Authors : Leila Ebrahimi, Hasan Askari Shirazi and Ahmad Askari
Abstract: Drought is a complex and dissonant natural phenomenon which its intensity changes and its occurrence take place slowly. This phenomenon is not predictable and different factors such as geographical location, atmospheric conditions, etc. are involved in its occurrence. Drought adversely affects peoples living conditions, economy and agriculture. Using monthly and annual precipitation data received from Iran Meteorological Organization and also data from National Centre for Environment Prediction-National Centre for Atmospheric Research (NCEP-NCAR) of the United State and using statistical methods, three cases of autumn drought periods in 1998, 1995 and 1996 have been identified and using synoptic maps received from this centre, atmospheric flow patterns, including high-pressure and low-pressure were studied. Maps used in this study are the daily maps with levels of 500 and 1000 HP in a network with a single degree latitude and longitude including 00-75 Northern degrees and 00-90 Eastern degrees. According to the statistical analysis conducted in this thesis, for different stations (31 stations) during the 30-years period, the results display a very dry climate in most of the stations and the order of dry years is as follows: 1998, 1995, 1996, 1983 and 1990. Among these years, we had a very little rainfall in the autumn of 1998, 1995 and 1996 in most of the stations. It was due to a high amplitude blocking ridge on Iran region which we had studied and discussed in this study.
Leila Ebrahimi, Hasan Askari Shirazi and Ahmad Askari, 2020. The Study of Frequencies of High-Amplitude Ridge Blocks with Incidence of Falls Droughts over Iran (Case Study: Between 1976-2005). International Journal of Tropical Medicine, 15: 47-51.