| In order to understand the mechanism of climate change in Changsha Zhuzhou Xiangtan Metropolitan and provide scientific theory basis for optimizing the planning of urban green space system,eight experimental schemes are developed,which used satellite land use remote sensing data from USGS and MODIS in1993,2001,2009 and the land use planning of 2020,and NCEPI°x1°FNL lattice data of typical summer weather in July 18,2009 and typical winter weather in January 13 to 14,2009.The WRF platform,combined dynamic frame with the Urban Canopy Model,is used to simulate the situation.The main elements,including surface energy balance,wind field,thermal field,humidity field and boundary layer height,are analyzed.The result shows that:Lake Dongting and the eastern mountain forest play a controlling role in the climate of the metropolitan when the average wind speed is less than 4m.s-1.In the second half of the night in summer,wind flows from the south higher cold area to Lake Dongting area.Cold air concentrates on the east of the Changsha,Zhuzhou and Xiangtan cityand flows through Zhuzhou city,then flows to Lake Dongting area.In the morning,there are three air sunken center in northeast and southeast area where cold air sinks.Arising center of warm air is formed in center between Changsha,Zhuzhou,Xiangtan Cities,where the air rise.Cold wind from the higher elevation mountains flows to theCZT Cities and dissipate in the northeast.In the afternoon,wind flows to the warmer land in southwest area to Lake Dongting area.On the middle of the night,South wind turns to west and east wind from mountains descend and flow to Changsha,Zhuzhou,Xiangtan Cities,then form southeast wind in west area.In the afternoon of winter,wind flow from Lake Dongting to the south.In the middle of the night,south wind begin enhance and flows to the core area ofCZT cities.The north wind begin to flow to the west deflection.In the past midnight,the east wind flows toCZT cities.Part of the wind flows to the northwest of Lake Dongting,and the other flows to the southwest area.In the morning,a strong air rising center is formed in the north of the Changsha and a strong air descending center is formed in the Liling City.The west wind in the northwest part is concentrate on the core area.Then,a part of the wind is deflected toward the northwest area,finally the wind flows to the Lake Dongting and other warm areas.The urban heat island circulation inCZTCities has a significant impact on the climate of urban metropolitan.In summer morning,with the surface temperature rising,the center of Chang-Zhuzhou-Xiangtan Cities forms a warm air rising center,the air rises,and the cold wind of the higher mountainous area in the east and west flow toCZT Cities,then spread to the northeast and form CZT urban heat island circulation,guiding the cold air to the central area and reducing the heat island strength.In the morning of winter,with the surface temperature rising in the core of metropolitan,two strong air rising centers are formed on the west side of Changsha and west of Xiangtan.In the north of Changsha,there is also a strong rising center of air flow.In the vicinity of Liling,there is a strong sinking center,the west wind of the northwestern part and east wind of the eastern part to the core area of CZT cities flow through this area.One part of it turn to southwest deflection,the other part turn to the south north deflection,and then flow to the Lake Dongting and other warm areas.Therefore,Urban heat island circulation is formed,which plays a major regulatory role in urban climate.The land use pattern has a significant impact on the climate of the metropolitan.Froml993 to 2020,the average daily upward heat flux in summer has surged an aggregate of 5.45w.m-2 and an average annual increase of 0.21w.m-2.The daily upward heat flux is 44.85w.m-2,46.01w.m-2 in2009 and 2020 with the daily average upward heat flux increasing by 1.15w.m-2.In winter,the average daily upward heat flux is 38.17 w.m-2,38.35 w.m-2 in the year of 2009 and 2020,increasing 0.17 w.m-2 with an average annual increase of 0.02 w.m-2.The cumulative growth of average daily surface latent flux in summer is 7.05 w.m-2,and an annual increase is 0.26 w.m 2.The cumulative growth of average daily surface latent heat flux in the year of 2009 and 2020 is 13.50 w.m-2,16.64 w.m-2 and an annual increase is 0.15 w.m-2.With the expansion of the city,the upstream circulation of the heat island circulation is shifted southward and the descending center is moved northward.The average wind speed decreases in winter and summer.and the decline decreases with the expansion of the urban city.The overall still show a decline trend.The average daily wind speed in the year of 2009 was 1.74m.s-1,1.7m.s-1 and decreased-0.04m.s-1.The average daily wind speed decreased by-0.15m.s-1 in winter and the average annual descent was-0.006m.s-1.The average daily wind speed is decreasing by-0.05m.s-1 in 2020 comparing with that in 2009.The daily average temperature is on the rise,but the rise is weaken with the urban metropolitan expanding.The average daily surface temperature increased slightly,increasing by 0.37k,.The average annual growth is 0.014m.s-1.The average daily temperature was 305.64k in 2009 and 2020,decline-0.01k.The diurnal mean surface temperature cumulative increased by 0.03k in winter and annual average growth has increased by 0.001k.In the year of2009 and 2020,each of the daily average temperature is 275.50 k,and 275.52 k,which has increased by 0.02 k.The daily average specific