| 286 | | 2. so far, only bottom surface-mounted obstacles are permitted (no holes or overhanging structures), and |
| 287 | | |
| 288 | | 3. the obstacles are fixed (not moving). |
| 289 | | |
| 290 | | These simplifications transform the 3-D obstacle dimension to a 2.5-D topography. This reduced dimension format is conform to the Digital Elevation Model (DEM) format. DEMs of city morphologies have become increasingly available worldwide due to advances in remote sensing technologies. Consequently, it is sufficient to provide 2-D topography height data to mask obstacles and their faces in PALM. The model domain is then separated into three subdomains (see Fig. 3): |
| 291 | | |
| 292 | | A. grid points in free fluid without adjacent walls, where the standard PALM code is executed, |
| 293 | | |
| 294 | | B. grid points next to walls that require extra code (e.g., wall functions), and |
| | 286 | 2. the obstacles are fixed (not moving). |
| | 287 | |
| | 288 | With revision -r2232, the topography implementation is completely revised. |
| | 289 | Starting from this revision, overhanging structures as for example bridges, ceilings, or tunnels, are allowed, i.e. topography does not necessarily be surface-mounted. |
| | 290 | If no overhanging structures are present, the 3-D obstacle dimension reduces to a 2.5-D topography format, which is conform to the Digital Elevation Model (DEM) format (DEMs of city morphologies have become increasingly available worldwide due to advances in remote sensing technologies). |
| | 291 | In case of overhanging structures, however, 3-D topography information is required to mask obstacles and their faces in PALM. |
| | 292 | |
| | 293 | The model domain is then separated into three subdomains (see Fig. 3): |
| | 294 | |
| | 295 | A. grid points in free fluid without adjacent surfaces, where the standard PALM code is executed, |
| | 296 | |
| | 297 | B. grid points next to surface that require extra code (e.g., surface parametrization), and |
| 300 | | Figure 4: Sketch of the 2.5-D implementation of topography using the mask method (here for ''w''). The yellow and red lines represent the limits of the arrays ''nzb_w_inner'' and ''nzb_w_outer'' as described in Sect. [wiki:doc/tec/topography topography implementation], respectively. |
| 301 | | |
| 302 | | Additional topography code is only executed in grid volumes of subdomain B. The faces of the obstacles are always located where the |
| 303 | | respective wall-normal velocity components ''u'', ''v'', and ''w'' are defined (cf. Fig. 1 in Sect. [wiki:doc/tec/discret discretization]) so that the impermeability boundary condition can be implemented by setting the respective wall-normal velocity component to zero. |
| 304 | | |
| 305 | | An exception is made for the 5th-order advection scheme, where the numerical stencil at grid points adjacent to obstacles would require data within the obstacle. In order to avoid this behavior, the order of the advection scheme is successively degraded at respective grid volumes adjacent to obstacles, i.e., from the 5th-order to 3rd-order at the second grid point above/beside an obstacle and from 3rd-order to 1st-order at grid points directly adjacent to an obstacle. |
| 306 | | |
| 307 | | Wall surfaces in PALM can be aligned horizontally (bottom surface or rooftop, i.e., always facing upwards) or vertically (facing north, |
| 308 | | east, south or west direction). At horizontal surfaces, PALM allows to either specify the surface values (''θ, q,,v,,, s'') or |
| 309 | | to prescribe their respective surface fluxes. The latter is the only option for vertically oriented surfaces. Simulations with topography |
| 310 | | require the application of MOST between each wall surface and the first computational grid point. For vertical walls, neutral |
| 311 | | stratification is assumed for MOST. The topography implementation has been validated by [#letzel2008 Letzel et al. (2008)] and [#kanda2013 Kanda et al. (2013)}. [#park2013 Park and Baik (2013)] have recently extended the vertical wall boundary conditions for non-neutral |
| | 303 | Figure 4: Sketch of the topograhy implementation using the mask method (here for ''w''). The yellow and red lines represent the limits of the arrays ''nzb_w_inner'' and ''nzb_w_outer'' as described in Sect. [wiki:doc/tec/topography topography implementation], respectively. |
| | 304 | |
| | 305 | Additional topography code is executed in grid volumes of subdomain B. The faces of the obstacles are always located where the |
| | 306 | respective surface-normal velocity components ''u'', ''v'', and ''w'' are defined (cf. Fig. 1 in Sect. [wiki:doc/tec/discret discretization]) so that the impermeability boundary condition can be implemented by setting the respective surface-normal velocity component to zero. |
| | 307 | |
| | 308 | In case of 5th-order advection scheme, the numerical stencil at grid points adjacent to obstacles would require data which is located within the obstacle. |
| | 309 | In order to avoid this, the order of the advection scheme is successively degraded at respective grid volumes adjacent to obstacles, i.e., from the 5th-order to 3rd-order at the second grid point above/beside an obstacle and from 3rd-order to 1st-order at grid points directly adjacent to an obstacle. |
| | 310 | |
| | 311 | Surfaces in PALM can be aligned horizontally upward facing (e.g. bottom surface or rooftop), horizontally downward facing (e.g. undersurface of bridges), or vertically (facing north, east, south or west direction). |
| | 312 | At horizontal surfaces, PALM allows to either specify the surface values (''θ, q,,v,,, s'') or to prescribe their respective surface fluxes. |
| | 313 | The latter is the only option for vertically oriented surfaces. |
| | 314 | Simulations with topography require the application of MOST between each surface and the first computational grid point outside of the topography. |
| | 315 | For vertical and horizontal downward-facing surfaces, neutral stratification is assumed for MOST. |
| | 316 | The topography implementation has been validated by [#letzel2008 Letzel et al. (2008)] and [#kanda2013 Kanda et al. (2013)}. |
| | 317 | [#park2013 Park and Baik (2013)] have recently extended the vertical wall boundary conditions for non-neutral |
