New aerodynamic parameter prerequisites for UCM inclusion, and other improvements

By Varquez ACG, Nakayoshi M, Kanda M

                 I.            Revision of the estimation of bulk transfer coefficients

The default urban canopy model relies on two z_o values, for the roof and the canyon. Both are calculated from fixed building morphology using the method of Macdonald et al. (1998). Because the urban parameters were fixed depending on the default building classification, two fixed values were assigned for a homogeneous classification. For example, in high-density residential areas, z_o values of 0.13 m and 0.33 m were used for the roof and canopy, respectively. Consequently, the local bulk transfer coefficients for the roof and canyon were also calculated separately (Chen et al., 1997).

New aerodynamic parameters were derived to represent the canopy as a whole, disregarding the individual effects of the roof and canyon. For consistency in applying the distributed z_o, a top-down scheme (Kanda et al., 2005) was employed wherein the surface layer bulk transfer was estimated and used to calculate, via weighted averaging, the transfer coefficients for each individual surface (roof, wall, and ground). This scheme is valid for applications generally focused on the surface layer at the mesoscale.

References:
Chen F, Janjić Z, Mitchell K (1997) Impact of Atmospheric Surface-layer Parameterizations in the new Land-surface scheme of the NCEP Mesoscale Eta Model. Boundary-Layer Meteorol 85: 391-421.
Kanda M, Kawai T, Kanega M, Moriwaki R, Narita K, Hagishima A (2005) A Simple Energy Balance Model For Regular Building Arrays. Boundary-Layer Meteorol 116: 423-443.
Macdonald RW, Griffiths RF, Hall DJ (1998) An improved method for the estimation of surface roughness of obstacle arrays. Atmos Environ 32: 1857-1864.

              II.            Calling single-layer urban canopy model at grids with urban fraction > 0

The accuracy of urban fraction in a calculation grid is also critical for simulation accuracy (Lo et al., 2007). The urban fraction (urb_frc) is related to the resolution of land use category. By default, urban canopy model is employed for urban dominant grids. In this case, the grid will assume three possible urb_frc values, (0.5, 0.9 and 0.95, as specified in the urban parameter table of WRF), depending on the assigned building classification for the grid. Excess of the assigned urb_frc (i.e. 1.0 – urb_frc) is automatically assigned cropland/grassland category and will be handled by the land use model. For non-urban dominant calculation grids, only the dominant category was considered and other categories which can be present are disregarded. Each land use category assumes various parameter values when distribution is not available. A few examples of the parameters used in the land surface models available for each internationally recognised land use category are roughness length of momentum, green vegetation fraction, albedo, leaf area index, and emissivity – the accuracy of these assumptions is beyond the concern of this study.

The simulations conducted in this study use a 100 m resolution 15 category land-use scheme from the National Land Numerical Information adjusted to match the standard U.S. Geological Survey land use. Due to its high spatial resolution, determining a more precise land use fraction, as well as urb_frc, was possible. Thus, calling the urban canopy model subroutine no longer relies mainly on the dominant category but instead on the actual calculated urb_frc. In other words, grids will use the actual fraction calculated from the land use category and, likewise, proportion the influence of both urban canopy model and land use model in the grid. Since 100 m is not enough to accommodate vegetation within urban spaces (which is approximately equal to 0.10 according to local government measurements in Tokyo), an urb_frc upper limit and vegetation fraction (veg_frc) of 0.9 and 0.1 were set, respectively.

In addition to using actual land use fractions, a second dominant land use category scheme was introduced for grids with urb_frc > 0. The reason behind this was to ensure that the energy and momentum flux contribution of other land use categories are considered in a calculation grid. This was achieved considering only the urban category, most dominant, and second dominant land use categories for each grid. First, the grid is classified as urban dominant (urb_frc­ > other land use factors) or non-urban dominant (other land use factors > urb_frc). If a grid is urban dominant, only the area occupied by urban and the area occupied by the next dominant land use category were considered. For non-urban dominant grids, only the area of the most dominant land-use category and the urban area were considered. The ratio of urb_frc to the sum of the urb_frc and the other considered land use category affects how much the urban canopy model contributes to the surface fluxes in the grid. The other land use category is handled by the land surface model as mentioned earlier.

There were two advantages to this procedure compared with default WRF. First, the effect of other land use category in urban dominant grids was considered; likewise, the urban contribution to fluxes from the urb_frc in non-urban dominant grids was also considered.

References:
Lo JCF, Lau AKH, Chen F, Fung JCH, and Leung KKM (2007) Urban modification in a mesoscale model and the effects on the local circulation in the Pearl River Delta region. J Appl Meteorol Climatol 46: 457-476.

    　III.            Consideration of 3-D urban surface features in the urban canopy model

To a certain extent, 2-D urban canopy features is inconsistent with the new 3-D surface-derived feedback parameters. As a supplement to using 3-D-generated roughness parameters, a gridded sky-view factor was also introduced and calculated based on H_ave, λ_p­, and λ_f, using an equation regressed from a highly accurate 3-D scheme (Kanda et al., 2005a). Because the equation assumed infinitely long street canyons, the parameterisation of sky-view factor was consistent with the default model’s sky-view factor definition. The sky-view factor, from the floor to the sky, determined the amount of total solar radiation reaching the wall and ground within the canopy. λ_p­ is also essential because the direct solar radiation was evaluated from a weighted average according to the relative area of different canyons. The diffuse solar and downward longwave radiation were assumed to be isotropic.

References:
Kanda M, Kawai T, Nakagawa K (2005a) A Simple Theoretical Radiation Scheme For Regular Building Arrays. Boundary-Layer Meteorol, 114, 71-90

           IV.            Consideration of the effect of vegetation on local bulk transfer coefficient

The roughness length for heat is parameterised by default as a function of the roughness length for momentum. This function utilizes on the Reynolds number, z_o. and roughness length for heat to estimate the transfer coefficient. Kawai et al. (2009) introduced an updated parameterisation of roughness length for heat, which considers the advection effect, or the enhancement of the transfer coefficients due to vegetation. This updated parameterisation was included into the urban canopy model along with the inclusion of a high spatial resolution vegetation fraction. 

References:
Kawai T, Ridwan MK, Kanda M (2009) Evaluation of the Simple Urban Energy Balance Model Using Calculated Data from 1-yr Flux Observations at Two Cities. J Appl Meteorol Climatol 48: 693-715.

Contact Information: Alvin Christopher Galang Varquez, varquez.a.aa@m.titech.ac.jp