Analysis Of Urban Heat Island In Tianjin

In this paper, the temporal and spatial characteristics of urban heat island in Tianjin during the year 2008 is analyzed, using 14 automatic weather station observation data of Tianjin, NCEP/NCAR daily reanalysis data (1°×1°), data from the Beijing sounding station and conventional surface meteorological stations in Tianjin. We also study the characteristics of variations of the urban heat island under different weather conditions and emphasize on the correlation between urban heat island intensity and cloudiness, height of cloud base, wind direction, wind speed and relative humidity. Finally, the weather types and the types of urban heat island are classified according to the circulations, clouds, winds and the temperature differences between Tianjin station and 11 surrounding stations. The main conclusions are summarized as follows:(1) The urban heat island in Tianjin not only has significant diurnal and seasonal variation characteristics, but also is closely related to the method of the choice of urban and suburban stations. The daily variation trends of the urban heat island in Tianjin in the year 2008 and during its four seasons are consistent. Tianjin has strong heat island intensity at night and before sunrise, weak in the afternoon; the strongest heat islands are observed in winter, the second in fall and the weakest in summer. The daily characteristics of temperature difference between Tianjin station and 11 surrounding stations respectively in the year 2008 are as follows:generally, the average daily maximum urban heat island intensity decreases from the northwest inland stations to the southeast coast stations. During the daytime, the average hourly temperature differences between Tianjin and coast stations are weak heat island, suburban stations are no heat island and weak heat island, rural stations are weak heat island; during the nighttime, the average hourly temperature differences between Tianjin and coast stations are no heat island and weak heat island, suburban stations and rural stations are weak heat island and strong heat island. The difference of average hourly temperature difference between Tianjin station and 11 surrounding stations respectively in the four seasons in 2008 has the maximum value in fall, the second in winter, the minimum in summer. The average hourly temperature differences between Tianjin and coast stations are strong in spring and summer, weak in fall and winter, and that between Tianjin and inland stations are on the opposite, strong in fall and winter, weak in spring and summer.(2) The analysis of the relationship between urban heat island intensity and clouds, atmospheric humidity, winds and precipitation is performed. Also the relevance of heat island intensity and the meteorological factors such as intensity of near-surface temperature inversion, sea breeze circulation and the front passage is taken into account. Tianjin often has strong heat island when it has very small relative humidity throughout the troposphere, weak northwester and clear nights. The heat island intensity is often greatly reduced when the sea breeze and the front passes during the spring and summer, and the cold air transporting from the southeast and northwest respectively thus weakens the urban heat island in Tianjin. The average heat island intensity increases to a maximum value between three and four nights after a cold front passes and we also find that the urban heat island intensity is closely related to the distance of sea breeze propagating. The analysis of relationship between urban heat island intensity and cloudiness, height of cloud base, wind speed, wind direction and relative humidity is performed using multiple linear regression analysis method, and the results show that the wind speed is the most dominant meteorological factor, but the overall impact of the meteorological elements on the urban heat island in Tianjin is relatively limited.(3) Statistical analysis shows the most frequently weather types occurring in Tianjin station when it has different types of temperature difference between Tianjin station and 11 surrounding stations, and the most frequently types of temperature difference between Tianjin station and 11 surrounding stations occurring under different types of weather condition.