| چکیده انگلیسی مقاله |
IntroductionThe Urban Heat Island is a phenomenon whereby cities become warmer than the surrounding suburbs. In otherwords, there is a temperature difference between the cities and their surrounding areas. Generally, the UHI effectis a result of excessive and unplanned growth of urbanization. The behavior of artificial urban texture in terms ofabsorption of short-wave and long-wave radiation, transpiration, releasing of anthropogenic heat, and blockingprevalent wind is significantly different from that of the rudimentary nature. Hence, the Bowen Ratio in the citiesalters and the sensible heat increases. Surface geometry, on the other hand, decreases wind speed in urbanregions that plays a significant role in formation of UHI. Since the energy balance inside a city is altered, UHIintensity may change. This means UHI intensity is not spatially and temporally similar in different cities. It mustalso be noticed that UHI formation in a city usually has diurnal or seasonal patterns which are mostly affected bysynoptic weather conditions. There are three main synoptic and local climatology parameters that affect UHIformation: Air Pressure Systems, Cloudiness, and Wind Speed. Under stationary high-pressure systemconditions temperature differences between urban and rural areas become large. UHI intensity is largest in calmair and cloudless sky conditions and tends to disappear in cloudy and windy weather. Generally, synopticpatterns can be divided into three major conditions as stable, unstable, and mediocre. Unstable conditions reducethe heat island intensity by making turbulences which mix the air. Stable conditions, on the other hand, increasethe heat island intensity as they are calm and without air movements. Mediocre conditions can play two rolesdepending on their characteristics and wind properties.The urban heat island can lead to urban temperatures being 2–5C higher than those in rural surroundings.Studies have shown the difference in temperature between urban and rural regions (UHI Intensity: TUR ) isrevealed in minimum temperatures rather than maximums. Henceforth, the Maximum UHI intensity shouldusually occur after sunsets in urban areas. Other impacts of the Urban Heat Island could be intensifying pollutantconcentration over urban areas, altering local wind patterns, increasing humidity, forming cloud and fog, andchanging the precipitation rate over a city.Material and MethodologyIn this study, the influence of synoptic weather conditions on the intensities of the urban heat island of Tehranwas analyzed. Tehran is the largest and the most populated city of Iran, with an approximate area of 750 Km2and a population of 8 Million during night time. The city lies almost in the middle of the Tehran Province (1882Km2 of area) in the southern side of the Alburz Mountain and is limited to the highlands in northern and easternparts. On the southern and western parts, it is connected to the flat plains of Varamin, Shahriar and Karaj.To investigate the effects of synoptic weather conditions on the intensities of the Urban Heat Island overTehran, after a literature reviews, 24 days were selected from the year 2006; two days of each month of the year,one day with the highest and the other with the lowest air pressure over the urban area. After the homogeneity ofthe data derived from the surface station with those of the midlevel atmosphere (850 HPa, 700 HPa, and 500HPa) was examined, the climatologic data (including temperature, air pressure, wind direction and wind speed)for each day were gathered from different data sources: 1- Iran meteorology Organization stations includingsynoptic and climatology stations, 2- Air quality measuring stations including Air Quality Control Company(AQCC) and Department of Environment (DOE) stations. The location and distribution of the stations is shown in Fig. 1. After data refinement, the measuring times were transformed from Greenwich Mean Time (GMT) tothe Local Time (LT) by adding 3:30 to the GMT.The difference between the average temperature of the urban district (TU) and the rural area (TR) wascalculated by MS Excel 2007 for collecting the UHI intensity during different seasons and months and for all 24days at each measurement hour. The midlevel atmospheric data were gathered from National Centers forEnvironmental Prediction/National Center for Atmospheric Research of the United States (NCEP/NCAR). Then,all patterns occurred at mentioned days were manually observed and investigated. Then, the data including geopotentialmeter height, sea level pressure, wind characteristics, and temperature were analyzed using 2.5*2.5geographical grids. At the final step, the 4 days were chosen to represent the influence of the synoptic conditionson the heat island intensities. Two days, with the absolute maximum and minimum of UHI intensity, and twodays representing the total weather conditions of wintertime and summertime heat island. Fig. 1. Spatial Distribution of weather stations in the study areaResults and discussionAccording to the literature reviews, it was expected that during cyclonic condition the intensity of the UHIwould be reduced and inverse condition would be happened in anti-cyclonic condition. Figure 2 represents thevariation of the heat island intensities in the study days. As it could be seen, the absolute maximum intensity (8.9Celsius degrees) has occurred in July while the absolute minimum intensity (1.1) has occurred in January. It canalso be seen that the difference between the maxima and minima of heat island intensities have seasonal changes.While the difference between maxima and minima is the least in cold period, it is the most in the warm period. Infact, in the summer the maxima intensities raises more than those of the minima making the difference biggerthan what it is in the winter. It should also be noticed that the behavior of the minima and maxima issignificantly simultaneous. The maxima and minima almost increase and decrease together. Even in the summerin which the difference is bigger, the maxima and minima are closely correlated. Fig. 2. The variation of maxima and minima intensities of the urban heat island of TehranIn order to investigate the influence of the synoptic weather conditions on the heat island intensity for all fourdays, as mentioned previously, the sea level pressure map,wind field and geo-potential height was calculated(Fig. 3 and 4). Fig. 3. The synoptic weather condition for the absolute maximum (29th of July: A and B) and minimum (4th ofJanuary: C and D) time. A and C: sea level pressure (contours) and surface temperature in Celsius (coloredspectrum); B and D: wind direction (vectors) and wind speed in m/s (colored spectrum). Fig. 4. These are representatives of cold period (A and B) and warm period (C and D). A and C: geo-potential heightof 500 Hpa (contours) and sea level pressure (colored spectrum) for the occurrence time of minimum intensity of heatisland; B and D: geo-potential height of 500 Hpa (contours) and sea level pressure (colored spectrum) for theoccurrence time of maximum intensity of heat island.ConclusionIn this study the influence of synoptic weather conditions on the intensities of the urban heat island of Tehranwas investigated. The results indicated that the intensity of summertime heat island is higher than that of thewintertime. Furthermore, the correlation between the minima and maxima of heat island intensities shows theinfluence of the synoptic weather patterns on heat island intensity. In the combined maps it was revealed that thecorrelation between the maximum and minimum times of heat island intensity is much more significant in thewarm period while there are some inconsistencies in cold period. The reason for this condition could be thedifferent patterns of the atmosphere of Iran. In summer, the edge of Azores' subtropical high pressure is locatedin the midlevel atmosphere of Iran while there are several thermal low pressure cells near the ground. Thiscauses daytime turbulences due to the high radiation income and calm weather when the radiation effect islessened. However, the condition is almost the opposite in the cold period. In cold period, while there is a coldhigh pressure condition near the ground, the midlevel atmosphere experiences a relatively active pattern. Due tothe passing of westerlies, many unstable synoptic systems pass through Iran's atmosphere. The instability andvariety of passing systems increases the wind speed by which the heat island intensity is reduced or undergonevariation. Henceforth, the difference between the low level and midlevel atmosphere is the main cause for thevariation of the intensities of the heat island of Tehran. |