PALM-4U components

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.

The PALM-4U components are have been and will be further developed by a consortium of institutions within the framework of the funding programme "[UC²] - Urban climate under change", funded by the German Federal Ministry of Education and Research (BMBF). For more information, see ​​http://uc2-mosaik.org.

In the following an overview of the PALM-4U components is given. Note that some of these still undergo major changes and improvements at the moment. A final documentation and publications are planned in 2018 and 2019.

Building parameterization

The Cartesian topography in PALM is generally based on the mask method (Briscolini and Santangelo, 1989) and allows for explicitly resolving solid obstacles such as buildings and orography. The implementation makes use of the following simplifications:

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,

2. the obstacles are fixed (not moving).

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). In case of overhanging structures, however, 3-D topography information is required to mask obstacles and their faces in PALM.

The model domain is then separated into three subdomains:

1. grid points in free fluid without adjacent surfaces, where the standard PALM code is executed,

2. grid points next to surface that require extra code (e.g., surface parametrization), and

3. grid points within obstacles, where the standard PALM code is executed but multiplied by zero.

Additional topography code is executed in grid volumes of subdomain B. The faces of the obstacles are always located where the 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.

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. 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.

Simulations with topography require the application of MOST between each surface and the first computational grid point outside of the topography. For vertical and horizontal downward-facing surfaces, neutral stratification is assumed for MOST, even if MOST is strictly speaking derived only for upward-facing horizontal surfaces. This is simply attributed to the lack of knowledge in the literature about the best practice in this matter.

Missing:

• Coupling to LSM and USM
• Terrain height and topography

The technical realization of the topography and treatment of surface-bounded grid cells will be outlined in Sect. topography implementation.

RANS turbulence parameterization

Land and urban surface model

• refer to PALM's LSM

Radiative transfer in the urban canopy layer

• refer to radiation models in PALM

Indoor climate model

Chemistry module

Multi-agent system

External forcing by COSMO data

• refer to PALM overview's "external forcing" page and vice versa

Data input, output, and handling

• Coupling to the water balance model STORM for preprocessing of soil moisture
• Analysis tools and direct output of biometeorological quantities

soon.