Using ENVI-met To Simulate The Impact Of Green Spaces On The Thermal Environment In The Summer

The transformation of land surface from original natural land to cement,asphalt as well as other impervious land caused by the rapid urbanization,and the intensive human activities have made the changes of regional climate and weather patterns.One of the phenomena is the increasing of urban heat island(UHI).UHI can result in increased energy consumption,serve as a trap for atmospheric pollutants,contribute to increased urban smog formation,reduce thermal comfort and the quality of life,and generate socioeconomic impacts on communities.The role of urban green space in moderating urban climates is being studied extensively,with researchers consistently suggesting that an effective way to reduce or alleviate the effects of UHIs is to increase tree cover area and density.Taken Nanjing university Gulou campus as a case study,in this paper,Using the ENVI-met model and supported by the investigated data as well as remote sensing images,we analyzes the impact of urban green spaces on the outdoor summer thermal environment.The results will help city managers and planners to plan and design of green space with regard to the mitigation of increasing temperatures at the block scale as well as at the city or region scale.This paper is divided into six chapters.The first chapter introduced the research background and significance and review related research.The second chapter introduced the study area.The third chapter described the data and key technology and method used in this paper.The fourth chapter captured the mico-climate characteristics based on the investigated data by using HOBO meteorological stations.The fifth chapter simulated the outdoor thermal environment at the present and at the scenario which all of the green spaces was replaced by the impervious land using the ENVI-met model.The sixth chapter systematically summarized the main conclusions innovations,and put forward the problems of the paper and the possible improvement as well as research direction in the future.The main conclusions are as follows:1)The statistics analysis of micro-meteorological factors showed that:the highest temperature in the study area was at 15:00.Since the study area is located at the campus basin,and almost of the center in Nanjing,and the urban canopy may weak the wind speed,and the thermal energy may hardly spread around,which will result in the mean temperature is about 1.5? higher than that at the suburban.The observed air temperature is different among different land surface types.The temperature of cement road was 0.9? higher than the turf,with the maximum temperature difference being 1.8?,and 2.4? higher than Bainian Jing woodland and avenue where have good vegetation shade.The relative humidity of each station throughout the day showed a single valley curve,reaching the lowest value at around 15:00-17:00.Relative humidity of understory(Avenue and Bainian Jing woodland)were close,with 5%higher than the cement road,which showed that understory trees had a significant humidification effect.Wind speed at different underlying surfaces also showed significant differences.The wind speed was larger in open area than the area with vegetation cover,indicating that vegetation had the effect of slowing the wind speed.2)This paper,based on ENVI-met model,the thermal and humidity characteristics at present situation and the design scenarios(removal of green spaces)was compared on the selected 9:00 and 12:00,and the results indicate that:a)Horizontal air temperature field simulation results show that in the study area,at a height of 1.5m,there were three main air flow,which were from the school gate,Mengmin Wei Building and the library layer Gap,Pingcan Xiang.At 9:00,the current distribution of wind and temperature field at 1.5m showed a high consistency with the spatial distribution of the underlying surface.The surface temperature were quite different among woodland,grassland and impervious cover.After the removal of green space,the near-surface wind field has not changed generally except an increasement of the wind speed.But the temperature field had changed significantly.There was a hot airflow throughout the campus in the west of avenue and east of playground,because of lacking barrier effect of woodland.Vertical wind and temperature field,at 9:00,there were atmospheric vertical stratification in both cases.Parallel to the wind direction,due to the impact of the main wind direction,wind vector formed laminar flow.Perpendicular to the main wind direction,due to the thermodynamic properties differences among underlying surfaces,wind vector formed a swirl.By comparing two scenarios at two time,the results showed that after removing the green spaces,the temperature near ground increased,and the turbulence perpendicular to main wind direction was strengthened.b)At 9:00,humidity field was affected by the underlying surface and the wind field.The playground maintained a high moisture content because of the air flow from the understory,although it is impervious surface.By 12:00,the near ground evaporation was enhanced,and the distribution of moisture showed a consistency with the spatial distribution of the land cover types.Areas with shade trees in the area had a higher humidity due to the evaporation,approximately 2g/kg higher than playground.After removal of green space,the entire study area has become a "dry island",and the moisture content between two time were almost the same,which showed that the green space humidification effect was obvious.