The Effect of City Expansion on Climatic Conditions of Mehrabad Synoptic Station

The UN Population Demographics in 2014 has shown that there is a reversal of the demographic trend of urban and rural areas, with the world's largest population (54%) living in urban areas, and this trend is increasing, while by the year 1990 A small population lived in cities. This undocumented development leads to major changes in the environment, including the reduction of natural surfaces and the replacement of these with artificial and human land covers. Urban areas are often identified with impenetrable and constructional surfaces that often have a negative impact on ecosystems. Urbanization changes the natural landscape into human-made with different physical characteristics. Changing and transformation in surface land covers has an important effect on energy balance and local climate. As a result, urban climate is formed from the climatic factors of the city that are changing with the impact of urban factors over time.In order to study the effect of urban expansion on climatic condition of the Tehran, daily precipitation, minimum and maximum temperature data were extracted from Mehrabad synoptic station during a period of 50 statistical periods (1966-2016). Different methods and tests have been used to analyze trends and changes in climatic conditions. The trend in the cluster data series was studied using the Mann-Kendall test. The Mann-Kendall test, is a base-rating nonparametric test for trend analysis which was first used by Mann for public applications in 1945 and revised by Kendall in 1948 and presented in a different way. Then, RClimDex software was used to extract the process of extreme weather indicators. This software has been developed by Zheng and Yang (2004&2005) at the Canadian Weather Service's Climate Research Branch. In the present study, 23 indicators of ETCCDI indicators were used using RClimDex. Indicators are divided into five groups of indicators based on percentiles, periodic indicators, absolute indices, threshold indicators and other indicators (such as temperature change overnight).Based on non-graphical Mann-Kendall Test, both temperature parameters (minimum and maximum temperatures) have an incremental trend during the 1966-2016 period, which is more evident at the minimum temperature. While precipitation in the same period of time has been a slight downward trend. Also, based on the Mann-Kendall graphical test, when there is a significant trend in data, the lines ui and u'i interrupt each other. The results of the Man-Kendall test show that the increase in minimum temperature is higher than the maximum temperature. Moreover, during 1976-1986, Tehran experienced rapid physical growth, and this period coincided with an increase in temperature, which continued until the end of the period. Based on the analysis of the temperature extremes index, the cold days indices (TX10p) is facing a downward slope. On the other hand, the Hot Days Index (TX90p) faces an upward slope in the period of 1966-2016. In fact, it can be concluded that during the study period, the percentage of hot days in the Mehrabad station is increasing and cold days are facing a decreasing trend. These results are consistent with the studies of Rahimzadeh et al. (2011), Niw et al. (2006) and Brown et al., 2010, on the trend of the temperature range of the nightly temperature range. Conclusion In recent years, the spread of cities and their growth has had a great impact on the environmental conditions in different parts of the world. In the meantime, some cities have been experiencing rapid and significant growth. Urban change has today had widespread effects on urban climatic conditions. One of these effects is the change in the temperature of the cities (formation of the UHI) and the increase in night temperatures. The present research was carried out to investigate the effects of the expansion of Tehran on the climatic conditions of the Mehrabad synoptic station. The study of the climate conditions of the Mehrabad station shows that during the study period, the air temperature of the Mehrabad station has undergone a change. So that the minimum and maximum temperature is facing an increasing trend, which is, of course, more severe at the minimum temperature. The average minimum temperature of the Mehrabad station during the 1996-75 period was about 10.9 ° C, reaching about 13.7 ° C in the period 2005-2015, and on the other hand, the average maximum temperature during the 1975-1996 period was about 5.5 ° C 22 ° C and in the period 2015-2005 reached about 23.6 ° C. In addition to temperature averages, temperature indices have also changed significantly at Mehrabad station so that cold and warm extreme events have decreasing and increasing trend, respectively.