| 3 | | PALM-4U is frequently referred to as a separate model for the simulation of urban atmospheric boundary layers. However, from a technical point of view, PALM-4U are special components that have been developed to suit the needs of modern academic urban boundary layer research and practical city planning related to the urban microclimate and climate change. PALM-4U components are shipped with PALM and are available after installation of PALM. PALM-4U components are thus also available in PALM and might be used without being limited to urban area applications. Per definition, starting from PALM version 5.0, the user runs PALM-4U as soon as buildings are placed within the model domain. |
| | 3 | PALM-4U is frequently referred to as a separate model for the simulation of urban atmospheric boundary layers. However, from a technical point of view, PALM-4U are special components that have been developed to suit the needs of modern academic urban boundary layer research and practical city planning related to the urban microclimate and climate change. PALM-4U components are shipped with PALM and are available after installation of PALM. PALM-4U components are thus also available in PALM and might be used without being limited to urban area applications. Per definition, starting from PALM version 5.0, the user runs PALM-4U as soon as buildings are placed within the model domain and at least one of the following PALM-4U components is used: |
| | 4 | |
| | 5 | {{{ |
| | 6 | #!div style="align:'left'; width: 200px; border: 0px solid; float:right" |
| | 7 | [[Image(PALM-4U_logo.png,200px,https://uc2-mosaik.org)]] |
| | 8 | }}} |
| | 9 | |
| | 10 | * Energy balance solvers for building and paved surfaces |
| | 11 | * Radiative transfer within the urban canopy layer, including shadowing effects and multiple reflections between urban structures |
| | 12 | * Wall material model for heat transfer between atmosphere and building |
| | 13 | * Indoor climate module, predicting indoor temperature, energy demand, and waste heat |
| | 14 | * Chemistry module for the transport and conversion of reactive species |
| | 15 | * Model self-nesting that allows to increase either model domain size or to focus on near-surface processes |
| | 16 | * A multi-agent system for urban residents, allowing for biometeorological studies and escape scenarios |
| | 17 | * Quasi-automatic external forcing by COSMO-DE model data |
| | 18 | * A Reynolds-averaged Navier Stokes (RANS) type turbulence parameterization can be used instead of LES to reduce computational costs |
| | 19 | * Analysis tools and direct output of biometeorological quantities |
| 9 | | == Topography parameterization == |
| 10 | | The Cartesian topography in PALM is generally based on the mask method ([#briscolini1989 Briscolini and Santangelo, 1989]) and allows for explicitly resolving solid obstacles such as buildings and orography. The implementation makes use of the following simplifications: |
| 11 | | |
| 12 | | 1. the obstacle shape is approximated by (an appropriate number of) full grid cells to fit the grid, i.e., a grid cell is either 100% fluid or 100% obstacle, |
| 13 | | |
| 14 | | 2. the obstacles are fixed (not moving). |
| 15 | | |
| 16 | | Topography is realized in 3-D, e.g., overhanging structures as for example bridges, ceilings, or tunnels, are allowed, i.e. topography does not necessarily be surface-mounted. 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). |
| 17 | | In case of overhanging structures, however, 3-D topography information is required to mask obstacles and their faces in PALM. |
| 18 | | |
| 19 | | The model domain is then separated into three subdomains: |
| 20 | | |
| 21 | | A. grid points in free fluid without adjacent surfaces, where the standard PALM code is executed, |
| 22 | | |
| 23 | | B. grid points next to surface that require extra code (e.g., surface parametrization), and |
| 24 | | |
| 25 | | C. grid points within obstacles, where the standard PALM code is executed but multiplied by zero. |
| 26 | | |
| 27 | | Additional topography code is executed in grid volumes of subdomain B. The faces of the obstacles are always located where the |
| 28 | | respective surface-normal velocity components ''u'', ''v'', and ''w'' are defined so that the impermeability boundary condition can be implemented by setting the respective surface-normal velocity component to zero. |
| 29 | | |
| 30 | | 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. |
| 31 | | 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. |
| 32 | | |
| 33 | | Simulations with topography require the application of MOST between each surface and the first computational grid point outside of the topography. |
| 34 | | For vertical and horizontal downward-facing surfaces, neutral stratification is assumed for MOST. |
