Study On Characteristics Of Urban Wind Field And Its Applications On Urban Planning In Hangzhou

The process of rapidly developing urbanization has changed the surface roughness and spatial structures of urban areas in recent decades,and accordingly,induced the accelerating expansion of city and the changing structure of wind field and temperature field.Which are showing poor ventilation,obvious heat island effect and the worsening atmospheric pollution,etc.Atmospheric circulations at different scales affect the structure of wind and temperature field near the surface of city.On one hand,the large-scale circulation influenced by the monsoon circulation formed the dominant wind system near the surface of Hangzhou.On the other hand,under the background of weak circulation,it shows the multiple local circulations coupling in the complex terrain and heterogeneous underlying surface,resulting in a unique wind and temperature field structure.In addition,urban morphological parameters also affect the urban microclimate effect,such as windward surface coefficient,building structure and green space.This paper mainly studies from the following three aspects:(1)Based on the objective classification of atmospheric circulation patterns affecting Hangzhou,a typical meteorological year based on the circulation pattern was constructed,and the characteristics of urban boundary layer under the influence of seasonal dominant circulation patterns were analyzed.The urban heat island morphology and surface wind field structure in winter and summer were studied under the static and steady weather conditions.Then,the influence of frontal system on the transport and diffusion characteristics of urban air pollution was studied.(2)A set of recognition methods for mountain wind system for complex terrain conditions were constructed,based on meteorological observation data from 81 AWSs in Hangzhou.The coupling effect between mountain wind system and heat island circulation was quantitatively revealed.(3)Several data,such as climate data,the data of urban form and architectural spatial structure,and simulated data from the mesoscale numerical model,were used to construct the technical framework for the application of climate information in the urban planning of Hangzhou.Then the multi-level ventilation corridor system of Hangzhou is proposed.The main conclusions are as follows:1.The urban heat island(UHI)effect is obvious in Hangzhou.The SHI(strong heat island)events are accounting for 20.7% and the WHI(weak heat island)events have risen to 47%.In winter,the heat island effect is more obvious than that in summer,and atmospheric circulation pattern A and its related patterns(such as ANE,AE,etc.)are conducive to the emergence of strong heat island.In summer,the east and west sides of city are controlled by easterly and westerly wind respectively.In winter,the dominant wind in the urban area is mainly northerly wind.The average wind speed in the riverside plain east of the urban area is slightly higher,and the core urban area presents the characteristics of "weak-wind area".Under the static and stable circulation pattern,the urban "weak-wind area" expands obviously.The dominant wind direction in the urban area and its surrounding areas shows diurnal reversal,and the closer to the western mountains,the diurnal reversal becomes more obvious.In Hangzhou,high pressure control A(45.5%)and high pressure front AN(18.1%)are the main weather types,and the occurrence frequency of weather type of warm sector development(SE)is high to 50%.When a weak cold front or a warm sector develops in autumn and winter,it is easy to induce moderate and severe pollution.While there are few cases of high pollution in summer.Frontal system is an important weather type affecting air pollution in Hangzhou.The weak cold front with 24 h negative temperature variation between 0℃ and 2℃ or average wind speed less than 2m/s mainly caused the inflow of exogenous pollutants and the accumulation of pollutants in Hangzhou.The strong cold front with negative temperature change amplitude greater than 4℃and average wind speed greater than 4m/s was mainly characterized by pollutant removal.However,the warm front mainly shows the accumulation of pollutants,and the removal characteristics are not obvious.2.According to the formation and development mechanism of mountain plain wind,5identification methods of mountain plain wind based on ground observation were concluded,and a total of 309 cases of mountain plain wind affecting Hangzhou were screened from 2013 to 2015,with an annual frequency of 28%,mainly occurring in summer and autumn.In addition to the high pressure controlled weather type,the weather types of high pressure and the weak westerly are more likely to induce the mountain plain wind in Hangzhou area.Valley winds and urban breezes induced by thermal circulation have been the subject of numerous studies;however,their interactions deserve further observational study under different geographical conditions globally.We propose quantitative criteria for analysis of observations,including identification of valley and mountain–plain wind episodes,determination of the direction and duration of valley winds,and exclusion of suspect cases induced by strong synoptic systems.We selected a subset of 345 valley wind days that occurred during 2013–2015 in Hangzhou,China.Valley winds in the study area occur mainly in autumn and early winter(annual frequency: 28%).The wind systems dominated by mountain plain wind extend gradually father into eastern city by approximately 50 km.To the west(east)of the urban center of Hangzhou are small mountains(is a plain).At night,the directions of mountain–plain circulation and urban thermal circulation are consistent in the region from the mountains to western parts of the city,resulting in a strengthened mountain wind at the surface.Their interaction also delays the reversal of the mountain wind to the plain wind in the morning,and slightly advances the reversal of the plain wind to the mountain wind in the evening.Conversely,in the plain area to the east of Hangzhou,the interaction delays the time of reversal of the plain wind to the mountain wind in the evening.3.Due to the influence of seasonal characteristics of atmospheric circulation,local circulation and urban form,there are obvious spatial differences in the characteristics of nearsurface dominant wind in Hangzhou,which can be divided into 15 regional dominant wind spaces.Hangzhou’s urban ventilation corridor system consists of 6 primary ventilation corridors,11 secondary primary ventilation corridors and 12 urban green corridors.The primary ventilation corridors are the main ventilation space in Hangzhou,which are designated according to the northwest wind in winter,southwest and east wind field in summer and wind resource area.The secondary ventilation corridors are mainly distributed in the edge of the city,as a ventilation space extending into the city interior as the primary wind corridor,and permeate to the edge of the city through the mountain wind system with the western mountain and hill as the clean wind source.Due to the coupling effect of mountain plain wind and urban wind,mountain wind and urban wind overlapped with each other at night,increasing the wind speed on the western edge of the city.Urban green corridors are mainly located in urban areas and they are constructed based on the indicators from ventilation potential..