The climate in the central city is different from that in the suburbs, which is manifested in the following
The impact of human activities on climate is most significant in cities. According to the observation data of weather stations located in urban areas and surrounding suburbs at the same time, compared with suburban areas, urban climate has five "heat islands", "dry islands", "wet islands", "turbid islands" and "rain islands". "Island" effect.
Heat island:
On a clear and windless night, when entering the city from the suburbs, the temperature will rise significantly. The value is greater above the suburbs. Taking Shanghai as an example, the maximum temperature differences between urban and suburban areas were recorded as 6.8°C and 4.8°C respectively on winter nights and summer nights. Passing through the city and reaching the suburbs, the temperature drops sharply, showing a clear "urban heat island" on the temperature distribution map.
The reason for the heat island effect is, first of all, that except for a few green spaces in the city, most of them are artificially paved roads and scattered buildings and structures, forming many "urban street canyons" with different height-to-width ratios. ". Under the sunlight during the day, due to the multiple reflections and absorptions between walls and walls and between walls and the ground in the street canyons, more solar heat energy can be obtained than in nearby flat suburbs. If walls and roofs are painted darker colors, they will have less reflectivity and absorb more solar heat, and because building materials such as bricks, asphalt and cement boards have greater thermal conductivity and heat capacity, cities Street valleys absorb and store much more heat during the day than the ground in suburban areas. The underlying surface in urban areas has a large impermeable area. After rainfall, rainwater quickly runs away from drainage pipes. The water available for evaporation is much less than that of farmland and green fields in suburban areas, and the latent heat consumed in evaporation is also less. The solar heat energy obtained is mainly used for The underlying surface heats up, forming an "underlying surface temperature heat island." Then the heat is transferred to the air to heat it through turbulent exchange and long-wave radiation. The air is not good at absorbing solar heat, but it is easy to absorb the above-mentioned heat from the underlying surface and increase its temperature.
In addition, because energy consumption and population density in cities are much greater than in suburbs, they emit more man-made heat and greenhouse gases (such as carbon dioxide, etc.) into the air than in suburbs, which in turn contributes to the To form a heat island, the wind speed is generally lower at night than during the day, and the heat exchange between urban and suburban areas is weak. Urban street valleys store more heat during the day and dissipate heat slowly at night. The temperature drops slower than in the suburbs, so the urban heat island effect is more significant at this time. .
Dry Island, Wet Island:
The impact of cities on atmospheric humidity is relatively complex. Taking Shanghai as an example, the average water vapor pressure of 11 stations in the city from 1984 to 1990 was compared with the average water vapor pressure of 4 stations in the surrounding suburbs during the same period. Both urban areas were lower than those in the suburbs, showing the "urban dry island" effect. The average intensity of dry islands (meaning that the average water vapor pressure in urban areas is lower than the average water vapor pressure in suburban areas) is the largest in July. (0.56 hPa), with the smallest difference (absolute value) in January (only 0.02 hPa). However, the difference in water vapor pressure between urban and suburban areas has obvious diurnal changes. If the average value is calculated according to the four observation times of the day (02, 08, 14, and 20 o'clock), it is found that from April to October of the year, the average water vapor pressure in the urban area at 02 o'clock at night is greater than that in the suburbs at the same time Average water vapor pressure, obvious "urban wet island" appears. The average intensity of wet islands is maximum in August. Taking 1984 as an example, at 02:00 in August of that year, the water vapor pressure in the urban area was 0.6 hPa higher than that in the suburbs, with the maximum difference reaching 2.0 hPa. However, at 08:00 and 14:00 during the day, the opposite was true. The urban area was significantly lower than the suburban area. In particular, the dry island intensity is the largest at 14:00 in the afternoon. The average dry island intensity at 14:00 in July and August of that year was 1.7 and 1.6 hPa respectively. The maximum intensity of the dry island was 3.0 hPa. At 20:00, the water vapor pressure in urban areas and suburbs was different. Not big, weak dry islands often appear. This phenomenon of alternating day and night between urban dry islands and urban wet islands often occurs in many European and American cities during the warm season.