humidity decreased in summer and the daily average specific humidity increased,but,with the metropolitan area expanding,the extent of increasing and decreasing diminished.The daily average specific humidity decreased by 0.00105 and average annual decline by 000039,in the year of 2009 and 2020,each of the daily average specific humidity respectively is 0.01775 and 0.01771,which decreased by-0.00004.In winter,the daily average specific humidity increased by 1.52E-05 and the average annual growth rate of 5.63E-07,in the year of 2009 and 2020,each of the daily average specific humidity is 0.00099 and 0.00099,which increased by 0.84E-05.The average height of the boundary layer is decreased in summer and increased in winter,however,with the metropolitan area increasing,the extent of increasing and decreasing has diminished.The daily average height has increased by 29.05 m and the average annual also increased by 1.08 m.In the year of 2009 and 2020,each of the daily average height is 764.43 m and 764.48 m,which has increased by 0.05 m.The daily average layer height in winter has increased by 3.33m and the growth of average annual is 0.12m.In contrast to 2009,the daily average maximum height in winter decreased by-1.89 m in 2020.With the expansion of the city,urban morphology has a significant impact on urban climate.The heat flux of urban surface is basically at a high level in summer,which is obviously higher than surrounding area.The range is basically the same as that of urban expansion boundary.Most of the time is negative state in winter,and the difference with surrounding area is not obvious.In the summer,as the expansion of the city,the surface upward heat flux rises continuously and most of the time is positive state.the surface heat flux is 260w.m-2,270 w.m-2in the year of 2009 and 2020,the surface upward heat flux of the three urban areas is about 10-140w.m2 higher than Periphery.In the year of 2020,the surface upwards heat flux in inner city has a difference about 10-40 wm-2.The surface latent heat range of the CZT cities is in a relatively low state,and is obviously lower than the surrounding areas in summer,but it is not obvious in winter.In summer,the difference of latent heat is about 4 w.m-2 in 2009,and the difference of latent heat is about 5-10 w.m-2 in 2020.In winter,it is about 10-40 w.m-2 in 2009 and about 20 w.m-2 in 2020.Night wind speed in urban area is improving and daytime wind speed is slowing,which the difference is obvious from the surrounding area.Daytime wind speed in the urban area is lower than the surrounding area,but the difference is not obvious.In the year of 2009 and 2020,the night wind speed of summer in the three cities of Changsha,Zhuzhou and Xiangtan is respectively about 3.0 ms-1,2.0 m.s-1,which is higher than surrounding area;The wind speed of daytime is about 1.0-1.4 m.s-1 and 0.2-0.8 m.s-1,Lower than the surrounding area.In winter,the day wind speed in 2009 and 2020 years is about 1.2-2.0 m.s-1 and night at 0.5m.s-1,which is lower than the surrounding area.The heat island is formed at night in three cities of the Changsha,Zhuzhou and Xiangtan and the area improve along with the expansion of the city,heat island boundary keep pace with expansion boundary of the city,the intensity of heat island up to 2-4 ? in summer,it up to 1-2? in winter.The specific humidity in winter and summer is lower than the surrounding areas,but at night,the difference with the surrounding area is not obvious.In summer,the specific humidity of three cities in 2009,2020 is between 0.015-0.018,0.014-0.017,significantly lower than surrounding area and formation of dry island.In winter,the specific humidity of urban is slightly lower than surrounding area during the day,but at night,the difference with the surrounding area is not obvious.The summer in 2009 and 2020,the boundary layer in urban area increased significantly in afternoon,the layer height is higher than surrounding area more than 300m at 20 o' clock.River Xiangjiang and the green heart area have a significant role in urban climate regulation.The green heart area is an important channel that east wind converge to theCZT cities.The surface upward heat flux in green heart area and latent heat flux increase significantly,wind speed decreases,specific humidity increases,temperature decreases and boundary layer height increases,which have an important role in regulating climate.River Xiangjiang plays an important role in regulating temperature and increasing the specific humidity.Inthe summer daytime,the temperature of the River Xiangjiang is lower than urban area about 2-4k,therefore,River Xiangjiang has an effect of cooling down the temperature during the day in summer and rising the temperature during the day in winter,regulating the surrounding urban temperature.In addition,River Xiangjiang have a significant humidification effect on surrounding area and downwind area,which improving specific humidity about 0.001-0.004.The land use planning of the CZT Metropolitan cann't solve the urban heat island problem.The Daily mean temperture trends to increase gradually from 2009 to 2020 year,and the intensity of heat island is up to 2-4 ?.Therefore,it is very necessary that green space system and Urban ventilation channel are need to study forward.Although the green heart area has fouction of climate regulation to a certain extent,but it also need some sensitive studies to understand the mechanism of climate change influenced by differrent land use. |
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