Urban surface model (USM)

Main page of the urban surface model under construction. Click here for first information about capabilities and model steering. Also available, the related article PALM-USM v1.0: A new urban surface model integrated into the PALM large-eddy simulation model (Resler et al., Geosci. Model Dev., 10, 3635–3659, ​10.5194/gmd-10-3635-2017 ​10.5194/gmd-10-3635-2017)

This page is part of the Urban Surface Mod (USM) documentation.
It describes the physical and numerical realization of the USM.
Please also see the namelist parameters.

Overview

Since r19xx an urban surface model (USM) is available in PALM (see urban_surface_mod.f90​). It consists of a multi layer wall and soil model, predicting wall and soil temperature and moisture content, and a solver for the energy balance, predicting the temperature of the surface or the skin layer. Urban surfaces (building surfaces) are simulated using a tile approach. Each surface element consists of a fraction of bare wall/ roof, window and green elements (green roofs/ facades) with underlying soil layers (green roofs only) and a bare wall/ roof structure.

Energy balance solver

The energy balance of the urban surfaces reads

where C₀ and T₀ are the heat capacity and radiative temperature of the surface skin layer, respectively. Note that C₀ is usually zero as it is assumed that the skin layer does not have a heat capacity (see also below). R_n, H, LE, and G are the net radiation, sensible heat flux, latent heat flux, and ground (soil) heat flux at the surface, respectively.

The energy balance is calculated for each urban surface tile individually and the three radiation surface temperatures are combined together.

The parametrisation of the sensible heat flux, latent heat flux and ground heat flux of the wall/ window/ soil is equivalent to the Land Surface Model (LSM).

The wall heat and green heat model consist of prognostic equations for the bare, window and soil temperature and the volumetric soil moisture which are solved for multiple layers. The models only take transport into account that is orthogonal to the urban surface layer orientation and no ice phase is considered. By default, the wall heat model and the green heat soil model consists of four layers each (see Fig. 1 below), in which the orthogonal heat and water transport inside the soil is modelled.

Figure 1: urban surfaces (bare, window, green - horizontal and vertical) in PALM-4U

The physical properties of the urban surfaces and wall, window and green soil elements can be set using values from a building database where different types of buildings are defined. The insulation value of windows are there only characterized by the U-value and and the heat capacity and heat conductivity is evenly distributed (no real glas or gas layers are taken into account.

The absorption of shortwave radiation inside the window layers is calculated using an logarithmic absorption function and the absorption coefficient is calculated using the overal window transmissivity value. The heat that is absorpted within each window layer is taken into account by the wall heat model.

The green heat model calculates the transport of soil moisture but neglects the extraction of water from the respective soil layers.

Boundary conditions

Neumann boundary conditions are used for the transport of heat at the upper boundary (surface). The values are given by the energy balance. At the bottom boundary either a fixed temperature of the inner wall and window layers is set or the ground heat flux from the inner wall and window surface is used that is calculated by the indoor model (Dirichlet conditions).