of the negative effects of dust storms and their increasing trend in some dust sources, the study of these changes in the last two decades show the importance of the dust storms in recent years. Moreover, spatial-temporal identification and analysis of the properties of these dust particles is very important in order to manage this crisis and prevent the harmful effects of dust particles. In Iran, due to desert conditions, the presence of dust hotspots has always caused air pollution and reduced the quality of life of people. In recent years, some dust hotspots have been ambiguous about increasing the intensity of dust emission. In this study, using the AOD product of MODIS, which compute the dust intensity, and based on the annual frequency and averages of dusty days, the location of dust hotspots were identified and then the trend of dust intensity in each hotspots were examine. The results showed that despite the relatively similar climate, the trend of changes in these dust hotspots does not follow the same pattern and complex human activities and natural changes.

 In this study AOD product from MODIS with the resolution of 10 km was used to extract dust information then the frequencies of days with AOD greater than 0.6 per year were extracted. In addition to correctly calculating the average of AODs, calculating the number of days without data is also important in the results. The spatial and temporal distribution of the study period, were identified in three periods, 2000-2006, 2007-2012 and 2013-2018. The percentage of changes in each of the dust sources compared in different periods. The standard deviation was extracted to identify the areas most vulnerable to dust storms. Finally, to detect the quantitative distribution, the trend of AOD changes in the extracted dust hotspots was used to investigate the changes in the dust intensity trends.

The map of dust hotspots in the first period shows the main dust sources are in the north of Sistan and Baluchestan (Zabol) and south of Sistan and Baluchestan (Chahbahar), in the southeast of Semnan (Dasht Kavir), Damghan, Garmsar, Jazmourian, southwest of Hormozgan, (Bandar Lengeh area), south and southwest of Khuzestan, southwest of Yazd (Nayer), as well as parts of Qom, Ilam (Mehran), Isfahan, and south of Fars provinces. In the second period of study, many dust centers have become more intense and extensive. According to the map of dust centers in the third period of studies, compared to the first and second periods, the area of dust centers has decreased.According to the results, about half of the areas without emission has been turned into areas with dust with different frequencies in second period, and also about half of the area of ​​very high-frequency hotspots has been turned into other dust sources with less intensity in the third period. Also, the most fluctuations in dust intensity have occurred in Sistan, Jazmorian, southeast of Semnan, East Azerbaijan, Zanjan and Khuzestan provinces. The results of trend analysis of dust intensity in different dust hotspots show that despite the relatively uniform climate, the dust sources trends in different dust sources do not follow the same pattern.

Due to the geographical location of Iran and the existence of vast deserts, the wethear has always affected by dust sources of inside and outside of the country. In this study, using satellite data with appropriate resolution, the location of dust sources in three time periods were extracted. The changes of each dust intensity class in the second and third periods were compared with the first period so that regardless of location, changes in dust intensity can be evaluated in general. Then, using the standard deviation method, the dust hotspots with the highest percentage of changes were identified. Finally, the trend of changes was calculated by examining the trends of changes in 24 main dust centers. According to the results of the present study, many changes have been observed in some dust sources and the intensity of dust in many dust sources has decreased. While some sources such as Isfahan, and Khuzestan province due to the role of human factors such as agricultural activities as well as the reduction of surface and ground water and as a result of drought and changes in soil texture have an increasing in trend of dust intensity. Since a decreasing trend is observed in most of dust sources, eastern and southern parts of Iran, the results of this study indicate the key role of climatic factors in changes and fluctuations in dust emission in Iran. Because climatic factor can be the only factor which has a relatively uniform effect on the dust emission on a large scale of Iran.

The purpose of the present study was to identify the characteristics of temporal and spatial distribution of climatic phenomena are thunderstorms. Therefore, for the purpose of data on the frequency of thunderstorms in Tehran province with emphasis on temporal and spatial characteristics 7 state on thunderstorms were studied and analytically considered. The results showed the highest frequency of thunders toms in theyear2004 and the lowest frequency of occurrencewasin 2000; Mehrabad station and Chet garmostregions have had the lowest volatility during the period. Oscillating and linear trend and polynomial models when all stations except CHITGAR and geophysics has been an increasing trend. Maps of the spatial distribution and spatial patterns howled that Abali and Firoozkooh the higher stand lowest frequency geophysical thunder storms between stations in the period studied.