| چکیده انگلیسی مقاله |
Introduction An extreme event is an event that rarely occurred than normal condition. Extreme events, stimulate changes in natural and human systems to more than the average climatic conditions (Peterson et al, 2008). Extreme days (very hot and cold days) have impact on human and natural systems such as agricultural, ecological and etc. A heat and cold wave is a hot and cold period which lasts from a few days to a few weeks and that may be accompanied by high humidity for heat waves and extreme wind for cold waves. The impacts of heat waves that have occurred in the world are follows as: heart pains, severe shortage of water, heat stroke, rising food prices, power outages, increased natural disasters, outbreaks of insects such as mosquitoes, fire, loss of life and living organisms. Also, frost, frost damage to crops and mortality are the result of the cold waves. By far, there are many studies about this climatic events in the world which have been done for example by Panmao and Xiaohua, 2003; and Peterson et al., 2013. Many studies have confirmed that heat and cold waves are increasing and decreasing in recent year, respectively. In Oklahoma, the results of the effects of heat island after a heat wave in year of 2008 showed that heat-island at the core of the city is warmer than rural areas during the day 5/0 degrees and at night 2 degrees celcius and due to presence humidity and heat waves, higher value of heat is generated (Basara et al, 2010). Also studies about this extreme events were done by few meteorologist and climatologist researcher in Iran, for example Lashkari (2009) studied the case study of cold wave which took place over Iran in 11 to 16 December of 2003, he found that Siberian high pressure and decreased temperature are responsible for the extreme cold wave (Lashkari, 1387). By far, about cold waves and heat waves have not been studied in coastal regions of Caspian Sea. This research aim is to provide a general overview of heat and cold waves in this area. Methodology This research has studied heat and cold wave events using minimum and maximum temperature which received from Iran meteorological Organization. After sorting the data and examining them for selecting threshold, cold waves with 0.02 percentile and heat waves with 0.98 percentile were evaluated during 1961-2010 in southern costal of Caspian Sea over five stations namely Bandar E Anzali, Rasht, Ramsar, Babolsar and Gorgan. For better interpretation, wave periods, were divided into three periods fallows as: The waves of long-term (more than 6 days), medium term (3 to 6 days) and short term (less than three days). For synoptic analysis, NCEP/NCAR reanalysis data including sea surface pressure and geopotential height at level of 500 received from the website with 2.5*2.5 resolution for an area covered from 20 to 70 N and 10 to 80E in this region. All the data were analyzed in GRADS software. Then all the results are interpreted with respect to warm and cold waves. Results and discussion Overall, the results showed an increasing trend for minimum temperature and a decreasing trend for maximum temperature. Maximum frequency of cold wave have took placed in years: 1963, 1964, 1969, 1972, 1973, 1989, 1991 and 2008. But continuity of the waves is observable in the year of 1964 and 1972. Maximum frequency of heat wave have took placed in 1962, 1966, 1980, 1983, 2000, 2009 and 2010 while continuity of the waves have been recorded in the year of 1966 and 1971. Ramsar station has recorded maximum frequency of both cold and heat waves. Gorgan has recorded minimum frequency of heat waves while Rasht and Anzali are specified by minimum frequency of cold waves. The synoptic analysis of NCEP/NCAR sea surface pressure and geopotential height at level of 500 showed that Eastern Europe high pressure, Siberia anticyclone, Alpine mountain high pressure and Oral low pressure caused to cold waves. Arabia, Pakistan, Persian Gulf low pressure have created heat waves. Conclusion By now, research about heat and cold waves has been not done in coastal regions of Caspian Sea with maritime climate. This research addressed this waves in 1961 to 2010 period using weather stations and reanalysis data at five stations with the most comprehensive weather data. Different results were obtained for stations which linked to synoptic conditions. In addition to weather and NCEP/NCAR data, it is suggested that use of ERA-interim and satellite data such as MODIS land surface temperature which have different resolution, can be a great help in filling the gaps between stations and will improve the obtained results. |