The formation of the above phenomenon is closely related to both underlying factors and weather conditions. Under the sunlight during the day, the amount of water vapor on the underlying surface entering the low-level air through evapotranspiration (including evaporation and plant transpiration) is smaller in the urban area than in the suburbs. Especially in the midsummer season, crops grow densely in the suburbs, and the natural environment between the city and the suburbs The difference in evapotranspiration is even greater. Due to the roughness of the underlying surface (dense buildings and uneven heights) and the heat island effect, urban areas have stronger mechanical and thermal turbulence than suburban areas. Through the vertical exchange of turbulent flow, the amount of water vapor transported from low levels to the upper air in urban areas is greater than that in suburban areas. Both of these results in the water vapor pressure near the ground in urban areas being lower than that in suburban areas, forming an "urban dry island". At night, the wind speed decreases, the air stratification is stable, the temperature in the suburbs drops rapidly, the saturated vapor pressure decreases, a large amount of water vapor condenses into dew on the surface, and the amount of water vapor remaining in the low-level air is small, the water vapor pressure decreases rapidly, and urban areas have heat islands. The effect is that the amount of condensation is much less than in the suburbs, the turbulence at night is weak, and the amount of water vapor exchange with the upper air is small. The water vapor pressure near the ground in the urban area is higher than that in the suburbs, resulting in the appearance of "urban wet islands". This kind of urban wet island formed due to the different amounts of condensation in the suburbs is called condensation wet island. Most of them form within 1 to 4 hours after sunset. After sunrise, due to the increase in temperature in the suburbs and the evaporation of dew, they quickly transform into "urban dry islands". When urban dry islands and wet islands appear, they must be accompanied by urban heat islands. .
Through comparative analysis of the urban and suburban areas of Shanghai in 1984, the author also discovered the formation of urban wet islands in Shanghai. In addition to the above-mentioned condensation wet islands, there are also frost wet islands and foggy wet islands. Islands, wet islands in rainy days and wet islands in snowy days can only appear when the wind is light and accompanied by urban heat islands.
Turbid Island:
The solar radiation projected to the surface can be divided into two parts: one part is direct sunlight in the form of parallel rays, called direct solar radiation; the other Part of it is the light scattered in all directions due to the scattering effect of air molecules, suspended particles and cloud particles when solar radiation passes through the earth's atmosphere. It is called scattered radiation D. Under the same intensity of solar radiation, there are many scattered particles in turbid air, and the scattered radiation is stronger than that of dry air, while the direct radiation is greatly weakened. Meteorologists use D/S to represent the turbidity of the atmosphere (also known as the turbidity factor). There are more smoke and dust pollutants emitted by industrial production, transportation, and residential stoves in cities than in suburbs. Most of these pollutants are condensation nuclei that are good at absorbing water. Vertical turbulence is relatively strong in cities, which is conducive to the development of low clouds. A large amount of observational data proves that urban areas have more low cloud cover than nearby suburbs, which makes the scattered radiation in cities stronger than in suburbs, the direct radiation weaker than in suburbs, and the turbidity of the atmosphere is significantly greater than in suburbs. Taking Shanghai as an example, according to the statistics of radiation data in the past 27 years, the average turbidity D/S of the Shanghai platform is 1.17. The average turbidity D/S of ten suburban stations during the same period is 15.8% greater. On the turbidity distribution map of Shanghai, the urban area presents an obvious turbidity island, a similar phenomenon seen in many foreign cities.
Yudao:
There is a lot of controversy in the world about the impact of cities on precipitation. The United States has set up a dense rainfall observation network in the city of St. Louis, Missouri, and its surrounding suburbs in its central plains. It has used advanced technology to conduct observation studies for five years, confirming that the city and its downwind direction do have a "rain island effect." We have used data from more than 170 rainfall observation stations in Shanghai, combined with weather conditions, to conduct numerous case analyzes and classified statistics. We found that the impact of Shanghai city on precipitation is more obvious with heavy rains in the flood season (May to September). In Shanghai in the past 30 years, On the precipitation distribution map of the annual flood season, the precipitation in urban areas is significantly higher than that in the suburbs, showing clear urban rain islands. This phenomenon does not exist in the non-flood season (October to April of the following year) and annual average rainfall maps.
The conditions for the formation of urban rain islands are when the atmospheric circulation is weak and is conducive to large-scale weather conditions that produce precipitation in urban areas. Due to the convergence and rise of local airflow generated by urban heat islands, it is conducive to convection. The development of rain; the roughness of the underlying surface is large, which has a blocking effect on the sluggish-moving rainfall system, making it move slower and prolonging the rainfall time in urban areas; in addition, there are many condensation nuclei in the urban air, and its chemical composition Different particle sizes vary. When there are more large nuclei (such as nitrate particles), it will promote warm cloud precipitation. The influence of the above factors will "induced" the maximum intensity of heavy rain to fall in urban areas and downwind direction, forming an urban rain island.
To sum up, it can be seen that the "five islands" effect in urban climate is the unconscious impact of human beings on local climate during the process of urbanization. Studying the laws is not only helpful for urban weather and climate forecasting; it can also consciously improve urban climate conditions through certain man-made measures such as strengthening urban greening, adjusting energy structure, rationally planning urban construction, controlling urban air pollution, etc. To develop in a direction that is beneficial to residents’ lives and production.