| 35 | | |
| 36 | | In the PALM core, buildings are primarily realized as obstacles that react to the flow dynamics via form drag |
| 37 | | and friction forces by assuming a constant flux layer between the building surface and the adjacent air volume. A simple thermodynamic coupling is also possible by prescribing surface fluxes of sensible (and latent heat) at any of the building surface grid elements. |
| 38 | | |
| 39 | | The technical realization of the topography and treatment of surface-bounded grid cells is be outlined in Section [wiki:doc/tec/topography topography implementation]. |
| 40 | | |
| 41 | | |
| 42 | | == Urban and natural surface schemes == |
| 43 | | |
| 44 | | In order to simulate interactions between the atmosphere |
| 45 | | and the soil-vegetation continuum, an energy balance |
| 46 | | solver for natural surfaces in urban environments is essential to predict realistic |
| 47 | | surface conditions and fluxes of sensible heat and latent |
| 48 | | heat. When using the concept of the surface skin layer, |
| 49 | | where vegetation and bare soil fractions are considered |
| 50 | | to be flat and have a joint skin layer temperature, T skin , |
| 51 | | the energy balance reads |
| 52 | | dT skin |
| 53 | | C skin |
| 54 | | = Rn − H − LE − G , |
| 55 | | dt |
| 56 | | (3.1) |
| 57 | | where Cskin is the heat capacity of the skin layer, Rn is |
| 58 | | the net radiation at the surface, H and LE are the tur- |
| 59 | | bulent surface fluxes of sensible and latent heat, and G |
| 60 | | is the heat flux into (or out of) the soil. Fluxes are de- |
| 61 | | fined positive (negative) when they are directed away |
| 62 | | (towards) the surface. A full interactive land surface |
| 63 | | scheme (LSM) was recently implemented in PALM, |
| 64 | | based on the Tiled European Centre for Medium-Range |
| 65 | | Weather Forecast Scheme for Surface Exchange over |
| 66 | | Land (Balsamo et al., 2009, TESSEL/HTESSEL, e.g.) |
| 67 | | and was first applied by Maronga and Bosveld (2017). |
| 68 | | The scheme consists of an energy balance solver for |
| 69 | | T skin and a multi-layer soil scheme that takes into ac- |
| 70 | | count the vertical diffusion of heat as well as vertical |
| 71 | | water transport in the soil. Vegetation is fully parameter- |
| 72 | | ized, including root extraction of water from particular |
| 73 | | soil layers used for transpiration and a prognostic equa- |
| 74 | | tion for the liquid water stored on plants by interception. |
| 75 | | So far, however, all vegetation is treated to be subgrid- |
| 76 | | scale, e.g. the canopy has no vertical extent and is pa- |
| 77 | | rameterized by aerodynamic parameters such as rough- |
| 78 | | ness lengths and heat capacity and conductivity of the |
| 79 | | skin layer. The land surface scheme is also applied for paved surfaces, which only deviates in the treatment of vegetated surface by different material properties and imperviousness to water. |
| 80 | | |
| 81 | | In order to simulate realistic urban environments, an adapted version of the land surface parameterization is available for building facade elements. |
| 82 | | Essentially, this involves the solution of an adapted version |
| 83 | | of Eq. 3.1 for each urban surface element, such as build- |
| 84 | | ing facades, roofs and impervious horizontal surfaces |
| 85 | | like pavement. For solving Eq. 3.1, the radiative transfer |
| 86 | | in the urban canopy, including multiple reflections and |
| 87 | | shading from buildings must be calculated, which may |
| 88 | | be considered to be one of the main challenging tasks |
| 89 | | in urban surface modelling (see Sect. 3.1.7). In order to |
| 90 | | estimate the heat flux G into the material, all building |
| 91 | | facades must be coupled to a multi-layer wall model. |
| 92 | | This is further complicated by the fact, that facades can |
| 93 | | not only consist of solid walls, but usually also consist |
| 94 | | of significant fractions of windows and sometimes green |
| 95 | | elements. Windows in particular have significantly dif- |
| 96 | | ferent physical properties than solid (greened) wall, e.g. |
| 97 | | in albedo, and they also allow shortwave radiation to en- |
| 98 | | ter the building. |
| 99 | | A preliminary version of an urban surface model |
| 100 | | (USM) has been recently entered the PALM default code |
| 101 | | (Resler et al., 2017), which already includes an energy |
| 102 | | balance solver for solid walls (see Fig. 5). In the course |
| 103 | | of MOSAIK we will take this as a basis to add the |
| 104 | | treatment of windows and green facades using the tile |
| 105 | | approach. Also, we will couple the USM to an indoor |
| 106 | | climate and energy demand model (see Sect. 3.1.6). |
| 107 | | |
| 108 | | |
| 109 | | |
