Intercomparison between the Integrated Urban land Model and the 1 Noah Urban Canopy Model

Abstract. Urban land surface model (ULSM) is an important tool to study the climatic effect of human activity. Now there are two main methods to parameterize the effects of human activity, the coupling method and the integrating method. For the coupled method, the urban canopy model (UCM) was developed and coupled with the land surface model for the natural land surfaces. For the integrated method, the urban land surface model was built directly based on the traditional land surface model. In this paper, the Noah Single Layer Urban Canopy Model (Noah/SLUCM) and the Integrated Urban land Model (IUM) were compared using the observed fluxes data at the 325-meter meteorology tower in Beijing. Through the comparison, the key factors and physical processes of the urban land surface model which have significant impact on the performance of ULSM were found out. The results indicate that the absorbed solar radiation of urban surface was reduced by the solar radiation scattering, the absorption of building roof and wall, and the shading effect of urban canopy and tall buildings. Urban surface roughness length and friction velocity are important in urban sensible heat flux simulation. Urban water balance and impervious surface evaporation (ISE) are important in urban latent heat flux simulation.



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China now is experiencing an unprecedented urbanization movement. The  Henderson- Sellers et al. 1993Sellers et al. , 1995 was launched to compare land surface models. the parameterization of the urban energy balance processes, recently developed 71 urban land surface models could not parameterize the urban water balance 72 (Grimmond et al. 1986;Mitchell et al. 2001;Wang et al. 2013;Miao and Chen 2014;  However, these intercomparison studies only concern the impact of the bulk 77 model and UCM with different complexity on surface radiation and energy balances. 78 In this paper, different from the models used for intercomparison in Best and 79 Grimmond (2015)  observation. The radiation balance model could be described as follows: Where n R is the net radiation (W m -2 );  S is the downward solar radiation; is the upward solar 99 radiation (W m -2 );  L is the upward longwave radiation (W m -2 ).

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The energy balance model could be described as follows: Where H is the sensible heat flux (W m -2 ); L is the latent heat of evaporation 103 for water (W m -2 ); E is the evapotranspiration (W m -2 ); LE is the latent heat flux 104 (W m -2 ); G is the ground heat flux (W m -2 ); A is the AHR (W m -2 ), which used the 105 same diurnal cycle data for the two models.

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As this paper focuses on the fluxes in urban areas, only the parameterization 107 schemes of the fluxes in urban areas is given blow. The detailed parameterization 108 scheme of the models could be seen in relevant papers (Chen and Dudhia, 2001 The upward shortwave radiation is parameterized as follows: Where  S is the upward shortwave radiation (W m -2 );  S is the downward 126 shortwave radiation (W m -2 ); R is the normalized roof width; GT H is the 127 normalized building height; R S , B S and G S are the absorbed solar radiation by 128 roof, wall and road respectively (W m -2 ).

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The upward longwave radiation is parameterized as follows: Where  L is the upward longwave radiation (W m -2 );  L is the downward 132 longwave radiation (W m -2 ); R R , B R and G R are the absorbed longwave radiation 133 by roof, wall and road respectively (W m -2 ).

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The sensible heat flux is parameterized as follows:

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Where imp E is the ISE (mm s -1 ); R E , B E and G E are the evaporation from roof, 141 wall and road respectively (mm s -1 ), they are calculated as follows:

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The upward shortwave radiation is parameterized as follows (Dai et al. 2003): Where  is the albedo, which is defined as follows (Dai et al., 2003): Where dif vis,  and dif nir ,  are the albedo for visible and near infrared diffuse 172 solar radiation respectively, which are set as the same as the albedo of the saturated 173 soil with darkest color in CoLM, they are 0.05 and 0.1 respectively.

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The upward longwave radiation is parameterized as follows: Where  is the emissivity;  is the Stefan-Boltzmann constant (W m -2 K -4 ); 177 cov F is the fractional vegetation cover; g T is the ground surface temperature (K); 178 l T is the leaf temperature (K).

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The sensible heat flux is parameterized as follows: Where a  is the air density (Kg m -3 ); p c is the specific heat of dry air (J Kg -1 182 K -1 ); g  is the surface potential temperature (K); a  is the air potential temperature at reference height (K); ah r is the aerodynamic resistance for sensible 184 heat flux between the atmosphere at reference height and the surface (m s -1 ), which 185 could be calculated as follow: 186 Where v is the von Karman constant; * u is the friction velocity (m s -1 ); h f is 188 the integral of profile function for heat, which is associated with the thermodynamic 189 roughness.

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The ISE in the IUM it is parameterized as follows: Where p E is the potential evaporation (m s -1 ), which will be discussed in the next part of the paper. p E can be parameterized as 196 follows: Where a  is the air density (kg m -3 ); w  is the water density (kg m -3 ), which is 199 approximately equal to 1000; d r is the aerodynamic resistance for evaporation (s compute the road water temperature. The simplified shallow lake model can be 205 described as follows: Where w T is the road water temperature (K), w k is the thermal conductivity of 208 water (W m -1 K -1 ), e k is the eddy diffusion coefficient (m 2 s -1 ), w c is the heat 209 capacity of water (J m -3 K -1 ), and  is the solar radiation heat source term (W m -2 ).

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The ground heat flux is considered as the remainder of the energy balance Where W is the water depth on impervious surface (mm); roof I is the roof rainfall