Fundamental Studies On The Influences Of Rainfall On Urban Microclimate In Hot-humid Region

Urban climate and hydrological changes are the two key factors that affect the safety of urban ecosystems and the quality of human settlements,which are particularly significant in the hot-humid region.The two influence each other,but their rules of action are not clear.Based on the energy and balance principle and the development of the new rainwater temperature tester,this study discussed the effect of energy allocation and hydrological changes on the typical city underlying surface through continuous systematic observation in hot-humid region,and revealed the influences of rainfall on the urban microclimate.The main conclusions are as follows:(1)Rainwater temperature prediction and typical urban surface energy characteristics during rainfallThe results show that the new tester can accurately measure the near-surface rainwater temperature in various rainfall events.The air temperature and wet bulb temperature near the surface both decreased significantly before and after the start of rain,and quickly entered a stable stage.During the stable period,the near-surface rainwater temperature was consistently3.0?lower than the air wet-bulb temperature at a height of 1.5 m above the ground,based on which the rainwater temperature can be predicted by using the air parameters.In the rainfall events in autumn and winter,the impervious urban surface constantly released the heat storage of the layer beneath,and for the proportions of heat dissipation items,latent heat(34%-65%)was largest,followed by convection heat transfer(11%-34%),and net radiation(3%-21%)was smallest.The proportion of rain sensible heat(5%-19%)closed to that of net radiation when the rainfall intensity was small,while significantly increased up to 46%for large rainfall intensity.(2)Energy and rainfall redistribution characteristics of green soilDetermined the steady-state infiltration rate of greening soil.The actual moistening front'depth was 2.0 times the calculated result,and the Green-Ampt model performed better to predict the infiltration rate and the cumulative infiltration amount as was corrected accordingly.The depth of rain migration increased with the increase of rainfall,and was affected by the volumetric moisture content before rain.In the initial period after rain,the rainwater was mainly stored in the 0?400mm soil layer,and the main storage depth moved down with the date.The upward migration of deep soil moisture not only supplemented evaporation consumption,but also increased soil water storage.For the soil ground,most of the time,convective heat transfer and latent heat were positive,which was expressed as heat dissipation through transporting heat and water vapor to the air.Heat storage was negative,indicating that the heat storage of the layer beneath was continuously released through temperature changes(heat storage 1)and moisture diffusion(heat storage 2),the heat storage 2 released more heat.For the proportions of heat dissipation items,the latent heat(56%?72%)was the largest,followed by convection heat transfer(19%?28%),and the heat storage 1(10%?16%)was the smallest,the average of rain sensible heat was 17%in light rain days and 40%in moderate to heavy rain days.In most rainfall events,net radiation was negative,convective heat transfer,latent heat and rainwater sensible heat were positive,which was heat dissipation to the ground.Both heat storage items were negative and heat gained to the ground,indicating that the heat storage of the layer beneath was continuously released through changes in temperature and moisture during rainfall.(3)Rainfall redistribution characteristics of vegetation canopyThe mean ratio of throughfall,stemflow and interception in medium and small rainfall events were as follows:isolated plant trees were 37.5%,2.77%and 61.88%,street trees were21.05%,5.43%and 77.72%,throughfall and interception ratio of shrub were 28.59%and71.41%.The water storage capacity of isolated tree,street tree and shrub were 1.4mm,1.5mm and 2.0mm respectively.Throughfall,stemflow,interception,ratio of throughfall and stemflow were positively correlated with rainfall,and the intercept ratio was negatively correlated with it.(4)Energy allocation and model optimization for urban tree canopyTranspiration rate was similar to the trend of solar radiation and air water vapor pressure deficit,and had the strongest correlation with them.During the day when the solar radiation was strong,the net radiation proportions of convective heat transfer and latent heat crossly changed,the former first high and then low,the latter first low and then high,and the ratio of heat storage was close to zero.Transpiration consumed 40%(sunny days)to 60%(cloudy days)of the daily total energy,and worked continuously on cloudy days resulting in net absorbing heat from the surrounding air.During rainfall events,rain sensible heat and latent heat were positive,for the canopy to dissipate heat,indicating that rainwater continued to cool the canopy through direct heat exchange(rainwater sensible heat)and indirect effects(evaporation and transpiration latent heat),increasing the absolute value of the temperature difference between the leaf and air,thereby enhancing the cooling effect of convective heat transfer.The sensible heat model performed better(RMSE=74.54W/m~2,R~2=0.86,d=0.96)when using 0.11m/s as the critical air speed for free and forced convections.The optimized Noilhan-Planton model combined with the Penman-Monteith model could accurately predict the hourly transpiration latent heat of urban trees in rainless and light rain days(rainfall<3.2mm)(RMSE=22.08W/m~2,R~2=0.80,d=0.94).This study provides equipment,theoretical basis and fundamental data for urban microclimate,building thermal and moisture performance and urban hydrology research,and provide references for understanding and controlling the energy change characteristics of typical underlying surfaces under the influence of rainfall and their moderating effects on urban climate.