On The Sensible And Latent Heat Fluxes From Complex Urban Surfaces

Urban surfaces contribute a mere3%coverage of Earth’s land, whereas more than50%of the world’s population live in cities. With worldwide continuously increasingurbanization, it is projected that more and more people will be affected by the urbanenvironment. Emerging urban environmental issues have also been manifested by thescenario of climate change. Sensible and latent heat fluxes from urban surfaces hold thekey of urban environmental studies, with difficulty inherited from the complexity ofurban surfaces. In this dissertation, we first carried out analysis on the controllingfactors of general surface energy budgets, followed by work on observations andsimulations of urban sensible and latent heat fluxes to illustrate their impact on the localmicro-climate and the physical mechanism.The hysteresis effect in the evolution of net radiation and ground surface heat flux,a prominent phenomenon in surface energy budgets, was revisited by analyzing thephysical mechanism. Parameterization schemes of net radiation and ground surface heatflux were proposed to explain the hysteresis effect. The phase lag between these twobudgets was found to be determined by the ambient radiative forcing and soil waterflux.The work on the observational characteristics of urban sensible and latent fluxeswas carried out by a comparative analysis on the data collected at two nearby flux sitesin Beijing area, of which the Beijing site is set up in the urban context while theXianghe site is located in rural settings. The analysis results show that (1) for theincoming radiative forcing, the Xianghe site receives more solar yet less longwaveradiation than the Beijing site; and (2) for the turbulent dissipative fluxes, the Beijingsite dissipates more sensible yet less latent heat fluxes than the Xianghe site in summerand autumn, whereas the difference of the partition of turbulent fluxes between the twosites is insignificant in spring and winter.The work on the modeling of urban sensible and latent fluxes was conducted byimproving the land-surface process model PUCM in its hydrological processes and thegreen roof module. The determinants of urban sensible and latent fluxes have beeninvestigated via simulations in two aspects: one the intrinsic properties of urban surfaces, the other the extrinsic forcing factors. The simulation results indicate that (1) among theintrinsic properties, the roof albedo has the most significant impact on the sensible heat,whereas the green roof fraction dominates the latent heat; and (2) among the extrinsicfactors, the incoming solar radiation and soil water content are the determinants of theturbulent fluxes.The improved PUCM with specific Beijing settings was then coupled with theWeather Research and Forecast model to examine the effect of urban sensible and latentheat fluxes on the local micro-climate and to reveal the inherent mechanism throughsimulations in a heat wave period under scenarios of varying green roof fractions. It isfound by the simulations that (1) altering the water content conditions via green roofs isproven to be effective in modifying the local micro-climate in terms of near surface airtemperature, air humidity and wind speed; and (2) the changed redistribution ofturbulent fluxes not only affects the surface energy budgets vertically, but has impact onthe regional horizontal circulation patterns, which thus changes the local micro-climate.