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. The scheme consist of an energy balance solver for all different types of surfaces as well as an multi-layer soil model to account for vertical diffusion of heat and water transport in the soil. For natural vegetated surfaces, the energy balance solver will use the concept of a skin layer that has no heat capacity but considers the insulating effect of plants. In the absence of vegetation, no skin layer approach is used and the surface temperature is taken equal to the outermost soil, pavement, or wall layer.

Vegetation can be either defined to be sub-grid scale (e.g. short grass) and is then purely treated in the land surface scheme. For tall vegetation (e.g. trees), PALM-4U offers a 3D canopy model which is based on a drag force approach and a leaf area density distribution. The canopy model is thus fully coupled to the soil model and an energy balance solver for the leaf temperature is solve at all grid volumen with a leaf area density. Also explicit transpiration of the 3D canopy elements will be realized.

Urban surface representation

For urban surface elements (i.e. building facades and roofs), an adapted version of the land surface scheme was developed. It consists of an energy balance solver for the surface temperature and a multi-layer wall material model. The wall model follows a tile approach so that fractions of solid walls, windows, and green facades are treated separately. Details of the preliminary urban surface model are given in Resler et al. (2017).

Indoor climate and building energy demand

In order to calculate the interaction of the buildings with the atmosphere, a holistic indoor climate model is available in PALM-4U. This model predicts the indoor temperature and also calculates both the energy demand of each building as well as the waste heat that is released to the atmosphere. The model is integrated as an optional module that is coupled to the wall model by using the temperature of the innermost wall layer of the respective building facades as input parameter. Also, the transmitted radiation by windows is transferred to the indoor model. The indoor temperature is then calculated based on building characteristics (e.g. insulation, air conditioning, and heating). In return, the indoor temperature is transferred to the wall model as boundary condition, while waste heat from heating or air conditioning is fed back into the atmosphere as an additional tendency in the prognostic equation for temperature at the roof top (representing the typical location of chimneys and air conditioning units).

Radiative transfer in the urban canopy layer

Chemistry

Multi-agent system

Human biometeorology

Graphical user interface