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 and at least one of the following PALM-4U components is used:

• Energy balance solvers for building and paved surfaces
• Radiative transfer within the urban canopy layer, including shadowing effects and multiple reflections between urban structures
• Wall material model for heat transfer between atmosphere and building
• Indoor climate module, predicting indoor temperature, energy demand, and waste heat
• Chemistry module for the transport and conversion of reactive species
• Model self-nesting that allows to increase either model domain size or to focus on near-surface processes
• A multi-agent system for urban residents, allowing for biometeorological studies and escape scenarios
• Quasi-automatic external forcing by COSMO-DE model data
• A Reynolds-averaged Navier Stokes (RANS) type turbulence parameterization can be used instead of LES to reduce computational costs
• Analysis tools and direct output of biometeorological quantities

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.

RANS turbulence parameterization

As alternative to the turbulence-resolving LES mode, PALM-4U offers a RANS-type turbulence parameterization. In more detail, a so-called TKE- ε−parameterization (Kato and Launder, 1993; Lopez et al., 2005) is implemented, which is based on two prognostic equations for the turbulence kinetic energy (TKE) and its dissipation rate ε.

Nesting and coupling to large-scale models

PALM/PALM-4U has an interface that allows for using model output of larger-scale models as boundary conditions. The additional software package INIFOR is shipped with PALM and allows to process data output from COSMO (support for the ICON model chain will be added later) and provides suitable input data for PALM-4U. The processed data can then be used in PALM-4U either as Dirichlet boundary conditions (in RANS mode, default) or as additional tendency terms in the respective prognostic equations (LES mode with cyclic boundary conditions).

Moreover, a self-nesting ofPALM-4U is realized, allowing to use the model with a magnifier lens tool. Adaption of the already implemented model coupler for RANS-RANS nesting is currently under way. Moreover, in order to use COSMO model data with Dirichlet boundary conditions together with the LES mode, it will soon be possible to feed the COSMO data to PALM-4U in the RANS mode, and nest LES domains therein.

Land surface representation

For natural and paved surfaces in urban environments, PALM-4U employs PALM's land surface model.

Urban surface representation

Indoor climate and building energy demand

Radiative transfer in the urban canopy layer

Chemistry

Multi-agent system

Human biometeorology

Graphical user interface