SOLWEIG 1.0 – Modelling spatial variations of 3D radiant fluxes and ...

698 Int J Biometeorol (2008) 52:697–713
sky the upper hemisphere, both with an angle factor of 0.5
(e.g. Jendritzky et al. 1990; PickupanddeDear1999).
Although easier to use, this method is only reliable for
unobstructed open spaces, and obstruction effects should
be added using sky view factors.
Over the years, several different software have been
developed to simulate Tmrt in outdoor urban settings.
TOWNSCOPE (Teller and Azar 2001) is a CAD-based
software which, among other things, simulates spatial
variations of solar access and Tmrt in complex urban
environments. Thermal comfort can only be calculated on
a daily, monthly or annual basis and, since vector data are
utilised, the spatial extension is limited. The RayMan
software (Matzarakis 2000; Matzarakis et al. 2000, 2007)
is a tool which, along with Tmrt, simulates different thermal
indices. RayMan is a stationary model and can be run on
very few input meteorological parameters. RayMan is very
user-friendly and thus a popular tool for researchers, urban
planners and practitioners. Although commonly used,
RayMan has some principal shortcomings regarding the
calculation of the three-dimensional radiation flux densities
and surface temperatures and, consequently, the resulting
Tmrt (Thorsson et al. 2007). The three-dimensional ENVImet
model (Bruse 1999, 2006) simulates radiation fluxes,
temperature, wind flow, turbulence and humidity, as well as
Tmrt with high spatial and temporal resolution. The model is
grid-based. Because of its complexity, the model’s domain
size is restricted to 250×250×25 grids. It can be run on a
regular PC. Like the other models mentioned above, ENVImet
calculates Tmrt according to Fanger (1972); the
surrounding environment is divided into building surfaces,
the free atmosphere (sky) and the ground surface for which
the direct, diffuse and diffusely reflected shortwave and the
total longwave radiation components are taken into account.
At street level, the total longwave radiation fluxes
are assumed to originate from the upper hemisphere (sky
and buildings) and from the ground (lower hemisphere).
ENVI-met is still under development. To date, the model
has only been evaluated with respect to Tmrt for an east–west
oriented urban canyon (H/W=1) in Freiburg, Germany,
during a hot summer’s day (Ali Toudert 2005). The
evaluation showed good agreement between simulated
shortwave radiation fluxes and field data. However, the
simulated longwave radiation fluxes revealed discrepancies
with the field data by up to 50 Wm −2 , which played the main
role in the differences observed in T mrt, (–8°C in daytime)
(Ali Toudert 2005).
In this paper, the development of a new radiation model,
SOLWEIG 1.0 (solar and longwave environmental irradiance
geometry-model), which simulates three-dimensional
daytime radiation fluxes and Tmrt in complex urban settings
is presented. The first part of the paper presents the features
of the SOLWEIG 1.0 model and is followed by an
evaluation of the model using three-dimensional integral
radiation measurements.
Materials and methods
Model structure
The framework theory for Tmrt calculations used in this
study is based on the measuring procedure proposed by
Höppe (1992), in which each of the six longwave and
shortwave radiation fluxes (upward, downward and from
the four cardinal points) is considered. The meteorological
input parameters are direct, diffuse and global shortwave
radiation, air temperature and relative humidity. Spatial
variations of urban geometry are represented by a highresolution
urban digital elevation model (DEM) covering
the central parts of Göteborg (Fig. 1). SOLWEIG 1.0 is a
2.5-dimensional model in the sense that it applies a 2.5dimensional
DEM (i.e. x and y coordinates with height
attributes) in the calculation of Tmrt. However, the output
from the current version of SOLWEIG is represented in two
dimensions (x and y). The model was evaluated by using
three-dimensional radiation data from two different locations
within the centre of Göteborg, Sweden (57°N), during
different weather conditions and at different times of the
year. Radiation data obtained from the Swedish Meteorological
and Hydrological Institute (SMHI) are used as
parameterisation data in the cloudiness calculations. Sur-
Fig. 1 A DEM covering the central parts of Göteborg. The two
locations where integral radiation measurements were conducted are
marked (SITE 1 and SITE 2)